Grandpa's Tube Karaoke Machine Becomes Smokin' Guitar Amp

[@TheGuitologist]

I get lots of emails and messages about this, so click here to get Tube Amp Repair Essentials, including the RCA Tube Manual used in this video: kit.com/TheGuitologist/tube-amp-repair-essentials

[@MrBrymstond]

Awesome as always! I'm waiting for the kitchen blender guitar amp... lol

[@AudioAtmos]

The Guitologist: Excellent job on the amp. Those humbuckers really pushed that front end nicely. The one setting with the Tele sounded really nice also! The "kit" link is a great addition. Thanks!!

[@TheGuitologist]

Thanks, y'all. I appreciate the comments. The humbuckers definitely were the winners here. It's usually the single coils on these old amps. Nice to see hums get a win for a change. ;)

[@podrum7421]

Yeah... I was surprised how it "growels" on the 'buckers... That is a rare thing for this old amps, radios, PA's and just about everything you find with tube inside it and convert it... But the speaker played a great roll here. And BTW great video, because conversions are my kind of thing

[@grain_fed_beef8777]

Hey Brad, thanks for this link. I came here to ask you the current rating of the variac you use ... found it on the linked site. Thanks

[@JoelzombieThomas]

Quite possibly one of the most unique amplifier rebuilds/mods the world has ever seen. I'm glad I hung on to my 7-pin TV tube collection. Maybe one day there will be a schematic for this amp, it sounds phenomenal.

[@TheGuitologist]

Thanks for the comment, Joel. Glad you like it.

[@JoelzombieThomas]

Is she English?

[@stavrost6559]

One of the best overdriven tones I even heard from your videos. Thanks for sharing man

[@TheGuitologist]

Thanks, buddy. I appreciate that.

[@budandbean1]

Hey, I didn't see the very end until now, your daughter giving you a guitar. She is so precious. I just about teared up. I've a granddaughter the same age, almost seven, that I adore. You are a lucky man.

[@TheGuitologist]

Totally. I am blessed more than I deserve. That fact is not lost on me, and I thank the Lord in quiet moments for all that I have. Thanks for the comment, Buddy.

[@MrBrymstond]

I have a wife, 4 kids 2 boys and 2 girls, all grown now and 10 grandkids I'm blessed as well, but you can't ever show your weak spot what ever you do! My oldest grandson picks on me, he's 6'4" 330lbs 16 years old, so cute... He's a linebacker and wrestles, but I never showed all of my goodies! I trained him never knowing his nickname would be Kong... He took me out on the front yard to show me who's boss. I'm only 6'1" 240lbs and 56, I tried to tell him I wrestled my whole life, but he had to get some, telling me I'm old and he's right. I couldn't get him off the ground to save my ass!

[@TheGuitologist]

hahaha. I guess it's not all in the training.

[@cblguy63]

My fondest memories is the pride my grand-daughter showed when she sat beside me with the band in church.

[@scott729]

Awesome Brad... thanks for diving in and actually showing us your thought process and design. I am trying to learn some amp repair so I can service my own amps and maybe build/ convert as a hobby. Between you and Uncle Doug there really isn't any better teachers available in this format that I have found. You have a nice family by the way. Thanks again

[@themarcoharder]

Scott Waters Same. Uncle Doug and Brad have been pretty much my teachers in amp repair. Been able to do basic repairs because of the videos they put out.

[@TheGuitologist]

Thanks for the comments, guys.

[@EmbelekoBand]

The ending was the icing in the cake. What a touching moment. My two kids are grown and its moments like that when a parent wishes their children were that age again when you have to bend over with open arms for that great hug. Enjoy these moments as much as you can, they grow up fast. Rock on brother.

[@TheGuitologist]

Thanks, Gil. I don't deserve her. That's for sure.

[@chef5150dotpsd]

As somebody who has no electronics experience, I had no idea what you were talking about, none the less, your video made more sense than my AP European history class.

[@TheGuitologist]

European history in a nutshell.... Greeks kicked everyone's ass, then Romans kicked everyone's ass, then Byzantium kicked everyone's ass, then the French kicked everyone's ass, then the Dutch kicked everyone's ass for a minute until they bankrupted themselves over a flower, then the French kicked more ass again, then the British kicked ass for a while and launched an Industrial Revolution in the process, and eventually the Germanic tribes, who had been pushed and kicked and beaten for a could thousand years, got pissed off and kicked ass for a couple years until everyone got together and kick their ass and shame them into all becoming castrati shadows of their former selves. Also in there, people mistakenly thought attacking Russia with winter approaching was a good idea. It wasn't a good idea.

[@mediocrefunkybeat]

As a proud European, you pretty much nailed it.

[@jto1916]

As another proud European, I second that.

[@danadrian2752]

The Guitologist As an American, I am fat.

[@bucyruserie1211]

Wait, I remember this History crap, you missed the part about Hot Queen letting the peasants eat cake, and twins gone missing in Scotland Yard (which is in England, NOT Scotland???) turning up 4 century's later in a parking lot!

[@atwozeek]

The ending is AWESOME so beautiful! They grow up so fast, enjoy every second. THanks

[@Extremebutcher]

That amp is a monster! Great Work on it!

[@billgallagher1351]

This is the best damn inside view I've ever seen.. This guy seems young but has GREAT old knowledge.. Hey can we keep in touch.. I have a year or two left...till I die..

[@Johnny_Dregs]

I don't understand the majority of components and/or terms you use but there is something so dam magical about the process of refurbishing an amp or guitar.

[@wildbillhackett]

Me too! I had some electronics classes back in the 80's as part of my computer repair courses, but I don't really remember much of it. It's fun to watch though even if I don't understand it. Interestingly, I feel the same about women's pole-vaulting.

[@ColaDose52]

Was really cool hearing your wife giggling in the background and you chuckling. Cool amp.

[@rayislooking2]

Great video. The knowledge you have of amps and modding them is awesome. Love the purple rain and your daughter is so sweet. Enjoyed it Thanks

[@surfboardsigns9214]

Your videos gave me the confidence to modify a Mint Blues Deluxe Reissue I picked up last week. Put in the Fromel full kit and wouldn't you know I turned it on and nothing. I was able to find my bad solder joint using the lessons learned here. Thanks

[@TheGuitologist]

Cool! I'm glad you got something useful out of watching my channel.

[@MrBrymstond]

1990's Food Processor Becomes Smokin' Guitar Amp

[@TheGuitologist]

That's next week. Then we turn the microwave oven into a microphone.

[@utubehound69]

What about a Talk box outta found objects?

[@jimbeaux4988]

Oh good lord! I can't believe you got that thing to sound like that. Bravo!

[@mountainmanmusic3846]

My grandfather took the mail order electricians course from GE in the 40/50s. He had several tube radios he had built in his little shop. Man the tubes that laid around that place would be with a fortune. Not too mention the knowledge I let slip away. I had a Champ back then and he would repair it for me using all he same books I see you use. He good old days...the funny part is your resemble him in his ukulele her days!✌🏻thanks for the video

[@MrBritrider]

I’ve been watching your channel a lot as of late and you inspired me to have an old Webcor model 1058 stereo amp converted into a guitar amp. It sounds fantastic! Thank you for sharing all your knowledge. You’re a heck of an amp technician and amp owners living near you are fortunate. Plus you’re a heck of a guitar player to boot!

[@curbmassa]

I had one of these. They were used with movie projectors, phonographs and as portable PA systems for cattle auctions in the Chicago stockyards. I did a trade for one at a guitar show that had low output but was clean and complete. It sat around for a few years until I found a website by a guy that had a collection of 150 small tube amps and he said this was his favorite. Brought it to a friend of mine who's really good with old crap and he got it working great and I used it for many gigs and rehearsals. Awesome amp that's fairly easy to find on Ebay. I'd highly recommend one for recording, harmonica or guitar. I'd love to hear it plugged into a 12" cabinet with a Jensen.

[@MaxStax1]

I love Guitologist videos, have watched many of them.I don't even work on or use guitar amps, I troubleshoot and fix entirely different types of equipment.But there are many similarities. I just find Brad to be fun to watch and listen to. He's really cool in my book.Just sayin'.

[@RuneTheFirst]

RMA was the Radio Manufactures Association, later RETMA - Radio, Electronics, and Television Manufacturing Association. They set standards for parts, safety,, publications (schematics and manuals) and other things related to a common standard for members. Webster Chicago made wire recorders, tape recorders, record changers, PA amplifiers, and complete systems (record players/radio consoles) etc. They became known as Webcor around 1955. They faded in the 60s and the brand ended in the 70s after that being slapped on pre-made stuff from other manufacturers. Once public domain the name appeared once in a while on off-brand imported junk like Crosley did in the 80s.

[@cdshull]

Just picked up a working one today for $123 (OK, and $30 shipping) 😊 And thanks for the schematic and mods/fixes.

[@TheGuitologist]

You may want to do something different with yours, but hopefully my experiences give you some directions to consider. Good luck!

[@lroy730]

Great Job! Nice to see you stick with the 6AU6 most would just slam a 12AX7 in. But you got it to sing out nicely. The RCA Tube Manual is the bible. I fond it useful for using some Hi Fi Tube amp noise reduction techniques on guitar amps. You don't need a Noise gate Metal heads you just need a better amp.

[@kardRatzinger]

The biasing of the output valves via the OT secondary is actually a pretty clever way to introduce negative feedback without having an actual loop.

[@TheGuitologist]

Yes, I think you're right. A couple others have pointed that out too and I agree. I also think bias is changing with the fluctuating resistance of the OT secondary inductor.

[@roncarter2188]

Brad I've been watching you do repair and mods for years and I have learned so much that I'm thankful for in doing my own repairs and mods. That being said, you have pretty much reworked this whole amplifier, keeping the transformers and tubes, but totally redesigning the whole amp for guitar and making it sound awesome. That being said I would love to see you get into the whole complete manufacturer of guitar tube amps. Linzy Amps, now there's a logo!

[@drmaawenz]

The approach using sec. windings for current feedback is used in the leslie 147 etc.

[@fongy200]

You guys get all the cool stuff in the US here in Blighty it's mostly crap solid state modeling amps. I would be grateful to have that little Bastard your having a look at, it looks great, front and back. Definitely cool.

[@brentstewart59]

That’s a monster right there. Great job sir ... like holy frickin hell man watta beast

[@josephcote6120]

Comment made at 13:24.... I bet the cathode bias setup is also the negative feedback circuit. Off to watch the rest and see if I spoke too soon.

[@fongy200]

It even has a label saying 60 cycles, 60 cycle hum. What a superb vintage amp.

[@mikemoyercell]

love watching you shred your guitars and the tube amp really sets the mood!

[@hifijohn]

That output is very clever I think the old tube mcintosh amps used something like that.Basically the cathode voltage is in series with the output out of phase so its acting as negative feedback.

[@douro20]

Magnavox speaker, and I'm sure it's newer than the amp. The 6AT6/12AT6 was designed for use in the second IF of radio receivers. It was replaced in most applications by the superior 6AV6/12AV6. 45V is quite a low plate voltage; typically those run with 100-250v plate voltage.

[@barryg41]

What more from a vid that restores, rebuilds and reengineers an amp. Then plays riffs from all genres. Even my favorite metal rock crap. Lol Your daughter was just whip cream to great cake and ice cream. Thanks Brad

[@TelecasterLPGTop]

When I was a kid I bought an Ibanez Rickenbacker copy but couldn't afford an amp. Soon I grew tired of playing without an amp and I cast my eye on my Dad's HMV valve radio. Sat down with a lead plugged into my guitar and bare wires at the other end testing each wire in the radio until I got sound. Installed a 1/4 inch input socket and an extn speaker into the radio and BINGO my first all tube amp. It sounded great with lots of overdrive when the tubes got hot.

[@garetkonigsfeld2]

This is an older video now. But it's funny how things go full circle. The threaded pop rivets in the Amp are now an as seen on T.V. thing I've seen these days. As usual awsome video thanks for sharing 👍.

[@MarkTillotson]

That biasing of the output tube cathodes via the transformer output centre-tap is just a simple way to do local negative feedback on the output stage. After all its a PA amp so was designed to be low distortion.

[@TheGuitologist]

+Mark Tillotson I agree

[@brittisaacrossman]

Hey Brad, great work on this conversion! Your sense of detail is astounding on this - let the use of the RCA bible be of note to the young and home-gamers! I think I'll borrow the idea of the 1482 output section for my Bell & Howell conversion! It's got a 12SJ7 and 12SN7 - should be a beast!

[@TheGuitologist]

The RCA manual should be on everyone's shelf. It's one thing you really can't PDF. Books are not dead!

[@trillrifaxegrindor4411]

you definitely have a golden tone amp,sounds wicked with a tele and liquid gold with humbuckers

[@jeff49605]

this amp has a natural "bite" to it, like if you used a compressor with short attack. it sounds really good!

[@matthewf1979]

Hey Brad, silcone high temp tape works great for tube dampers. A $5 roll wil make enough rings for 10 tubes. I cut it right down the middle length wise and it's very effective. Just make sure it's 500 F or higher. You're a great singer brother! I love the technical stuff a lot, but the demos are the icing on the cake! I would love more singing too.

[@patprop74]

33:00 Pretty cool the way the leads glow like that. not sure which kind of lighting you were using back then, but makes for a cool effect.

[@jimbrennanguitar]

That Roadstar sounds great with that amp!!!

[@michaeloberb2120]

I had bad surprises with using tube-socket terminals as terminals for other parts. Some tube-manufacturers connect "unused" pins internally for other purposes...Had some surprises with 6BQ5´s of different manufacturers! So watch out when changing the tubes.

[@TheGuitologist]

Never had that specific problem, but definitely watch out what you read online about this or that tube being compatible with this or that socket because if the manufacturer tied something to the unused pins and the different sub tube uses those pins, you might be listening to Dragonforce...and in "Through the Fire and the Flames".

[@michaeloberb2120]

Absolutely! The specific problem occurred, if i remember correctly, when i tied pin 1 to pin 3 to be able to use 6P15P in the same circuit without changing a lot. It was kind of a test-bed circuit... Long story short, the RCA and Telefunken/Valvo 6BQ5 's have pin 1 connected to grid 1. JJ 6BQ5`s and Tungsram 6BQ5 `s don`t...Did cost me a Valvo one.

[@Iamadroid]

The Guitologist and what would be wrong with some through the fire and flames?

[@electroKrunch]

Turned out sweet! You made history on that one!!

[@bob123surfer]

I think the odd biasing setup tied to the secondary of the output transformer is for signal linearity. Lots of organ amplifiers from the fifties and sixties used a similar setup. It's similar to a super linear setup if my logic is sound.

[@TheGuitologist]

Yes, that's kind of my thought too. I think biasing in that way reflected voltages from the secondary back to the primary and probably helped maybe compress output spikes. It's an odd design feature, for sure. First time I've seen it.

[@paolocanali3361]

I've seen something like this in Hammond tone cabinets F40. They use a separate center tapped secondary winding for the output tubes cathode. The goal is perhaps both to get some NFB, and to help cancel out imbalances in the output transformer magnetic fields. Secondary winding should be wound in a way that the DC cathode current will induce a field opposite to primary. Maybe this has been done because some people were changing only the bad/worst output tube, leaving the other almost worn-out tube in place until it failed completely. On single-ended designs (some Philips tube radios had this arrangement) it enabled the use of a smaller core, lowering manufacturing costs.

[@83roadstar]

I love the amp,and especially the Ibanez RS 1000 ,great video!!!

[@TheGuitologist]

I see from your username you're a fan. Very cool, brother. Thanks for watching! I feature that guitar in several of my newer vids.

[@83roadstar]

I have an 83 Roadstar RS 1000 ,I bought it in 1984 , the pickups were already changed to Dimarzio X2N and a Super Distortion for the neck pickup, coil taps , shielded , I've been dabbling in the electronic world Myself ,but not to the extent that you are... much respect!!!

[@jamsguitar]

Still working my way through all the old vids just wanted to say nice southern rock version of Purple Rain. I fall into full shred mode like that after a pre jam smoke session. ;)

[@JohnIainMcFarlanewaspfactor]

Rewire,explanation,test,info,demo.All good bro.

[@douro20]

The 6AU6 is a good choice since it is very much identical electrically to a 6SJ7. The 6HR6 will give even more gain but nowadays is rare and expensive due to its use in the FM section of high-end Fisher receivers.

[@DavidBrown-it9ig]

Nice playing! That amp turned out to be smokin' as well!

[@jasonkeen712]

Great sounding amp. That ending was too damn cute. Another great video, sir!

[@bucyruserie1211]

I was fortunate I suppose, that an RCA Receiving Manual was included, when I acquired my !st lot of tubes +tester from an obsolete powerhouse that we (my job) were doing a demo on. It was built in 1957, many nos tubes I saved from the garbage, but the most valuable thing was that manual. Lots of information (as you showed), that you just can't find anywhere else. Great job Brad, on that little amp, I love the pentode tube. Don't know why, but the pentode's always have a dirtier, grittier sound when pushed to me..[[[

[@TheGuitologist]

Most of the other books you'll ever buy on the subject of tube amplifiers are just re-wordings of the RCA Manual. Even if you don't use the tube data, just read the opening chapters and learn a ton.

[@vsopowered6477]

freaking awesome channel you have sir!

[@charlesshaw9090]

Another very interesting video. The Vox AC30 amplifier has always had a treble cut tone control between the PI plates, which is quite common on non USA guitar amps from the ‘60s on. The high value resistor and small value, high voltage capacitor on the primary winding is not a Zobel it is a ‘conjunctive filter’ to prevent the primary impedance rising too far at higher frequencies and is used a lot in guitar amplifiers. A Zobel network is a low value high wattage resistor, similar in Ohms and wattage to the speaker itself, and a high value low voltage capacitor across the OT secondary, to counter the effect of the speaker coil inductance, often used in speakers with crossovers. They can both do a similar job to remove high frequency tizz if correct values are used but the Zobel dissipates significantly more power. Oddly the originals in the conjunctive filter on this amp, 100k and 2n2, will have no effect at all with ~8K primary on the OT unless that particular speaker has a 10-15x rise in impedance within audio frequencies, and will do nothing to protect the tubes. The odd OT secondary arrangement provides negative feedback on the 6V6 cathodes. As the output signal rises so do the cathodes which reduces the gain. It reduces unwanted distortions and controls the output stage better, much like any other nfb. Sorry for the length of this comment but, like you, I believe knowledge should be shared.

[@TheGuitologist]

A Zobel Network (or Bucherot Cell) in this case, are a type of conjunctive filter. They work with either the inductor in the speaker coil or the inductor on the primary of an OT. That's been my general understanding of it. I'll admit I'd need to hit the books again to defend this argument. Thanks for the insightful comment.

[@TheGuitologist]

...and the reason the values of the resistance change from primary to secondary of the OT is because of the impedance transformation.

[@ipsurvivor]

I think I preferred what looks like a hum-bucker guitar... the red one through that amp... just my personal preference... amazing stuff...

[@TheGuitologist]

I think I agree in this case.

[@danielsaturnino5715]

I got my hands on a reel to reel tape recorder with an ef86 and 12ax7, maybe I can make something like this as a small practice amp. Thanks Brad

[@TheGuitologist]

Cool. Outta be a good one!

[@murattaner7384]

ı say there is definitely a market for amps this size that sound like this, Every electric player needs one. Big cograts :))))

[@michaelmarini3123]

I say it turned out nice and it is a perfect example of how this is a labor of love...

[@JoelzombieThomas]

Yes! I found a wire recorder and got it for free from a junk yard out of a car. Unfortunately, I tried converting it to a disc sander with it's motor and flywheel... that didn't go very well, and I tried to hang onto the amplifier, and that didn't work out either, so here I am appreciating someone who knows WTF they're doing.

[@amoruzz]

That little amp sounds pretty good over the phone. 👍

[@garystevens6831]

Absolutely love watching your vids, I'd be intrigued to see what happens if you get a Carvin Belair or Nomad combo and indulge your creative licence modding them. :)

[@williamsquires3070]

The cathodes of the output tubes connected to the center-tapped secondary may be some sort of negative feedback, or a hum-canceling arrangement. The 6AT6/6AV6 (or the 12-volt cousins) are usually the detector/1st-AF tube in All-American-5 tube radios; weird choice in a PA/guitar amp.

[@kevin2960]

That tone circuit is almost exactly what a VOX cut switch is. You should try leaving it. Fender did the same tone on there very first amp the Model 26 Deluxe. Also the 6au6 is a miniature 6sj7. So you can copy the First Fender Champ Input or many others that used a 6sj7. You can even go close to a ef86 style input but the ef86 and 5879 are way better pentodes for audio.

[@Nobody92421]

I saw what you did there... Pointing with your middle finger at 18:35... I do that too.. Except I turn it upwards and usually throw an empty beer can at said device that I'm working on...

[@rebelcat420]

Damn that sounds sweet! No high gain amp or any pedals can give that kind of tone! And yes, pentode preamp tubes fell out of favor for the dual triode due to the difficulty in finding ones that were not microphonic. Unfortunately, although easier to find, the dual triode doesn't touch the tone of a good preamp pentode.

[@TheGuitologist]

I agree. A pentode up front gives a certain rawness and sparkly character on the top end. It's more...ALIVE.

[@TimmyP1955]

Those rivets are Rivet Nuts. I've never seen them for wood, only for metal. If you want to assemble sheet metal pieces with machine screws instead of self-tapping screws, you use Rivet Nuts.

[@TheGuitologist]

They can clearly be rounded off at the other end with the proper tool.

[@INDYOSKARS]

That was nice and the guitar playing groovy. As George Formby used to say "it turned out nice again".

[@matthewf1979]

I had to screen shot the cathode bias bit. WTF, OVER? (For the new viewers below) Paraphrase inverters have more gain than a Long Tailed Pair but are hard to balance. The GA-40 is a perfect example for a single pentode driving a PPI 12AX7 and two 6V6's.

[@TheGuitologist]

And GA-20T. Same as GA-40 on Channel 2. This was in the back of my mind on this one.

[@matthewf1979]

The Guitologist I tried messing around with large value screen bypass caps, what a difference it makes in gain levels. 680nf with a 25uf cathode bypass is better than 220k or 470k plate load resistors and half the bypass cap values. I think pentode sound better when run at higher voltage too, 220k resistors drop a 330v node to under 100v. 100k only drops about 100v.

[@matthewf1979]

The Guitologist I tried messing around with large value screen bypass caps, what a difference it makes in gain levels. 680nf with a 25uf cathode bypass is better than 220k or 470k plate load resistors and half the bypass cap values. I think pentode sound better when run at higher voltage too, 220k resistors drop a 330v node to under 100v. 100k only drops about 100v.

[@lilipearl1]

Hi Guitologist. Please can you do a video on a 5F1 circuit with the schematic and a wiring diagram? I'm only learning and I know there are a lot of videos explaining the 5F1 circuit but if you did it with a wiring diagram as well, it might be a little easier to understand. It would be great to be shown what resisters and caps affect the tone, voltage readings and so on. I love your videos so much. You and uncle Doug have some of the most informative videos on KZbin. Now he has retired your the only one carrying the flame.😀😀😀. Thanks again for all the time you give up to show us how amps work and sound. Cheers from Australia. Daniel.

[@TheGuitologist]

Thanks for the compliments and suggestion. I have no plans on a 5F1, but you never know what will come.

[@niekdriesschen3376]

dude, so much gain, it´s insane!

[@guitarocd9984]

How come the amp makers can't make one like this that's all we want.

[@williamsquires3070]

Another handy book to have is, "Tab Guide to Vacuum Tube Audio" (ISBN: 978-0-07-175321-0) All the projects have full schematics and parts lists, and several chapters at the beginning that explain the electronics of vacuum tubes, construction techniques and tools, some tube pinouts, etc...

[@jabelsjabels]

I love these conversions! Keep doing them, get weird with it!

[@seanodonnell9826]

Holy crap that thing sounds amazing!

[@TheGuitologist]

I apologize for the schizophrenic playing at the end. :P

[@kimsoundmusic]

The Guitologist It's probably good that you get that out of you. It might be just what you need to give you steady hands for the next repair. Wish I understood tube amp repair. I have an old Masco MAP18 hat needs a complete rebuild. That amp sounded great then one day the output dropped to about 50% but it still worked. One of the tube sockets was broke and the tube was loose. Started cutting in and out while playing so I pushed down on it a little and it went quiet. It didn't make any pops, no buzzes, no mad scientist noises, no burnt smells. Nothing. Of course that I unplugged it right away and it's been collecting dust since. Maybe someday I'll buy all new parts and go for it myself. LoL. I might.

[@ebozyn]

It would be cool to play that amp going through that 4x12 it's sitting on, but with a tube screamer used as a boost, or just a boost in front. So you have this little amp that was used at some hazard county pumpkin fest in the 1950s and now you're playing Racer X's scarified through it. hehe fun stuff.

[@timcook9988]

RMA 375 sounds like it is Webster Chicago calling out their Radio Manufacturer's Association code (pre-cursor of EIA). 375 matches the prefix of the "Prod No" on that label inside the cabinet. See www.triodeel.com/eiacode.htm for a mention oif EIA to RETMA to RMA.

[@TheGuitologist]

Thanks for the comment, Tim. I believe you are right. The RMA did jog a memory, but not loose enough to get any information out. Too many cobwebs up there.

[@timcook9988]

No probs. Cool project. I had not seen you go that far in a mod!

[@charleslambiase5670]

Do what I got to tell you something besides being a serious guitarist you are an amazing technician and I'm a bass player but I'm about sound and I'm about old amplifiers and I have a few Little Gems here and people always say to me why do you like these low powered amplifiers because they are your best friend in a recording studio especially that I play an elaborate style of fretless bass but then I left my ass off of some of the things you say but it's a total learning experience as well thank you man, for making some of this Mayhem public!!!!!!

[@Patriot31743]

I want to have a T Shirt made that says “Now Let’s Dial It Up On The Variac” lol 😂 love the channel!

[@TheGuitologist]

Check my store in the description.

[@DAVIDGREGORYKERR]

Whole scale capacitor replacement required with Polypropylene Metal Foil 600v wkg, What about 2x ECC88 duel triode valves for the first and second stages and PL83 for the power output stage just a suggestion.

[@TheGuitologist]

These are the caps I use as default, unless instructed otherwise. Great value for money. Only issues I've ever had has been in installing them. Sometimes if you're rough with them, the legs can pull off. www.tubesandmore.com/products/capacitor-630v-polypropylene-radial-leads

[@crazylegslife]

purple rain. that was goooood.

[@graxjpg]

The double death cap, aka peter griffins bunker busting mega ultra super death cap. Like stickin a bunch of m80s together. Holy smokes that thing sounds soooo killer.

[@jimmydeanfishboy3734]

That sounds awesome, wish I could build my dream amp

[@fortj3]

Those red Bakelite knobs are the dog's balls.

[@TheGuitologist]

Yeah, woe betide to he who loses one though. You'll never find a replacement.

[@PaulinTaegu]

A Bogen K10 has a 6AU6 input 12AX7 PI and PP 6AQ5s (6V6s little 7 pin twin brother) check it out if you want. In its original PA configuration it is mild, but with some slight modifications, it becomes a wild beast of a guitar amp. LOL With our tube amps it is always best to work backward. That is to start with the PS and output sections and then work on pre-amp and input sections. The cathode biasing of the 6V6s is unusual but it does sort of make sense if you consider that it is providing a NFB (negative feedback) loop to the output tubes instead of the PI or input sections. ;^)

[@TheGuitologist]

Good call on the Bogen. Might have been helpful had I known that in advance. On the cathode biasing thing...I think it might also be changing the bias slightly as the tubes begin to conduct more, resistance will increase in the secondary winding, because inductors are dynamic, and therefore, the bias will fluctuate...almost a sort of compression or limiter. But yes, some NFB going on there too. In hindsight, I probably could have played around with converting it all and leaving that in place, but the amp is pretty cramped to work in. Made more sense to remove it and go with what I know will work.

[@PaulinTaegu]

Mirroring the K10's 6AU6 input configuration and TS might be something to consider if your customer is not completely satisfied. ;^) LOL Nice job!

[@TheGuitologist]

heheheh...never know, may have to.

[@PaulinTaegu]

I've also read many complaints about the 6AU6s microphonics etc. But I've got a massive inventory of them in my salvaged tube stock. All manufacturers and production eras are present in my stock. LOL I have not personally experienced much microphonics from them in my circuits. And to tell the truth, I personally really like the sustain/gain and distortion this little pentode produces as an input tube. The other input pentode I sometimes use is the 5879, even worse microphonics IMO, and more expensive. Almost everyone uses 12A_7s in the input/preamp stage. The input pentodes sound different and not like all the other amps. LOL Another input tube that may just be my favorite yet is a 6DJ8 (low mu twin triode) in a cascode (Totem Pole) configuration. You won't see that in any guitar amps that I know of. It is mostly found in Hi-Fi phono preamps, but its clean clear output and frequency response are outstanding IMHO.

[@TheGuitologist]

You'll notice in the video, I am biasing the front end to squeeze the most voltage gain from the stage, so I think that contributes to microphonics at higher volumes. There are no problems at all at lower volumes, and the clean tone is very nice and sparkly.

[@InyoTim]

I've got one of those that was given to me. I found a connection from the factory that was never soldered. The amp had been passed around because it would quit working at times. After fixing the bad connection, it makes a great amp. Creamy break-up. The tubes that are in it are the best from the 50's and still work fine.

[@jorgegomez6736]

That roadstar was a beauty

[@JJ-JOHNSON]

another job well done, sounds great, thanks for sharing Brad. 👍👍,🤘👀🤘.

[@MrBrymstond]

I wonder if that domino looking Mika Capacitor still works proper I never found a bad one.

[@TheGuitologist]

It was probably fine. I've never found a bad one either.

[@frankscassi4960]

Damn, that thing sounds great

[@TheGuitologist]

Thanks, Frank. I hope the owner likes it too.

[@randyheath8566]

Great Video Brad

[@BassySB]

That is just amazing! I have never seen one like that before. I would be too afraid of messing with. I have a feeling I'd damage it.

[@alecjahn]

Double the danger, double the crap. It's the statement of the great risk of double-death-cap.

[@paulkerr2298]

dude,, you got to build that guitar.. she's done the hard work for you. :)

[@TheGuitologist]

Butterflies and all! :D

[@budandbean1]

Thanks for such a great video, it's so nice to have your expertise as you work on the various circuits. I read what you said about books but a lot of my tube learning material I get in pdf on the tubebooks.org website. Loved Purple Rain...

[@TheGuitologist]

Nice website. I may consult it sometime. I use the net a lot, but that RCA manual is indispensable to have at your fingertips.

[@budandbean1]

You are totally correct, some day I'll put all the hyperlinks in, but for now, it is a pain to try to find something.

[@Cancun771]

It's not dancing in the actual rain in the Superbowl halftime with two long-legged lovelies in eight-inch heels but by golly. Wasn't expecting _that._

[@themarcoharder]

Interesting point re the RC network in series with the OT primary. I want to read up more on it. Is it Zobel or Zoe-Bell network?

[@TheGuitologist]

Zoebel. It is the guy's name who discovered the effect. I think he worked for AT&T or Bell Labs.

[@jussomdude9357]

I own one of these. I'll need it repaired. Will it be able to power a fender m8 4x12 cabinet?

[@Phoenix-vx3wf]

Billy Gibbons would love this...