'69 Princeton Reverb.....Badly Burned, Brutally Botched, and (finally) Reborn

This video records the challenging restoration of a 1969 Fender Princeton Reverb which was badly burned in a fire. The chassis was rescued by the owner, who hand-built a beautiful, original-looking replacement cabinet with a nice Jensen speaker and new Accutronics reverb tank......then, very sadly, it was turned over to a local "technician" who proceeded to mis-wire it badly, with several short circuits, missing wires and connections, and improper component values, creating a dysfunctional nightmare.
In desperation, it was sent to me for repair.....and presented one of the biggest repair challenges I have ever faced. This video follows the step-by-step elimination of (hopefully) all the previous mistakes, resulting in one of the best sounding PR's I have ever heard. I hope you agree :)
In response to viewer requests, I have focused on relating schematics to actual circuits, given detailed explanations of why certain repairs work, and thoroughly discussed pertinent tube amp circuitry details......all in an attempt to maximize the educational content of this video.
If you enjoy advertising-free videos like this, please subscribe to my channel and consider becoming a Patreon patron at: / uncledougsvintageamps or making a PayPal contribution to [e-mail]. Also if you have any vintage tube amps or gear that you would like to contribute as possible video features, please provide your e-mail address and I will contact you.
Thanks so much !!!

[@UncleDoug]

NOTES: 1.) I have upgraded to a broadband Internet provider, with uploads that are now 10X faster. I believe this has resulted in improved audio and video. See what you think. 2.) I did not realize during video production that the owner actually hand-built the replica cabinet, using vintage Fender Tolex. It completely fooled me. I thought it was an original replacement.

[@gerthalberg9735]

Hi Uncle - right on regarding video & audio

[@freepress8451]

Another fine video - do you celebrate Pancake Day (Shrove Tuesday) in US?

[@UncleDoug]

Thanks guys. No, we only celebrate Catnip Day......every day :)

[@peterespada6226]

I just discovered your videos a couple months

[@peterespada6226]

ago and am amazed with your deep erudition regarding electronics. Were you once a college professor or teacher in this field? I’ve learned so much so easily that I had always wondered about watching your videos...

[@Bbendfender]

Hey guys, I am the owner of this little Princeton Reverb. I just had to save it when I saw it. I'm not a tech so I let a "friend" look at it. He tried but couldn't do the job correctly. Along with this Princeton Reverb I also got a '63 Fender Bandmaster that was in the same burned condition of the Princeton. When I can afford it, I may get it refurbished. I know it will be another great amp. I am thankful to Uncle Doug for his great work.

[@UncleDoug]

Thanks for your comments, Bob. I hope you enjoyed the video. The PR is now one of the best sounding ones I have ever heard. We can talk about the Bandmaster. Hopefully, it hasn't been worked on previously. Also, the replica cabinet you constructed is truly outstanding. You have done us all a great favor by saving this badly damaged PR from the landfill.

[@izzysantiago978]

The 2 thumbs down are the 2 idiots that messed up the wiring on this amp.

[@UncleDoug]

.......and their reward will be a colonoscopy with the unbiased 6V6 that blistered my finger :)

[@HasteFrankie]

@@UncleDoug ha! they deserve it. Hope that heals up quick. thank you for the great content as always!

[@freepress8451]

@@UncleDoug Ouch

[@jagtone]

LOL!

[@WCM1945]

@@UncleDoug _However..._ You _did_ notice the cherry-red plates, didn't you? Shouldn't that have given you a clue as to the temp of the tubes? I share you dissatisfaction with the "tech" who did the original work... He either didn't check his work for function, or just didn't give a s**t. I made some wiring errors in my time, but I would **_*never_* have let an amp in that condition leave my workbench.

[@coronadofishingclub3478]

Warm regards Uncle Doug. I enter into chemo therapy for lymphoma tomorrow and I hope to see some more videos to help me pass the time in the hospital. I wish your videos had been available 20 years ago when I started teaching myself about tube electronics, I could’ve avoided many mistakes. Thank you so much for taking the time to clarify and demystify this stuff! Once I beat the cancer I’m gonna start a new amp build and I owe it to you for re kindling my fascination for this wacky stuff. Cheers, Tim.

[@UncleDoug]

We are all wishing you the very best for a successful treatment and full recovery, Tim. The world needs all the tube amp lovers it can get. Your presence is definitely required.....so get well quick !!!

[@willford8475]

Good Luck, Mate. 👍

[@lousekoya1803]

We wanna see you back here in shape soon !!........Regards!...........Lou from Canada !!

[@FishnMusician]

Tim, get well soon man. You have folks pulling for you, whether you know it or not. Hoping for the best! Take care, and the very best of luck to you and yours.

[@tatialo37]

Praying for you!!!

[@kendavis8046]

Great trouble shooting and restoration of a classic amp. Thanks, Uncle Doug. If there is a future Sergio Leone, this amp can make the soundtrack! (To most young folks, that is likely a totally obscure reference.) Thumbs up!

[@UncleDoug]

You're welcome, Ken. The next time Sergio is over to visit with Jack & Ollie, I'll suggest it to him :)

[@renegade44040]

Don't like to but in...but let's give credit where it's due. Ennio Morricone is Sergio Leone's composer.

[@goodun6081]

A modern Sergio Leone movie will be titled *Twice* Upon a Time in the West. The music will be by Eno Morracowbell.....

[@kendavis8046]

@@goodun6081 He has a fever! And the only cure is Morracowbell.

[@leonardodic3po607]

It seems like the wiring of this amp was done by Sergio Aragonês!

[@smasica]

I'm reluctant to slag anyone else's work, but that amp tech must have worked on the amp after a rough night at the club. Wow, rookie mistakes galore. Doug, I respect your methodical, meticulous craftsmanship. I learn a bunch with every new video.

[@UncleDoug]

I agree, TM. In this case the previous work is condemnation enough :) Thanks !!!

[@deadfreightwest5956]

_"With the axe in one hand, and the guitar in the other... which sounds redundant..."_ - I burst out laughing at that one. I don't even play guitar, but I got it.

[@UncleDoug]

Thanks, DF. Fortunately, I didn't get mixed up and play the axe while chopping with the guitar :)

[@ronniecastellanos8223]

@@UncleDoug Uncle doug I need app doctor I've been using amp for music on my phone during family functions and she started to smell smoky what do I do help Uncle duggy

[@JalopyTechnology]

I saw Uncle Doug at the ER the same day he had his finger repaired. In my case it was an index finger nail was charred by a 6v6... We both sat in the corner wearing dunce caps.

[@bubbakav]

Everyone needs an uncle who knows his way around an amplifier.

[@UncleDoug]

Amen, David. I didn't have one.......so I had to become one.

[@vhollund]

I love watching this pretending i understand more than half of what's going on Maybe if i watch enough I'll learn it

[@soulvaccination8679]

Thats what i do...Ijust keep watching it and watching it and all of a sudden i get a moment of clarity .

[@Dennis-mq6or]

@@soulvaccination8679 Same here!

[@paulcargo4233]

Lets review today's lesson: Electricity like most people will take the path of least resistance.

[@UncleDoug]

So it seems, Paul :)

[@guitarocd9984]

My father was a ham radio operator. He built his own radios. I came home one day he was sitting in his chair with a spaced-out look on his face. He grabbed the wrong thing. Electricity went through his hand and blew a 2 inch hole in his leg. The doctor said it could have went to his member. LOL

[@UncleDoug]

Thank God he wasn't sitting on a metal stool, GO.

[@guitarocd9984]

The only thing that happened from it was he was mellow for a couple weeks.

[@UncleDoug]

@@guitarocd9984 I guess a shock once in a while is good for the soul :)

[@budandbean1]

Thank you Uncle Doug, this was really interesting. Excellent troubleshooting, really a fine job! Thanks for the tips about the virtual center tap too, you’ve gone over this a few times before but this time it really clicked. I just happen to have a solid state PA amp that has been humming a tiny bit since I bought it years ago that I’m going to see it this might knock that hum out of. Greatly appreciated, Buddy

[@UncleDoug]

You're welcome, Buddy. Thanks for all your very nice comments.

[@Magikflame]

@@UncleDoug I've just thoroughly enjoyed watching this vid and subbed. Yes I came across this type of thing many many years ago where there was a say 220 ohm pot across the filament winding with the wiper to earth marked as a.... Humdinger !!!! which I believe was in an old reel to reel....

[@daveogarf]

Uncle Doug, I'm AMAZED that you've managed to find such Talented kitty cats! String bends, complex chords, etc. seem to be a snap for them! Thank you so much for showcasing them and their talents. That amp could've been ever so painful to repair, but by following the schematic, coupled with your pre-existing knowledge of the beast, you found small but important errors in the circuitry.

[@UncleDoug]

Thanks for your kind words, Dave. Yes, we were lucky to have adopted kitties who were also excellent musicians :)

[@TheGuitologist]

100% agreed on tube mismatching.

[@UncleDoug]

Thanks, Brad :)

[@fiddlix]

The Guitologist I could not agree more.

[@Dennis-mq6or]

Matched tubes might have eliminated that very small bit hum that was left in the amp because the hum would have been cancelled in the output transformer The only way to know for sure is to ground the input to the phase inverter. One other possible solution would have been to add a 100 ohm potentiometer between the two 100 ohm 'phantom ground' resistors with it's wiper attached to ground. Then adjusting that pot might be able to 'null out' the remaining hum.

[@johnnybravo4704]

Job nice done.. Bravo Dough..

[@UncleDoug]

Thanks, Johnny.

[@daleburrell6273]

27:31...the amplifier hums because it doesn't know the words(?)

[@KC-cl1vr]

I have been to circuit design church with the preamp back-pressure reference

[@UncleDoug]

Those pesky electrons have to be herded like cattle :)

[@0richbike]

A little bit of knowledge... Did the "rewired" amp start the fire?

[@UncleDoug]

If it had been allowed to remain "on" for very long, I think the unbiased 6V6 would have probably incinerated the entire block. Actually, I think it was a house fire and the amp, which was in good shape at the time, was a victim......not a cause.

[@russellesimonetta3835]

Like uncle doug sez if the amp looks altered keep the fire extinquisher handy and the fire department on speed dial.

[@goodun6081]

@@UncleDoug , I personally know of several cases where solid state, transistorized audio amplifiers or Hi-Fi stereos that were DC coupled and that did not have output relays, or even fuses, between the amplifier and the speakers, actually failed sufficiently to send full B+ ( anywhere from 35 volts to 150 volts, depending on the amplifier) through to the speakers and caused them to catch fire. Whenever we fix an amplifier like this at the stereo repair shop I work for, we add fuses inside the amp or supply appropriately-sized fuses and fuse holders and instruct the customer how to add them into the system between the speakers and the amplifier. The danger is if the stereo is left on while someone is away from home, or at night when everyone is sleeping, you risk Burning Down the House and maybe the occupants with it. Fortunately, most modern amplifiers , at least those used in a home audio situation, will have time delay relays that not only let the amplifier stabilize for a couple of seconds before turning on the output to the speakers, but that also disconnect the speaker wires from the amplifier internally if they sense a significant fault, any DC voltage that would cause speaker damage or worse. It bears mentioning that many high power PA amplifiers do not have relays and might not have speaker protection fuses either. A tube guitar amp, by comparison, is unlikely to set the speaker on fire, unless the output Transformer short circuits, however A Fault In The Amplifier can easily burn resistors and capacitors, then the wiring and the cabinet catches fire. or the tubes get so hot from lack of negative bias that they become cherry red glowing radiant heaters and set the cabinet and speaker on fire.

[@timlilly]

Great Stuff Doug very interesting as usual

[@UncleDoug]

Thanks, Tim. That's good to hear.

[@johntempleton6368]

You're spot on about the matching of the output tubes. A small to moderate difference in idle current does provide more harmonic content. I've found that once the difference goes beyond the 5 - 8 ma range that a bit of hum creeps in because the common mode rejection of noise is reduced. Great tip. Your trem mods are another killer tip that I've happily borrowed and used. Love your videos.

[@UncleDoug]

Thanks so much, John. Yes, excessive mismatch can result in reduced hum cancellation and core saturation......so it must be kept within reasonable limits.

[@fiddlix]

I have been doing this as well for years. We are talking about making music. Not reproducing it...!!

[@hallanvaara6106]

I haven't tried mismatched output tubes but Doug's theory about them makes perfect sense to me.

[@Geopholus]

This is a well known effect in electronic music (pulse width variation) if the upper going, wave is related to the bottom going wave by 3/4 or 4/3 it adds some pleasing even harmonics, though I'd rather be in control. of that by my playing style, picking closer the the bridge or the neck.

[@timka880057]

Well hello, Uncle Doug! Once again a great amplifier fix. It was exciting to know you had another video uploaded. By the way the quality of the video looks good, great audio as well. Another amp turned out sounding great thanks to you. I just fixed a Marshal solid state for my brother. I forgot what model. The volume pot shaft had broken off, Luckily I had another pot with the correct ohms in my junk box. I couldn't solder it straight to board like the other but I was able to solder in wire leads to the board and secured the pot in place. There were other things like the power would just randomly cut off while the switch was still on. I found it needed a new power cord but I didn't have one available so I found the open was close to the cabinet. My first thought was, if I cut the cord, it would shorten it quite a bit by the time I stripped it and resoldered it back inside. But it came to me, I just cut the cord where it was open, put the cord through the grommet, stripped back about 3-4 inches, soldered the leads to the leads already inside that go to the transformer and switch, used heat shrink tubing and secured it with wire ties and wallah! It wasn't too short after all. Luckily it was an open close to the cabinet. He's happy with it! And I didn't have to buy a new cord. I think I learned that from you or someone.... Lol

[@UncleDoug]

Thanks, CT. I'm glad you're having success with your own repairs. That's our goal :)

[@UBOOFTB]

Dear UD. It is Saturday afternoon, 1955. The band is practicing for the dance that night, and the tube dies in my amp. What do I do? I go down to the drugstore and buy a tube, pray when I put it in the amp it will work.Thank God, it works!. I can play at the dance tonight. Didn't even think about tube matching for a minute. :>) I love your videos don't ever stop!!

[@UncleDoug]

Those were the days, UB. I remember the big tube tester at the local Gunning Casteel (drug store) where you could test your tubes and buy new ones for 50 cents apiece.

[@grb1184]

@@UncleDoug My dad Ray had one at his stores, Colony Drugs and before that Orange Hill both in St. Pete Fla. My love for his love of this stuff brought me here.

[@WileECoyotey]

I remember tube testers in the 5&10 stores and Ace hardware .

[@mspysu79]

Nice work resurrecting that one! What a botch job that was. Turned out real nice in the end though.

[@UncleDoug]

Thanks, M79 :)

[@midnightraiin4035]

grab your surf board, we've got REvERB:)

[@UncleDoug]

Darn right, MR. Time for Jack to "Hang 8".......dew claws don't count.

[@Scodiddly]

Wow - that Princeton is the near twin of mine. And interesting about tube (mis)matching. I would expect you are correct, and I figure that as Jack’s guitar tech you would have been corrected by now if matching matter that much.

[@UncleDoug]

Thanks, SD. Mild to moderate mismatch can indeed produce a very pleasing tone.....but too much can cause annoying hum.

[@ObiTrev]

"Do it right or don't do it at all" - Everyone who does it right

[@gitargris]

Wow! The audio is 100x better! Thanks! Now we can really bask in the sounds of all your great old amps!

[@UncleDoug]

Thanks, OR. I changed to a broadband Internet provider (120Mb/sec) and now the videos take 20 minutes to upload instead of 4 hours. I think the slow upload was degrading them.

[@justinbouchard]

The alliteration in the title is fantastically fundamental for fixing fender flubs.

[@UncleDoug]

It sums up some serious shortcomings in the circuit :)

[@jagtone]

@@UncleDoug -- serious circuit silliness.

[@UncleDoug]

Amen, Ben :)

[@iskandertime747]

Well done sir, well done.

[@goodun6081]

@@UncleDoug , Snappy soldering, savvy skills, scientific study, and steely resolve surely solve somebody else's stupid snafus.

[@tomk1tl]

Hey UD....I installed new matched set of 6V6's in my Princeton RI. One of the former tubes starting internally arching. I finally got the tubes out and put them in a tube tester...at first it measured OK then swung to bad...then good...then bad. Almost the same thing with the other 6V6. I got a NOS 6V6 and checked it just in case there was an issue with the switch in the tester...it was OK. The new MS/6V6's are from Tube Doctor. The others were Russian 6V6's which were supposed to be good. I checked the PC board making sure that no burnt resistors or bulging caps...all looked fine and got the tube bias set @ 20-21mV. Fender says 23mV @ pin 8 of the 6V6's. One was 20mV and the other 19mV so that is very close.I also got my 6G15 Reverb unit finally working as it stopped last week. I removed the tubes and checked them out...seems the 12AT7 @ the input side, one half of the tube was not working but somehow did pass the signal thru the 6K6/12AX7. The section of the 12AT7 was in the "dwell" circuit. Once I replaced it, the unit works A-OK...talk about a Dick Dale reverb sound...WOW ! I'm still contemplating on installing the HV caps under the chassis instead of on top of the chassis like Fender does.Looking forward to your next video !

[@UncleDoug]

You may recall that I had a similar problem with a Fender amp in for repair a few months ago......an output tube that checked out OK, then bad, then OK, etc. I don't recall seeing this occur with vintage tubes. Congratulations of fixing the 6G15. It is a wonderful device :)

[@tomk1tl]

UD....PART II to the Princeton RI tube dilemma......I installed the new set of matched tubes (Tube Doctor) I got from StewMac and so far, sounds great. I set bias @ 20mV as Fender suggests 23mV for the 6V6's. Hope this is adequate and notice that the amp top does not get "HOT" like it used to with the other tubes. I always look forward to your videos. I sure wish I could donate to your PATREON but I'm on a limited budget (SS) !Also, I notice a slight hum from 6AG15 when hooked up to 5F2 amp I built. I believe some of the orange drop caps should be checked out for "orientation" as one end has a " - " point and may be pointed to the wrong direction on the PP board. Mr. Carlson's lab has a video on this issue.

[@jagtone]

Some people who never crank the reverb to that extreme will sub a 2M2 for the 3M3, which gives a more useable range for the reverb and gives the amp a bit more gain. But 3k3--OMG, no. Anyway, nice job on the forensics and the fixes. What great amps those are.

[@UncleDoug]

Thanks, Ben. Good advice :)

[@jeffbenson6102]

The prior (tech) was dead certain that he had the right amount of zeros when he read the values of those resistors, I always feel better if I confirm the readings with an ohm meter. Nice job Doug!!

[@UncleDoug]

I tend to think he had an eyesight problem, Jeff. It would also explain the wiring errors and all the burns on the insulation.

[@rogerraven6976]

Thank you for explaining the importance of the beaver dam in reverb setup.

[@tombryan1]

Lowering the 3.3 meg to 2.2 or even 1.5 meg opens up the preamp power delivery and still leaves plenty for the reverb. Just adjust the cap for high pass preferences.

[@UncleDoug]

Thanks for your helpful input, Tom.

[@chokkan7]

I'm fairly certain that Mojotone has interchangeable panels such as you're seeking, Uncle Doug. In addition, if needed, I can put you in touch with a vendor who can do these at very reasonable prices, although he doesn't attempt to do Fender copies...let me know... In an age of oppressive conformity such as we find ourselves, your occasional blasphemy is most welcome...

[@UncleDoug]

Hopefully, the owner will follow through with either Mojotone or your source, C7, to obtain a replacement rear panel. Yes, indeed, we specialize in blasphemy......indeed, we revel in it :)

[@washedupguy2759]

Great video, Uncle Doug! And thanks for elucidating my personal mantra: "it takes twice as long and twice as much effort to fix stuff that has been screwed up by the previous guy". After years of designing large-scale mixed-signal analog/digital IC's, I finally reached the point where I routinely quoted $500K and 12 months to fix a piece of shit that was already designed (and they weren't even sure if the goddamn schematic matched the silicon), OR $250K and 6 months if they let me scrap the existing incompetent design and let me do it myself from scratch (and with my guarantee that it would work perfectly). One would think the geniuses in upper management would choose the latter, but noooooooooo. They always went for option 1 and then didn't understand when it was still a piece of shit 12 months later.

[@UncleDoug]

Thanks, WUG. I agree that option 2 makes much better sense, but then whatever manager was responsible for option 1 would look bad. I wonder how much money is wasted both in business and government by the necessity to "save face" :)

[@cawfeedawg]

Doug you have super powers with bringing these beauties back to life.. we don't however want to see you in a cape and tights. 🤣

[@UncleDoug]

Thanks. No you don't, CD. In fact several of the neighbors have restraining orders against my donning such attire :)

[@zopilotesky3014]

Another great video, and I hear a definite improvement in your guitar playing. 👍😉

[@UncleDoug]

Thanks, ZS.....twice.

[@W.E.]

04:07 How does a Fender amp chassis code of A 22264 mean 1969? Inquiring minds want to know. :)

[@erikjohansson1814]

Wayne Elliott I’d love to know the answer too...

[@budandbean1]

You have to look this one up. Fender Silver-face Princeton Reverb amps with code A21500 to A25500 were built in 1969. I have a little cheat sheet but there are tons of web sites that list the Fender number scheme.

[@UncleDoug]

Thanks, Buddy. You are right on the money :) Since the serial number was close to 21500, you can assume it was made early in the year's run.

[@W.E.]

@@budandbean1 Thanks guys; I had looked at this page superficially: www.superiormusic.com/page195.htm ... and saw "S" relating to 1969, but ... if one goes further down on the page on the right, the TRUTH will be found!

[@budandbean1]

@Wayne Elliott You know, if I had read that page, written like it is, I’d be a bit confused too. 😉

[@leedress2187]

50 bucks an hour. 75 if you want to watch, 100 if you started fixing it yourself.

[@AROSFC]

....150 if you start giving sugestions...🤣🤣

[@UncleDoug]

......and $200 per hour if you turned it over to an "amp assassin" to be colossally screwed up.

[@PrinceWesterburg]

Ace video - its nearly 3am but I'm so happy! Thanks for the plate current maths and tremolo mods, much appreciated.

[@UncleDoug]

You're welcome, Prince :)

[@jonathanhorne6503]

EIA 989 code is Marvel Electric Corp. Chicago.

[@bemi-ton]

luckily enough there was no (virtual) center tap before as the plate was shorted to the filaments ;)

[@carlhart9604]

Mismatched tubes, as detailed here, create the asymmetric distortion that emphasizes the second harmonic, and multiples. This is the secret sauce of the Rangemaster Treble Booster. i.e. The greatest British guitarists sound, Brian May, Tony Iommi, Rory Gallagher, KK Downing, Glenn Tipton, Richie Blackmore, Eric Clapton, Marc Bolan, Billy Gibbon's (USA) etc.

[@carlhart9604]

Fatter, richer and more "liquid" tone. In a word "Singing" 👍

[@UncleDoug]

Thanks for your helpful input on this topic, Carl. Hopefully, guitarists will experiment with mismatched output tubes and see for themselves.

[@drteming70]

Hear, hear. I purposely mismatch my output tubes in my push-pull amps. More sweet 2nd order harmonics.

[@guitarocd9984]

I know nothing about circuitry in amps. but was your statement based in science. Or just what you hear.

[@UncleDoug]

I'm glad to hear that I am not alone in this discovery. Let's hope people become more aware of the benefits of mismatching :)

[@UncleDoug]

@@guitarocd9984 Both. I have seen harmonic spectral analysis from perfectly matched vs mismatched output tubes. The spectrum is more complex for the mismatched......primarily even-order harmonics. To my ear, mismatched yields fuller, more musical output. The bottom line, however, is how it sounds to you. Do your own experimentation and see what you think.

[@guitarocd9984]

Okay I only commented because this was a new one on me. Good to know I'll definitely try it. My dad used to laugh at me when I used to tell him there is a difference in tubes. He was old school. He was dealing with televisions and Ham radios not guitar amplifiers. He's passed on but I never could convince him.

[@UncleDoug]

@@guitarocd9984 Don't be afraid to experiment with speakers either. Try a ceramic and an Alnico in the same 2 x 12 enclosure. You'll be surprised by how great they sound together.

[@walshy2116]

I just got it! He reminds me of Lt.Joe Kenda, his voice and words. Lol.

[@UncleDoug]

I'm not sure who that is, Walshy, but he must be a really dynamic stud.

[@sting1111]

Botched and baked, a classic!

[@mikecamps7226]

Interesting video.... Your description of the wet/dry mix part of the circuit is interesting. You didn't mention the cap in parallel with the 3.3 meg resistor is bypassing highs around that attenuating resistor. This is the part of the circuit that determines the ratio of wet & dry signal mix.... If you experimented and disconnected the reverb circuit completely, both the signal send to the 12at7 grid and the 12ax7 plate out to the phase inverter......and ran the dry signal straight into the phase inverter without the 3.3 meg attenuating resistor and the parallel cap...….the gain of the amp would be incredible. Essentially the Princeton is a single channel tweed deluxe amp with the modern tone stack (add trem), and the Princeton reverb is the tweed deluxe with added reverb (and trem)…...all be it not cathode biased power tubes. This would have been the evolution of the tweed deluxe amp crossing over into the tolex era. But straight up as is in design, the Princeton & the PR are no tweed deluxe as they padded down the signal sooo much via this wet/dry mix part of the circuit

[@mikecamps7226]

I should add in.....the modern tone stack has a lot of signal loss as compared to the tweed deluxe and the simple tone pot used on that circuit. So if you did a switch to drop the tone stack ground, which is the typical mesa boogie trick from back in the day for their "gain" boost......it would be more close to a tweed deluxe in performance. Though the tweed deluxe doesn't really have a gradual swing in gain throughout the whole travel of the dial...….with that modern tone stack in place there should theoretically be a good swing in the dial for gain......a Princeton on steroids

[@UncleDoug]

Thanks for all your interesting comments, Mike. You're right about the gain being huge without the the 3.3Meg resistor......I have the neighbor's wrath to prove it :)

[@mikecamps7226]

@@UncleDoug When you discovered the wrong resistor in the dry/wet mix......you were getting soo much dry in that mix ratio. But that mix ratio is interesting to play with and rework to a degree. I have clients that just use a dash of reverb in their "sound", and its a shame that they are not getting the full gain capability of the stock circuit. So I might change the ratio so they bring up the reverb on the dial to get the same amount of reverb they would normally use if the ratio wasn't adjusted. So instead of a reverb knob on 2.5/3, the adjustment would bring the reverb dial up to like 6/6.5 for the amount of reverb they normally like.....and dialing up further doesn't wash it out with extreme reverb...….everybody is different. My personal preference is really wet reverb......but that's me. But since this is a cathodyne phase inverter......it needs to be pushed to bring out the harmonic content that its capable of......the factory Princeton isn't pushing it to full potential. Joe Walsh is involved with DR. Z to produce a clone of the tweed bandmaster 3/10 amp, and Joe talks extensively with the guy from Sweetwater about the amp & why its "THE" amp. SO a Princeton is right up that alley in concept.....and not deafeningly loud. I couldn't tell if the Jensen speaker in the cabinet is a real Jensen or a reissue modern Jensen......that makes for the sweetness of this amp combined with the circuit. The 6V6's have a certain character when pushed and the circuit is right.

[@mikecamps7226]

I guess I'll answer my own question in retrospect upon looking back......that's a fender reissue Jensen speaker.....which is a nice speaker. The reissue black face PR's usually have a C10R, a client of mine had a reissue he bought for his stage rig, and then acquired a real black face PR, so he's using both now as his stage rig...….I rebuilt and tweaked both of them. Makes for a nice professional stage rig. He has both mounted on a bar and set up on a tripod at chest level. For his real black face, we went for a weber Jensen clone.....which had its thing. Between that weber and the Jensen reissue.....each had their thing but they were not exactly sonically the same...both sweet in their own right and a great blend on stage.

[@chrisleech1565]

Just had an idea for a series Uncle Doug, how about a Build-along-with-Doug ...DIY_Tube_Amp and one effects pedal. Plus a Speaker box or integrated like the Princeton etc. One of my favorite amps is the Roland Jazz Chorus 120 and I would love to make a smaller version of same for instance. It has stereo chorusing between each of its twin 12" speakers OMG it sounds good!! You could consider a thru hole kit build from Aliexpress maybe. Just a thought, but you could manage several vids out of that venture and have a blast at the same time.

[@UncleDoug]

Thanks for the suggestion, Chris. I have posted several video series that show all the steps in building an amp at home......strictly all-tube and point-to-point wiring.

[@Murry_in_Arizona]

Doug how tough would it be to modify a standard tremolo ( Ive got a 65 princeton reissue) or build a seperate pedal for the early 60's harmonic tremolo? I think the Lads at That Pedal Show with blues player Joey Landreth started a trend last year when they featured Joey and his vintage amps as a half a dozen new pedals with harmonic tremolo have come out.

[@UncleDoug]

If by Harmonic Tremolo, you mean the Fender circuit used in amps like the Brownface Concert, it would be a somewhat challenging multi-tube build. I would recommend that you watch my video on building a Rick-Tone Tremo-Boost. It's a relatively easy build with an inexpensive power supply and great tube tremolo effect.

[@Murry_in_Arizona]

@@UncleDoug "Harmonic Tremolo" is the part that's fascinating me U.D. The amp in question was a brown 1960 Super. Supro makes a pedal that approximates it ($250-$300) I'm at the beginner stage probably not a beginner project. FTR I've watched most of, if not all of your episodes ;0) long story short, I'm a 60 yr old retired/disabled Phx cop somehow Andertons music UK showed up in my YT feed 3 years ago and I got hooked on their guitar paradiso episodes which lead to following Mick Taylor to That Pedal show which lead to you and D-Labs when I started investigating tube amps after taking the guitar back up (starting as therapy for my hands after spinal cord injuries) after a 35 year hiatus.

[@julesl6910]

Greetings Uncle, What is your technique for checking the values of say a capacitor while it's in circuit? In my experience other components connected to the one you're testing can skew the values... do you remove them? I have an Atlas LCR and ECR that are designed for this purpose and they're fairly accurate, but can this goal be accomplished with just a multimeter? Jules

[@UncleDoug]

I've had good luck testing capacitors in circuit, Jules. Testing leakage of coupling caps requires that one end be separated from the circuit.

[@phonotical]

Woo

[@UncleDoug]

Hoo !

[@utviewer1231]

Hi Uncle Doug, Love the videos, it is especially helpful when you switch between pictures of the actual circuit board and then show the equivalent section of the schematic! When replacing the original power cord with a 3 wire cord, does it make any sense to have the incoming hot (black) wire go through the fuse first, then the power switch and then to the transformer lead? It seems like it might offer a little more protection but perhaps I'm missing something? The power transformer on my Princeton is showing 368V AC going to pins 4 and 6 of the GZ34 and then 454V DC coming out of the GZ34. Pretty high comparing those numbers to the schematics. However, by the time we get to the plate dissipation, I have 10.16W and 9.48W on the 6V6's. These are Tube Amp Doctor (supposedly matched) 6V6's which are rated 14W, so those numbers seem OK and the amp sounds great! Not sure if I have a problem there with the transformer or not? Any thoughts? Thanks a lot! Tom

[@UncleDoug]

Thanks, Tom. There are two schools of thought about whether the fuse should be before or after the on/off switch. I prefer it to be after, so that the entire circuit is "off" when the switch is off......reducing the chances of a shock or arc while changing fuses. Others want the fuse first, constantly energized but protecting against a short in the on/off switch (which I have never seen in over 40 years of experience). You are free to make your own choice. The plate voltage being applied to the output tubes may seem high, but their plate current and dissipation are governed by the -DC bias voltage being applied to their grids. In your case, it appears to be the proper amount. There is nothing wrong with your power transformer.

[@utviewer1231]

@@UncleDoug Excellent, thanks for the quick reply uncle Doug!

[@billmiller7138]

Interesting, for blues harp I've been running intentionally mismatched output tube pairs for many years. Stumbled on it accidentally and it just sounded better to me.

[@UncleDoug]

I'm with you, Bill. The reason choirs sound so good is that everyone has a different voice.....and the blend is both complex and rich.

[@pharmerdavid1432]

@@UncleDoug Brilliant analogy, and we too love your videos!

[@philr6865]

IMO the comments about previous bodge-artists also apply to software (that's my trade). Thanks, and KUTGW!

[@stratdude83]

The bias is set to about 900 watts! That had me rolling 😂

[@codaalive5076]

Why do they rewire if amp only needed component replacements? Makes no sense because company made enough prototypes to conclude wiring works fine, repairman can't say the same for one off. Anode touching filament shows person who worked on it didn't care much about such a nice original amp, it can't happen by mistake we all do... 6% output tube mismatch should be fine although not far from what is max allowable for output transformer, THD, sound, etc. I love 6V6's sound, preferring it by far to EL84 clean or driven. Nice video, thanks.

[@UncleDoug]

Thanks for all your interesting comments, CA.

[@billedwards2657]

I’d bet mojotone would have a rear face plate.

[@UncleDoug]

I'll let the owner know, Bill.

[@Starcrunch72]

@@UncleDoug Yup $24.60 SKU # 9131521

[@Bbendfender]

@@Starcrunch72 I ordered a back plate yesterday for the amp.

[@melvynobrien6193]

Those CBS silverface amps were considered crap back in the late 60s and 70s. Just like their guitars. That's the reason pre-CBS Fenders skyrocketed in price. I'm sure you'll fix it up to sound better than the original CBS garbage.

[@UncleDoug]

This is a common misconception, Mel. The truth is, CBS's negative influence was not felt until the mid to late 70's, and even then mainly in the larger circuits. The '68 to early-70's Silverfaces were virtually identical to their Blackface predecessors, and were excellent amps.

[@justinoneil6971]

Hey, I talked to Jack Allen today about Darrel Braun’s review of the Stage Right 15 watt. He mentioned sending you some stuff and here we are...opening it up! What a crazy world we live in today

[@UncleDoug]

Yes, Jack was kind enough to send me the Alamo Futura and reverb tank shown in the video. It is indeed a small world.

[@justinoneil6971]

Uncle Doug I can make it even smaller, or at least more coincidental. I, too, live in Oklahoma and my son is named “Jack Allen O’Neil” Almost spooky.

[@Mauitaoist]

I just finished rebuilding my Princeton thanks for all your help I have a 1966 original vintage

[@elliotbradley]

sounds pretty good :)

[@UncleDoug]

Faint praise, Elliot, but thanks. Hopefully I can feature a Dumble Overdrive some day and achieve an unqualified "good" :)

[@Chuffin_ell]

I have a good idea, the owner saw blueglows packing video. I sent blueglow an amp with awesome packing and consequently he went on to make a video documenting the way I packed his amp. Me, yeah...

[@bamboosa]

@Uncle Doug - nice. S'funny how the things I was nuts about in 1969 were laughed at and now they are pricey treasures. I wanted a Princeton in 69 and I was 16 and I had no money. I have a beautiful and incredibly loud and clear Mitchell from g.k.w. decade - 10-inch Electro Voice. I accidentally left it on for a week and it stopped working. I replaced the one fuse that I could find and nothing. It weighs, literally, half as much as I do and it's tiny. She shut down about ten years ago. If you choose to come to Hollywood I could share some magical guitar "bypasses" (the b.s.) that are so simple people literally don't believe how easy it is to play fluid or chunky or light and fluttery or z.z. boogie or whatever. I expose all the rockstar secrets and I play guitar. Come to Hollywood. I'll give you the amp to do whatever's right or not. The one guitar seminar that blows all the poop away and shows you how to play like yourself is of course free because that's what I do. It only took me six decades to figure it out because if a gypsy guitar player can make the gods weep and sing with two fingers on his fretting hand? Do I need a mythological Hot Jazz Hero from the thirties to show me how to play my way? Anyhoo I much enjoy your passion and electronically I understand nothing. I've been told that I must go purchase a soldering iron and solder and I am much afraid however I have engaged the last 77 days in building a shimisin/CBG - a Japanese thing and my first build has demanded to be electronified with a couple of pezio-electric transducers and a volume knob and a strap-pin jack and I look at the tiny bag of electric guitar guts and I am afraid. I am building a sublime three-string thing and she deserves the best wiring in the world and I don't know how to bloody solder. I do fab portraits and play guitar. Aloha.

[@UncleDoug]

I'll keep your offer in mind, B. Meanwhile keep listening to your Django Rheinhardt records :)

[@bamboosa]

@@UncleDoug - thanks. Aloha.

[@bradt.3555]

I work on guitar amps and totally agree about tube matching. In the early days of guitar amps they just put in tubes. Also all the fuss over biasing and tone. Biasing is to make the tubes run properly, it's not a tone control.

[@whatsstefon]

Brad T. 100% this is what I tell everyone. This is also why I kept my Bassman 50 stock with the bias balance. I can throw in anything that works and in 5 minutes have an amazing matching pair. Hahahaha. If I want to adjust the bias, I can quickly swap out the resistor. No big deal.

[@bradt.3555]

@@whatsstefon, That balance pot was a really great idea, and if you want a minor mod just change the resistor and add a bias pot, then you have the best of both worlds. I had one of those bassman heads years ago. Also had a Peavey classic 50 which was fixed non adjustable, and they ran them hot. Very little screen resistance and about 410 plate volts, I made it adjustable. Had that amp about 20 yrs, just sold 6 mos. ago. I see a lot of people on utube who don't understand it's not the voltage that kills a tube, it's the current, (as long as it doesn't arc inside).

[@whatsstefon]

@@bradt.3555 absolutely correct. You can often exceed plate voltage. But I do try and maintain to spec anyway.

[@bucyruserie1211]

Hi Uncle Doug, It's always a good day when I see a new video posted by you and your gang of felines.. I'm having my doubts that the Princeton was working for a while, then suddenly stopped functioning.. What type of tech cuts off the center tap for the 6.3 winding :/? Not sure what did more damage, the fire or the previous "repair"? You had to be a real sleuth to track down all the errors.. Great repair video... Sounded fantastic. I like the burned back, makes a good story! Be well, Tom

[@UncleDoug]

Thanks so much, Tom. As usual, your comments are accurate, concise, and greatly appreciated.

[@russellsmith4615]

Thanks for doing these videos, Uncle Doug - I've learned so much from your video tutorials - they're thorough, well explained, and entertaining. I've been doing refurbish work on 3 Fender amps of my own as well as 3 others by friends and band mates thanks to you an a couple of others who are so kind to share their wealth of knowledge.

[@UncleDoug]

You're welcome, Russell. I'm glad the videos were helpful.

[@drsproc]

At 20:45 a wire is clearly unsoldered on that tremolo tube. (Green middle) when it was wiggled.

[@UncleDoug]

I'll check, Dr. S.

[@drsproc]

On second look it might just be mechanical wiggle of the pin... so... not so clear after all.

[@CoquiAudio]

all those things touching each other means that either the guy has not good vision or he does not believe in old good glass magnifier

[@UncleDoug]

Probably both, VM.

[@fartzerelli1385]

I worked with plenty of hack techs in my years and I can say this, just when you think you've seen the worst there's always something worse! The hack who wired this amp is nowhere near as bad as some of those I've worked with.

[@UncleDoug]

I'll take your word for that, FZ :)

[@DeadKoby]

Be nice to "Technicians".. we're not all bad!!! LOL

[@UncleDoug]

I have great respect for good techs......like you DK :) It's those other guys I'm upset with.

[@strangersound]

Best teacher ever. Thanks, Doug. You make a circuit easy to understand. :) As for the people who wasted money on an incompetent tech...the end result is seeing Doug fix it and teach us the whole she-bang and then some (43:00). So, I'd say that was money well spent. :)

[@UncleDoug]

Thanks so much, SS. We really appreciate it :)

[@mgb961]

fabulous educational video...so generous..many thanks

[@ResoBridge]

Here's the thing, even with a perfectly matched set of output tubes, if the D.C. resistance of the two halves of the output transformer primary windings is not a perfect match (and if its not a bifilar wound transformer they won't be) then with a single common bias supply the plate current and dissipation for the two output tubes won't be the same.

[@Finom1]

Thank you for another fantastic educational video!!! Your the best, we want more.

[@UncleDoug]

You're welcome, F1. More is on the way :)

[@repairfreak]

What more can I say? You nailed it again Uncle Doug. Please do me a big favor, when you pass to the great gig in the sky, please mail your brain to me. I could really use one that has been fully uploaded with usable data. 😁👍

[@UncleDoug]

Wow, thanks so much, Mike. I'll do it, if you promise to give the pup in your icon photo a nice head scratching from KC, Jack, and Ollie :)

[@repairfreak]

You got my promise on that my friend. Just make sure to take extra care with the packaging, please put plenty of foam padding in the box your brain is being shipped in. Thanks again 🧠 👍

[@UncleDoug]

@@repairfreak You're welcome, Mike. The postage should be fairly cheap since the box won't weigh much :):)

[@repairfreak]

Uncle Doug Well if this is the case... I then feel it incumbent on me to start my brain on a rigorous training, and low-carb diet immediately! 😂👍

[@joedavidson9811]

Why do some tube sockets have those diagonal turbine-looking slots instead of just holes for the pins on the tube?

[@UncleDoug]

Different sockets are made differently, Joe. It's just the way they are designed.

[@joedavidson9811]

Uncle Doug Thanks!

[@wakerickman]

regarding the non functioning reverb and your repair, i look at it in another way and not as a back force, forcing more signal to the reverb circuit. it semms to me if the critical 3.3M resister is only 3.3K it looks like that the 3.3K resister is simply puting a low resistance "short" accross the input to output of the reverb circuit, compared to the 3.3K resister, which will obviously reduce the amount of reverb attained. would you also aggree/ understand on this diferant view point as to what is happening in the reverb circuit. ?

[@henryhunter5026]

What a headache! If someone has been messing around with the wiring it usually makes things very difficult when fixing an amp. It’s not just a case of checking that everything is the right value and in the right place but you have to check out all the solder joints. One good thing about Fender amps is that the eyelet board layouts are pretty well documented, a messed around with point to point circuit is an absolute nightmare.

[@russellesimonetta3835]

That amp tech did some really sketchy work but that deaf dumb and blind tech sure plays a mean pinball.but he did replace with good quality tube sockets with imoroved connection. Those look like belton,s and used machine screws and bolts instead of leos standard of sheet metal screws. But he didn,t use the trick of using nail polish as lock tite.

[@timothyandrews2157]

Awesome Job Uncle Doug. I'm appreciating your videos so much more, after slowly putting what you've taught me through the years, to good use. Thanks again.

[@UncleDoug]

Thanks, Tim. The learning process is definitely cumulative :)

[@magavsschwaga7834]

This was a repair for a customer or you have? I'd love a Princton. I watch .a lot but I didn't watch the beginning to learn if you were keeping this. If you don't have a compiled list I understand but consider I'm not attempting to spend a lot.

[@UncleDoug]

It was a customer's amp, Kurt, who asked me to try to resurrect it from the trash heap. Fortunately, it turned out great and has been returned to him. I have sold all my own Princeton's and currently have none for sale. Like you, I would love to have another one......or ten :)

[@TheHayruss]

Hi a potentialy interesting demonstration could have been to swap the output tubes between sockets and recheck the settings to see the values follow the tubes or see if it was circuit induced differences? Just a thought love ur Vids cheers Big H.

[@davidausterman5915]

...and botched. I'm glad to see you resurrected this one before the bad re-wire job finished off what the house fire started.

[@dhpbear2]

The last thing you want to hear from a plumber: "Who was the last person that worked on this?" (24:28)

[@UncleDoug]

.....or your dentist :)

[@kenhancock8931]

thx uncle doug n little 1's. I'd trust anything you told me ;-) you know when there's snow on the roof you have been around and know a few thing's. hope you have a great day....

[@UncleDoug]

You're welcome, Ken. Actually the snow blew away and now the roof is almost bare :)

[@bruceeverett1993]

You almost( I say that with great respect) make it look easy. Glad there’s guys like you around! Thanks

[@UncleDoug]

Thank you, Bruce.

[@Tonetwisters]

I sure wish you lived in MY neighborhood. I don't mean to be unkind ... but really competent amp techs are hard to find; particularly, who are not rude and downright mean to their customers. I admire their knowledge and skill sets; but holy cow, they need to go to finishing school in the worst way ...

[@UncleDoug]

I guess they've been breathing solder fumes for too long, TT :)

[@Tonetwisters]

@@UncleDoug LOL! The do have a tough job. Knew one who was paralyzed on one side ... imagine soldering stuff and holding things in place ...

[@UncleDoug]

@@Tonetwisters God knows, it's hard enough with two (reasonably good) hands.

[@JagStar]

WOW! Only half way through the video and I can't believe how botched this was. I'm sure the original tech was a hobbyist and had the best intentions but, let alone the things that should have been obvious when sound testing, there are so many mistakes even an experienced hobbyist should not have missed. Maybe this was his first repair?

[@UncleDoug]

Possibly his first. Hopefully his subsequent efforts improved dramatically (we can only hope) ;)

[@conjering]

Uncle Doug love all you're videos. I noticed that meter you are using Mastech MS8268. Do you recommend it? What do you like about? What don't you like about it? I'm looking for one right now and a little hesitant in buying a Fluke for 150. $ Thanks

[@UncleDoug]

I recommend it, C, based on its price and quality. It's a great bargain

[@ashscott6068]

Are you sure someone rewired it? Looking at the filament wiring, maybe some old parts were just piled up and got accidentally connected in an earthquake or house fire. That's what it looks like. And all the soldering iron burns! And...did they use plumber's flux?

[@UncleDoug]

Unfortunately, it had definitely been worked on prior to being sent to me, Ash.

[@diabolicalartificer]

What a nightmare repair. noticed some iffy soldering @35:53 on the LHS tag, between the green heater/filament wires and black ground wire to chassis looks iffy, guess you don't have the amp anymore though. Cheers, DA.

[@UncleDoug]

Thanks for the tip, DA. Hopefully, it won't be a problem.

[@Geopholus]

Doug, beautiful repair job, and the video is very helpful to anyone not familiar with the Fender Silver Face Princeton Reverb or many of the other similar Fenders from this period. I intend to as much as possible extend the grace to You that You have not shown to the well meaning and apparently pretty attentive nouveaux "tech" who "built" or rebuilt this particular amp. Doug Your complaints against this poor fellow who probably rebuilt this entire amp using a new board kit, are exaggerated. He rebuilt the entire amplifier, as far as I can tell, as only the transformers (but not the reverb driving xformer), the chassis, and perhaps a few jacks or pots and outboard hardware are original. How do I know? I know the Fender parts from this period of time. All the caps in this build are currently favored for new builds, and only the two blue moldeds, in the tone stack, were available at that time this amp was originally factory built. I have been repairing these amps (Fenders) for 55 years (1000's of them). The original board would have been coated in thick paraffin (which may have been what caught fire) and all the resistors would have been phenolic body carbon comps at 10 %, The coupling caps would have been mostly blue moldeds and several yellow ceramic discs. All the wiring would have been in thick plastic jacketed white wire. There would be no tubular polypropylene caps, no 2 % metal film resistors anywhere, no orange drop caps, and no small black sprague or amp doctor cathode bypass electrolytics. Yes his soldering suffers from the currently available tutorials online, as he needed a heavy duty iron or gun and good old 60% tin/40 lead rosin core solder, and knowing how to melt solder on the preheated joint rather than apply melted solder... etc ,... etc... but overall, it looks like a labor of love, and for someone who was probably self trained with perhaps less than 200 hours of soldering experience (at least into eyelets with fairly heavy leads,... not a "butcher' job at all. Specifically the worst error was in leaving no wire to the grid of one output tube. The input jack problem is easily understood as FENDER DID short two pins together on input jacks 1 & 2. They would be the high side input of #1 and shorting jack tab of #2. since both the input hi and the shorting jack tabs are shorted together when no jack is in place, unless one is careful, this is a very easy mistake to make, particularly because many knock off, "pretend" "switch craft " jacks have the tabs arranged in a different order. The claimed error of using a 3.3K resistor for mixing the dry and reverb signals, I think, is mistaken. As this is a blue body 1 or 2 % Asian metal film resistor with 5 stripes, certainly the color code, as I read it of orange, orange (3&3)black (0), yellow or green (4 or 5 (number of zeros)) and red (I think means 2 indicating 2%) does not indicate that it is a 3.3K resistor. If You look at an earlier point in the video (5:40) You will see that the (should be) 3.3 Megohm is indeed orange, orange, black, green, red. That is, as far as I can tell, 3,3,0(third significant digit) 5 (zero's ) & red (2 %), which You replaced with the same value 3.3Megohm . I think it worked the 2nd time because the 470 K reverb mixing input resistor, was such a bad connection, it was completely disconnected, and got resoldered when You installed the 2nd 3.3Meg 10% carbon comp. As far as the tremolo not working, It may have been deliberate or a mistake, as many players want to eliminate the tremolo as they feel it destroys the tone ,.. and in this case the tremolo circuit also is the only path through which the output grids sees bias so You can't disconnect the entire thing. So in case the guy who built this amp is watching, I, .... by the imaginary powers, invested in me, as a kind hearted old tech, ... exonerate You of all charges of butchery or other forms of contemptible behavior, rather I commend You. I'd give You a job in my shop anyway, and with a few pointers I'm sure everyone would be happy with Your skills. Uncle Doug, please avoid future trips to the ER. (never happened but makes for great click bait),. and don't attack Your poor clients amps with an AXE unless it is one of Your lovely guitars. All joking aside a very informative video. Also Doug please get a Variac and a 'scope, they come in handy.

[@UncleDoug]

Thanks for your input, Geo.

[@brianingram2068]

@Uncle Doug At 32:48 you talk about why the 3.3MΩ resistor is important to the level of the reverb; I wanted to get you to think about how it works differently. Rather than "back-pressure" this 3.3MΩ forms a voltage divider with the 470kΩ resistor that runs from it to the Reverb pot. On the Reverb pot side, "ground" is seen through the Reverb pot and is the actual chassis/circuit ground. If we follow from the "input side" of the 3.3MΩ, we see a 0.02µF coupling cap then a 100kΩ plate load and then a filter cap + terminal. That filter cap is "same as ground" for a.c. voltages. Now the reverb signal coming out of the tank is very weak and that's why it goes straight to a gain stage at V3B. When the Dry and Reverb signals get mixed, the Dry signal would still overpower the Reverb signal unless something is done (you could check this by measuring the voltage of the Dry signal at the 3.3MΩ and comparing to the Reverb voltage at the Reverb pot wiper). The way this is corrected is applying both signals to a voltage divider, but from opposite ends. The divider is made up of the 3.3MΩ and the 470kΩ resistors that meet at V3A's grid. The Dry signal is reduced to 470k/(470k+3.3M) = 12.5% its original strength, while the Reverb signal is reduced to 3.3M/(470k+3.3M) = 87.5% its original strength. When the 3.3kΩ was wrongly in-place, the Dry signal was 470k/(470k+3.3k) = 99.3% full strength, while the Reverb signal was cut to 0.7% its full strength!! And as you found, it was as if the reverb circuit was nearly dead. So the 3.3MΩ is not used for "back-pressure" (because V3A's grid draws no current; it's a piece of metal sitting in a vacuum and usually with a negative bias to keep it from attracting electrons from the cathode), but to set the relative levels of the Dry vs Reverb in a voltage divider.

[@UncleDoug]

Thanks for your very elaborate explanation, Brian.

[@hippyamp]

Great video as always Uncle Doug. Thanks for making them.

[@UncleDoug]

It's our pleasure, HA. Thanks for watching.

[@rogerstorrs8679]

Ohh its been TOASTED ! I had a funky Solid state PEAVEY bassamp that was .. REAL toasted too 'back in the day' (pre- Simelight Club wreckage) Melted control knobs, could hardly ready any cap values.. I DID actually get that one GOING - but never got all the crackles out :)

[@UncleDoug]

Thanks for sharing, Roger.

[@edwardebel1847]

With your tremolo mod being what would seem to be the gold standard...it just sounds "better" or "normal", one has to wonder if anyone actually listened to the original tremolo before wiring and selling thousands of these amps. Can you imagine the opening guitar lick of "Born on the Bayou" with the tremolo speed on 8 or higher on the original tremolo circuit design? Jus' sayin' 'z'all...wish I was back on the Bayou, rollin'...you know...

[@UncleDoug]

Thanks for your input, Edward :)

[@glenwhatley4125]

With respect to the better tone from slightly mismatched outputs in a guitar amp, i agree. Some think you also need to match the phase inverter triodes, too (in the long tail phase inverter). All very silly for a guitar amp. Now in high end hifi amps that are shooting for minimum distortion it actually makes sense to match outputs and balance the phase inverter. They also charge dearly for hand tested/balanced, and microphonic checked tubes. Just retubed a McIntosh MA2275. 2ea KT88's and 3 12AX7 Gold Lion tube set per channel came close to $500.00! The amp sells for around $6000 when new. The owner didn't even blink. Can you see those tubes going intro a guitar amp?!!

[@UncleDoug]

No, I can't, Glen, but people continue to pay extra for closely matched output tubes for their guitar amps. I guess it's good for business.....but I consider it to be almost as futile as spending excessive amounts on so-called "tone capacitors". Until someone can complete a blind study that demonstrates an obvious tonal improvement with the high-dollar caps, I will remain a skeptic.

[@charris939]

Hi Uncle Doug, lot's of rookie errors there, and I consider myself to be not much more than rookie myself! I am certainly in agreement with you that mismatched output tubes add colour to guitar amps. I hope the owner didn't pay too much for all of that dodgy work! All the best Colin.

[@UncleDoug]

Thanks so much, Colin. I'm not sure how much he was charged......I was afraid to ask :)

[@rogerhowell9474]

What would you say if Fender approached you about making a series of amplifiers. Based upon your re-design or revamping of the best circuits from classic amps and rolling them into class A and push pull single speaker style cabinet. They do it all the time with guitars like my Jimmy Vaughan edition strat. Why not amplifiers. Take care of friend. P.S. Hamfest in Midland,TX March 16th I have over 150 NOS potentiometers and other stuff I'll be pedaling. You never know what you will find. Roger, San Angelo

[@UncleDoug]

I get the feeling that Fender is quite confident in their own capabilities and thus has no need for help from an old, eccentric guy with a bunch of cats, Roger. However, should they change their minds.....we're ready !!!

[@yrulooknatme]

looks to me that a part replacer guy got his hands in that and missed a wire and caused shorts etc.... but did not have a clue... and I doubt if it ever worked right....it at all.... But you actually had to go thru the whole thing piece by piece... SOUNDS BETTER THAN EVER DOUG!!!!

[@UncleDoug]

Thanks, YRU. That is our goal :)

[@rogertyler3237]

I Was Told By Using Your 2 Middle Fingers You Can Play Some Pretty Familiar Cords Like D E All You Need To Do Is Put You'r 2 Middle Fingers On The A & G Strings & Leave You'r low E String Open.

[@UncleDoug]

Thanks for sharing that with us, Roger :)

[@TucsonBillD]

A couple of comments, Doug... regarding the speaker, I would suggest that during the fire which damaged the amp originally, the speaker was burnt. I suspect that the speaker basket was repainted flat black (hence no ID information) and reconed. As far as your rebuilding of the amp goes, during the initial phase of review of the condition of the amp showed me (and, I'm sure, you) that the "soldering" of components was, at best, sloppy. You picked up on most of the other incompetint "work" on the amp. Whoever charged the owner for their "efforts" charged way too much. Anyways, this amp is now one of the best sounding '68 Princeton Reverb amps I've heard in a long time... Congratulations on a job well done.

[@UncleDoug]

Thanks so much, Bill. I'm glad you share my appreciation for the excellent tone from this resurrected gem.