Conversion of a J.W. Davis P.A. Amp into a Tube Guitar Amp

[@kennethgroves6552]

Uncle Doug thank you. I purchased a 1960s vintage classroom record player. It utilizes a 6t9 tube. I have changed from the hi-z input for the stylus, into a jack for my guitar. I used my linear power booster in front of it and it distorts all by itself. I have nowhere near the knowledge you do. However this thing works and I have you to thank for it. God's blessings to you and family.

[@UncleDoug]

Congratulations on a successful project, Ken. We're glad the videos have been helpful :)

[@TheGuitologist]

Cool amps, Doug! Great project. Not surprised the disc cap removal made no noticeable difference. Disc caps were used in Valcos as coupling caps and those amps,as we know, sound fantastic. :D

[@UncleDoug]

+The Guitologist Thanks, Brad. Yes, it was a foolish experiment that I regret. I still have the disc caps and am going to re-install them the next time I have the amp out and open. This just adds fuel to my conviction that caps are caps, as long as they are within spec and fully functional.

[@nandouy]

Thank you for all the knowledge, dedication and grace you display and share in all your videos Doug! greetings from Uruguay!

[@UncleDoug]

You are quite welcome, Nando. Thanks for your very nice comment :)

[@hardcorechronicles]

Pensaba que era el único loco en este país para hacer estas locuras jajajjaj

[@jfjoubertquebec]

I think one of the reasons I keep coming back to this channel with joy, might be a surprise to some: American technology and craftsmanship. Frankly, I miss it ! My dad used to say, if you can buy the American version of anything do so, it will be bigger, better built and last a lifetime. Bonjour from Québec!

[@UncleDoug]

Thanks so much, JF. It's nice to hear comments that compliment our country, especially from those who don't live here. Rusty, Jack, and I really appreciate it.

[@djfrank59]

Another excellent video! This was a good demonstration of the use of grid stoppers. I've added them to numerous amplifiers that lacked top end. They round the tone out greatly. I just want to add a short note to all of your subscribers: I'm an active audio electronics technician who does amplifier repairs for a living. I FULLY endorse Uncle Doug as not only an extremely competent technician, but teacher as well. Just a few short years back, I was teaching a vacuum tube electronics class in a local vocational school 2 nights a week, and I used Uncle Doug's videos as a valuable teaching aid in the class room. The class consisted of kids in their mid to late teens/early twenties. His videos gave a clear understanding of amplifier operation and theory made easy to understand, and the kids were able to grasp the concepts, which also helped me to be a better teacher. (Rusty gained extra fans as well) :) There is a lot mis-information out on the internet and you have to read, watch, and listen with discerning eyes and ears. Not all of the info out there is correct. But, you can put money on Uncle Doug. He speaks the technical truth to the fullest...Frank Ferraro--Audio Craft Electronics...

[@UncleDoug]

+Frank Ferraro Wow, thanks so much for the nice comments and great endorsement, Frank. Rusty, Jack, and I really appreciate it :)

[@usedguitargearmodsmusic5628]

Thanks to your instruction, I was able to convert a Bogen Challenger HF10A into a great sounding clean guitar amp.

[@UncleDoug]

That's great to hear, UG. Keep up the good work :)

[@rebelcat420]

Those Davis PA heads have some of the best sounding output transformers I’ve ever heard. Rebuilt one of these heads about 20yrs ago, and loved the tone I got from it

[@UncleDoug]

Thanks for your input, RC.

[@cliffbadger1]

I've installed a fair amount of J.W. Davis products over the years, especially notable were the Dr. Eugene Patronis-designed loudspeaker systems. When I saw your Gibson BR-1 video I was delighted as it was my very first guitar amp, purchased used at a rummage sale for $50.00 around 1967 or so. I sure would like to find one today.

[@UncleDoug]

Thanks for sharing, Steve. Have a great Christmas !!

[@orange70383]

This was perfect, I rarely find someone who is so very clear and explains exactly what needs to be explained. It's videos like this which makes youtube a real treasure. Thank you sir'

[@UncleDoug]

You're welcome, Orange. Thanks for your very nice comments. Rusty sends his regards to your icon pup.

[@bullthrush]

Another outstanding tutorial. The hand acting was subtle but appreciated, as was Rusty's comments about the capacitors. :)) As for Jack, ( in best Barney Fife voice) he's a nut! :)) A good sounding amp with beautiful factory wiring.

[@UncleDoug]

+bullthrush Thanks, Andrew.....glad you liked it. Like all actors, my hands are now whining about wanting to direct. I guess I should have been a traffic cop.

[@flurng]

Howdy, Uncle Doug! It's truly a blessing that Rusty has found in you a far more than passable assistant for producing his remarkable tube amplifier videos - after all, as capable as he is, he's only ONE dog, and occasionally, he needs SOMEbody to pick up the slack! Suffice to say, your performance has been exemplary, as are you wonderful videos! Well done, Sir!

[@UncleDoug]

+flurng God knows I try to follow all of Rusty's orders. He's a harsh taskmaster, but always fair, and I've learned a lot from him (he told me to say this). Also......we're glad you enjoy our videos :)

[@4hammonds672]

thanks for sharing your knowledge uncle Doug, i've seen a lot of your videos on how you are able to restore even the most insignificant looking old and ugly looking amps, i am truly amazed' many of those guitar amps i've seen you getting them to sound so good totally blow my mind, i just can believe it i got so much passion for tube amps and i wish i can do the things that you do with amps that look no good no more, my hat out to you sir. good job! me, you and this amp share the same thing in common... and that is been from the same state! have a safe memorial day weekend with rusty and jack

[@UncleDoug]

You're welcome, 4H, and thanks for your very nice comments. Rusty, Jack, and I wish you the best for this Memorial Day holiday.

[@jimblack5726]

Hi Doug, what a brilliant presentation. I've loads of gear & don't need another amp but I just enjoyed your video. You're what we limeys call a "national treasure." & a right southern gentleman. Yee_Haw!

[@UncleDoug]

Thanks so much, Jim. Please watch the rest of our 250 videos ;)

[@davidperry9417]

Charlie gives Dolly a claws out swat on the tailbone when he catches both her paws in the chow chassis. She jumps like it was high voltage! Glad to see all the jolts you took through the years have made you and Rusty immortal...I started to worry a little bit this winter when you went into hibernation...must be those Texas winters...now I know better for next year! Dave from N.J.

[@UncleDoug]

+David Perry We were just spending time on other hobbies and projects, David. A little rest helps us come back fresh and creative.

[@mikeelmore6040]

Uncle Doug great videos with both Davis amps. Each video you produce gives me much needed knowledge and confidence with my own tinkering and tweaking. Thanks for your efforts to teach and entertain.

[@UncleDoug]

+Mike Elmore You're welcome, Mike. Rusty and I are glad the videos are helpful :)

[@budandbean1]

Thank you Uncle Doug, another very interesting job. This was very cool to see the proposed mods and how everything worked out! See you next time...

[@UncleDoug]

+Buddy Martin You're welcome, Buddy.....glad you enjoyed it.

[@donaldfilbert4832]

One other comment. I noticed the same thing on my Hammond AO-44 conversion. After I completed the conversion - it worked - but it just didn't sound right; muddy and overdriven in a bad way. Then I noticed the absence of a grid stopper resistor - and added in a 68K. That fixed it very well !!! Seems many of these non-guitar tube amps have the same absence of a grid stopper resistor !!

[@UncleDoug]

+Donald Filbert Thanks for the input, Donald. It's definitely a deficiency to watch for in any PA to guitar conversion.

[@Bigjoedo66]

Great Video!! I noticed right away the grid stopper resistor was missing. However, I never would have guessed it made as much difference. Thanks for another great video. Joe

[@UncleDoug]

+Bigjoedo66 Thanks, Joe. Yes, it made a huge difference, eliminating the unpleasant overdrive distortion in V1a. I intend to experiment with higher resistance grid stoppers, i.e. install a 1 Meg pot and tune for best tone.

[@HBSuccess]

You are a master sir - and I dig the Rusty and Jack comic relief. I've always enjoyed tinkering with electronics but in my 40+ years of gigging with tube Amps never learned squat about them. So now I'm transitioning to 'semi-retired PITA tinkerer guy" and want to catch up. Your vids like this one - walking down that schematic and clearly/concisely explaining is INVALUABLE so all I can say is 'thank you' and I look forward to more. You have also inspired me to start some video production on my own that hopefully pays it forward and will help others on other topics where I have some chops to pass down. So thanks again !

[@UncleDoug]

+Tioga Fretworks You're welcome, TF. Rusty and I appreciate your nice comments and wish you the best on your future efforts in amp repair and video production.

[@geraldhinson]

Love all of the explanations of the circuit - phase inversion, center tap differences from usual, disk capacitors being legendary for bad tone, etc. And, the before and after tone experiments as the circuit is changed are super helpful as well. Great video.

[@UncleDoug]

+Gerald Hinson Thanks so much, Gerald. Rusty and I are really glad that you enjoyed the video :)

[@geraldhinson]

+Uncle Doug Jack doesn't care if I enjoyed it, but he's a cat so that's to be expected.

[@UncleDoug]

You're right, Gerald. Jack doesn't care about much other than his food and cat box.

[@aerofart]

BIG IMPROVEMENT. Well done Uncle Doug I’m learning so much from you. I can’t wait to tear into my old Hoffman ST2 for a third time and try a few of these mods/upgrades. Don’t blame Rusty for eating all of Jack’s food. He must be tired of eating from the senior citizens menu.

[@UncleDoug]

Thanks, Aero. Good luck with your project.

[@olipito]

Hi Uncle Doug! I'm very happy you made new videos, and as always they're amazing! I particularly enjoy the way you explain technical stuff.

[@UncleDoug]

+olipito Greetings, Oli, and glad to hear that you enjoy the videos. Thanks for watching :)

[@skycarl]

Good stuff Doug. Always a treat to see a new vid from you. Carl

[@UncleDoug]

+skycarl Thanks, Carl. It's good to hear from you :)

[@randyschock7374]

Dude, your so cool. Its hard for me to be envious of another man, but your electrical skill are amazing. I hope to be there one day. Thanks for wonderful videos that are greatly appreciated.

[@UncleDoug]

Wow....thanks so much, Randy. One thing's for sure, if I can do it, so can you :)

[@tomsamps]

What a great improvement from the beginning to the final product.

[@UncleDoug]

Thanks, TA :)

[@robertcovemaker9440]

Love your videos what I particularly like are your explanations of how things work. I repaired electric fork trucks for forty years and we had service manuals entitled " Sequence of Operations " This seems to be your method of teaching. Well done . Have you or anyone out there heard of a Morgan Rhein model 5230 pa amp,,My wife saw it at an antique store and bought it for me . She didnt know what it was but saw it had tubes so she got it. Its got a 5u4 , 2 6l6's 6sl7 6sj7 and a couple more that I don, recall, I need a schematic .

[@UncleDoug]

Thanks so much, Robert. We really try to present the information in a rational, orderly manner. I have never heard of that brand, but the tube complement closely resembles that of many similar PA amps. Perhaps a schematic from a more common brand might work.

[@els1f]

Really great! I've always wanted to give the tube PA to Guitar amp conversion a try, but never did. Very interesting video.

[@utubehound69]

+Elsif We used one for a Bass Amp years ago lol just Plugged & Played we didn't have uncle Doug to teach us right from wrong.

[@starlingave]

Thanks for the continuing education. Please let us know if you decided to make any other changes, thanks again, Bob

[@UncleDoug]

+starlingave You're welcome, Bob. I will probably experiment with the grid stopper value, since it made such a big difference by overcoming the initial overdrive distortion in V1a......temporarily inserting a 1 Meg pot and tuning it for best tone. I'll keep you posted.

[@td7456]

Neat stuff, Uncle Doug! I remember playing guitar through an old Bogen Challenger PA amp...it didn't sound great but it did amplify....and was louder than my Teisco Del Ray amp! lol! Hey, good to see Jack back...I heard they were getting ready to yank his SAG card...

[@UncleDoug]

+Tim Dockery Thanks, Tim. With a few modifications, I'll bet that Bogen Challenger could have sounded great. Yes, Jack was forced out of hiatus to maintain his SAG health benefits :)

[@RandySchartiger]

Great video man! Good to see Rusty is doing well and making exciting decisions! :)

[@UncleDoug]

+Randy Schartiger Thanks, Randy. Rusty is in rare form lately, possibly due to some fan mail from a poodle in Paris :)

[@treborheminway1196]

The viewers of your excellent video's really are treated to the "dinner and a show" equivalent circuit with your instruction and humor. Thanks for all of your high gain knowledge and wonderful entertainment.

[@UncleDoug]

You're welcome, Trebor. Thanks for the very nice comments.

[@paparufus4125]

I just picked up a Davis pa amp. Of course Uncle Doug has a video on them! Such a huge library of knowledge in Doug! All of your hard work is greatly appreciated by all of us mere mortals. My amp is a model 805-A. In my searches I cannot find a schematic or any other info. Any help from you or your viewers would be greatly appreciated.

[@UncleDoug]

You might check the Rider's Manuals for a schematic, PR. Good luck.

[@paparufus4125]

@@UncleDoug Thanks! I'll give it a shot.

[@stephenquail3168]

Loving it.rusty is a trip.Great repurposing.cool conversion.

[@UncleDoug]

Thanks, Stephen :)

[@mariannwatt2678]

When i was a kid i found a thorinson (spelling ?) 3 tube amp in a trash can in my alley . i used my 1 15 inch speakercab ( silvertone ) and it sounded monster with my. Fake les paul ha ha wish i still had it to send to uncle doug just for fun . youre the best uncle doug god bless !

[@UncleDoug]

Thanks so much, Mariann :)

[@Slugg-O]

Hi Doug. Your videos are always interesting, informative, and entertaining thanks to Rusty and Jack. It's good to see Rusty assisting you again with your experiments, which is more than I can say for my dog. Mine reached retirement age a few years ago and, due to a hidden clause he had included in his watchdog contract, is now entitled to free housing and benefits for the rest of his life. I just hope he's not registered to vote. Fantastic job on the PA. I've been interested in converting an old tube radio to a guitar amp but haven't come across any bargains yet.

[@UncleDoug]

+Slugg0matic Thanks so much, Randy. I believe that Rusty's contract has a similar clause....or at least he acts like it. Best of luck with future projects.

[@petedazer2556]

Doug, try a 120pf mica around the volume pot for a bright boost. I've done this with several PA to GA conversions and been pleased. Do it with a toggle switch, and tell everybody it was Rusty's idea! I love your videos, and have learned quite a bit from you, sir!

[@UncleDoug]

+Pete Dazer I'll give it a try, Pete. Thanks for the input.

[@charlesgwyn2115]

Brings back good memories... we used these amps in our hometown radio station for monitors throughout the studio. Still have 'Davis' speakers in use there! Good video.

[@UncleDoug]

+charles gwyn Thanks, Charles. They are indeed nicely designed and built. Good quality equipment......from Texas :)

[@charlesgwyn2115]

+charles gwyn BTW, we used the 70 volt output to wire 7 or 8 "house" speakers in each office area... so the radio station audio could be heard all through the building. I've never seen 70 volt output used anywhere else. Worked well for long distance runs of audio. Thought someone might like to know what the 70 volt output was used for.

[@UncleDoug]

Thanks, Charles. The constant voltage output is to eliminate the problem associated with the impedance of a large number of speakers in a system. Whether wired in series or parallel, it would be virtually impossible to match their impedance with a 4, 8, or 16 ohm output.

[@TheGuitologist]

The amp in this vid reminds me very closely of the Grommes Precision Electronics S-10 PA amp I recently converted. Almost identical schematic. In that one I decided to do away with the stock output wiring and go with a push-pull and experimented with the preamp. Something interesting came of it: PART 1 (Demo / Humbuckers): kzbin.info/www/bejne/jorJY51ofZ5qpq8 PART 2: kzbin.info/www/bejne/mJesiJprr716ask

[@UncleDoug]

+The Guitologist The Grommes project was quite interesting. I watched both videos and was impressed by the dramatic changes associated with each of the three different tone settings. I have heard that asymmetrical phase inverters produce more even-order harmonics, and it also appears that the asymmetry creates some very interesting tonal effects. Rusty and I will have to do some experimenting on this concept. Thanks so much for the links, the videos are excellent (as always).

[@charleslambiase5670]

Once again another gem you have re reproduced, if that makes any sense LOL along with the entertainment a true learning experience thank you Uncle Doug 🙏

[@UncleDoug]

Glad you enjoyed it, Charles :)

[@charleslambiase5670]

@@UncleDoug 🤞🤞🤞

[@tbdalva]

Coffee and a new UD video, doesn't get any better... well maybe beer and a UD video. This would be an awesome stereo setup with a delay pedal. Thanks for sharing.

[@UncleDoug]

+tbdalva You're welcome, TB. It's always great to hear from you. I wonder what type of echo effect you could achieve with the delay pedal applied to only one of the two? Also, since they are identical, they could also form a nice monobloc stereo system.

[@tbdalva]

+Uncle Doug Well, I experimented with this a while ago with two Fender Champs, sounded great in my living room.Had mixed results live because the PA was always out of my control.If you are curious, this is a song we did in a real studio, two Champs, one wet one dry. I had no part in the production, but it is the 2 Champ setup. Our friend Michael wrote this song and asked us to back him up.My son on Bass my wife on drums and me on Telecaster. The song is "The Jungle" hear it at www.reverbnation.com/michaeljdwyer

[@UncleDoug]

You and your family are quite talented, TB. I really liked the tune and the guitar work was excellent. The two-Champ setup sounds great ! Thanks for the link.

[@DaveHilts]

great job as always UD! I've got a couple of PA Amps laying around and this is just what I needed to push me to do something with them! Thanks and as always I love the Jack and Rusty show. And by the way, my garage has an old wood floor so I'm jealous of you concrete floor in yours. I also, sadly, now can't get the image of you rolling around on the concrete floor out of my mind!!! Dave

[@UncleDoug]

+Dave Hilts Thanks, Dave. I was just dusting myself off after a good roll. By all means, drag out those old PA amps and get to work converting them to fire-breathing Bandmaster specs :) Let us know how they turn out.

[@keiranbradley3222]

Hey Doug, always a treet seeing some quality US made gear, they don't make them like this no more sadly, give Rusty a scooby snack from me !!, Thanks from Scotland.

[@UncleDoug]

Greetings, Keiran. Always good to hear from the land of my ancestors:) Rusty says "Thanks for the snack".

[@jimmyhoffa5658]

Good morning from California:) I was attempting to find some videos discussing vintage Benson guitar amplifiers, As I Have recent acquired a model 300. I believe it’s a very early model judging by the scarcity of information along any format…. Thankfully I stumbled upon this video of Jack and rusty! It’s crazy that a dog and cat channel have a guy explaining tube amp conversions running in the background!!! The internet never ceases to amaze me;) If rusty or jack come across this comment, FEED THE MUSICIAN!!! But fair warning Jack… musicians will totally go for the cat food straight away😏

[@jimmyhoffa5658]

You got my subscription! Really thankful for this information! If you have any on my amp, please share!

[@UncleDoug]

@@jimmyhoffa5658 Thanks for commenting and subscribing, Jimmy. Sorry, but I have never even HEARD of Benson amps. Hopefully there is some info on the internet. Good luck.

[@lrdisco2005]

Excellent tutorial, really clear explanation for someone from the solid state generation.

[@UncleDoug]

Thanks, LR....glad you liked it :)

[@faultlessguitarsandamps1116]

Great video . I'm off to try my garage floor right now .

[@UncleDoug]

Thanks, FG. Jack suggests the area between the cars. Behind the cars is too risky......you might end up falling asleep and become roadkill when the wife heads out to Walmart.

[@schastain5]

Excellent video. I really appreciate you sharing your knowledge and taking the time to put together this Amp conversion.

[@UncleDoug]

+Scott Chastain You're welcome, Scott, and thanks for your nice comments.

[@Billo1281]

I've been really sick and just watching your videos have made me feel a little better. You are a great teacher and I enjoy your great videos, thanks.

[@UncleDoug]

+Billo1281 We're sorry to hear of your illness, Bill, but glad our videos are helpful. Get well soon :)

[@davidsigglekow1349]

I didn’t know any better, I’d say You’re an innovator! Nice work as usual.

[@UncleDoug]

Thanks, David :)

[@SNORKYMEDIA]

Fabulous as always Doug with the added advantage of additional veterinary market research.

[@UncleDoug]

+SNORKYMEDIA Thanks, Snork. Yes, Purina pays me a monthly stipend for my state-of-the-art investigations.

[@claucofer]

Good work Uncle Doug, just like you allways do. I´m workin' on a Optimus Ontario conversion into a guitar amp, I found that P.A. on internet for 100 Euros (85 Dollars I think). Is a 60Watts beast with a pair of EL34 and 3 12AX7, I´ll put some pics in your facebook page.

[@UncleDoug]

+Claudio Coello Thanks, Claudio, and best of luck with your project. I'll keep an eye out for your photos.

[@claucofer]

+Uncle Doug I found another P.A. in a web site from 1950, I put some pics on your facebook page, hope you like. It only cost to me 60 Euros (50 Dollars) and looks great.

[@UncleDoug]

I saw the photo and was really impressed by the design and appearance, Claudio. It looks more like a nice piece of furniture than a PA amp. Be sure to post some photos of it when you're finished.....including some interior shots. Thanks for posting !!

[@Zone1242]

Nice conversion Doug - I really enjoyed following along. Cheers, John

[@UncleDoug]

Thanks, John. We're glad you enjoyed it :)

[@audiotechlabs4650]

Great as always! We all are so glad to get another gem of an informative video. Good to see your team doing well also. Thankz

[@UncleDoug]

+Audio Tech Labs You're welcome, ATL.....glad you liked it :)

[@mixolydian2010]

Thanks a lot Doug, i was wondering what Jack makes of Rusty's food? This is definitely i project i would have a go at. As always your explanations are clear and and concise. All the best. Colin

[@UncleDoug]

+mixolydian2010 Jack is a complete snob. Rusty's food is beneath his dignity, and when he sees or smells it, he tries to bury it (the ultimate insult).

[@lmbsparky]

another great video.......thanks doug. ive converted 2 pa amps....thanks to your vids!!

[@UncleDoug]

+larry benben You're welcome, Larry.....glad to hear the videos are helpful :)

[@lmbsparky]

they are a lot more than helpful,,,, they are inspirational!!!!

[@ronniestump2001]

I have a J.W. Davis Model 405 amp/pa, I think a 1972 model, was the year it was made. It's been in the box all that time & has never been used.

[@d0u6la5m]

i did similar. changed out the 6eu7 to 12ax7. luckily the 6v winding have enough ampacity to handle the 12 volt heaters hooked using the center tap and paralleling to match the 6 volt. seems like they always matched 6eu7 6bq5 and 6ca4 together. i've seen it a lot. especially in old stereo phono radio combo systems and the lark amp to name two.

[@UncleDoug]

Thanks for your input, Doug.

[@aportilla007]

I enjoy your videos very much - thanks for taking the time to make them! It just occurred to me that it might be a good idea to write notes directly on a copy of the diagram.

[@UncleDoug]

You're welcome, Alex. I do write notes all over schematics, especially when scratch-building. In the videos, I use clean schematics for clarity.

[@rheidtech]

ahhhhh. the garage floor. the coolest room in the house...great vid UD.

[@UncleDoug]

+R HEIDEMAN Thanks, RH :) Jack and I spend a lot of time rolling around now that the temperature is rising for summer.

[@markbrooks7257]

Hey Uncle Doug, thanks for all your videos from a fellow Texan!

[@UncleDoug]

You're welcome, Mark :)

[@phillipevans4041]

Hey Uncle Doug, Just got done watching the tube amp bootcamp videos. Very Nice work love all the videos. I have aquired one of these Davis 106 amps, Unfortunately my was used at a bank for some type of alarm. I have removed all the unnessary items and am working on the modifications. Did you to ever do more work on the tone? If so we would to see :). Any way thanks for all the videos, keep giving Rusty those treats and meeting his contract, and Jack on his retrieving skills.

[@UncleDoug]

You're welcome, Phil. I haven't done any more work on the tone, but as I recall, Ben Prevo made some helpful suggestions (see below in the Comments). Good luck.

[@paulcowart3174]

Very cool I've got an old Bogen 440 projector conversion w two big bottle 6L6's Definitely needs a Drive w good EQ

[@UncleDoug]

Glad you enjoyed the video, Paul :)

[@sinecure45]

Thanks for another of your signature Master Classes, Uncle Doug. Unfortunately your very welcome demonstration comes a bit late for my benefit, as I had just finished botching the conversion of a very similar piece of antique equipment. After watching this video, I wonder if it might be salvageable after all.

[@bertuspa3grn]

Hello good Morning Uncle dog I again enjoyed the video...special the faseiverter i never seen it that before many thanks that share your videos with us greetings from Netherlands. Berus

[@UncleDoug]

+Bertus pa3grn You're welcome, Berus. Yes, indeed, this is an unusual type of phase inverter....very simple and economical. Thanks for watching and commenting :)

[@jessevanruller1319]

The added grid stopper resulting in more highs was a surprise to me. I would expect the opposite to happen. Can you explain why? Love your channel, Doug!

[@UncleDoug]

Since resistors are not frequency dependent, I would have expected no change.

[@tectalabyss]

Outstanding has always Doug. All my best. Bobby

[@UncleDoug]

+Bobby Tectalabyss Thanks, Bobby. Our best to you.

[@moseymen]

A very pleasant and informative video, again... thanks Doug. Regards...

[@UncleDoug]

+moseymen You're welcome, MM.....glad you liked it :)

[@germansanchez3577]

hi uncle doug!!!! cant thank you enough great video intresting information !!!!!!

[@UncleDoug]

+German Sanchez You're welcome, German.....glad you liked it :)

[@denniswalsh8476]

Good looking little amps. Same treatment for the second amp I presume. Great videos Doug.

[@UncleDoug]

+dennis walsh Thanks, Dennis. I'm not sure about the second one. The chassis is pretty rusty (no pun intended), so I may cannibalize it for a future build......the output transformer would be quite versatile.

[@sting1111]

A very interesting video! My question is why don't we concern ourselves with the differences between input impedance between mic and a guitar on the input side? Is it because we are talking about current at the nano amp level? Hopefully Rusty can shed some light here. Keep the videos coming! Sting.

[@UncleDoug]

Greetings, Sting. Many vintage amps do have inputs of varying impedance to match guitar pickups and microphones. Watch some of my videos on early tube amps and this is discussed.

[@rlibby404]

@11:25 I'm glad you didn't plug it in! I'm watching your videos for some preparation and inspiration before I start tearing apart vintage portable tube reel-to-reel decks. It looks like I picked a good starting place since I'm looking to make at least one of the three into a standalone microphone preamp. Any chance you would want to do a similar walkthrough for this type of thing? There are hundreds of old cheap tube reel-to-reel decks for very affordable prices online.

[@UncleDoug]

The conversion process is essentially the same for all such amps, RL.....add input and output jacks, fusing, 3-wire cord, and fine tune the volume/tone controls. How this is done may vary from amp to amp, based on the tonal expectations of the one doing the modification. However, If I do encounter a tape deck amp, I will make a video of the conversion.

[@Lost-xl6im]

I just rolled around on my back in the garage and I enjoyed it too ... 08:36

[@UncleDoug]

It's really good for the posture :)

[@benitomgomez3290]

Earthing.! ! 🤔

[@ijamsum]

Good going Doug , I wondered about changing the input impedence similar to guitar amps and some have a 32,000 ohm resistors and 68,000 ohms on the input if I remember correctly !

[@UncleDoug]

You're better off with 32-34K, IS. In Fenders, the 68K input resistors are generally in parallel.....so actually about half that value.

[@eugeneeaton7262]

Great video Doug I learn something on each video thanks.

[@UncleDoug]

You're welcome, Eugene.

[@robertbarcus2008]

Thanks for your time sounds great I learn something new every time and I'm pretty dumb

[@UncleDoug]

You're welcome, Robert.

[@santi308]

Nice video. Watching your older video recently, in which you explain the operation of three types of inverters, the Split Load, paraphase and the long-trailed pair phase inverters, buy how is it called this type of Driverless (?) Push pull inverter?

[@UncleDoug]

This circuit, like the Magnatone Melodier model 109, uses a configuration called "screen-fed self-split", in which you add a voltage drop on the screen grid of the tube being driven by the preamp, and the screen will work as a "second plate" with an inverted signal that can be taken from it and fed to the other 6V6 grid through the 0.1uf cap.

[@justinberry415]

Great video, Doug. I recently found a Bell Pacemaker PM 20 that I would like to convert to a guitar amp. Very informational like all your videos! I just need more test equipment. Thanks, Doug.

[@UncleDoug]

You're welcome, Justin, and thanks for the very nice comments. Best of luck with your PM20 conversion.

[@DavidGatto]

I used a 50's magnatone PA amp 6SL7 6V6 combo sounds sweet

[@UncleDoug]

They make excellent equipment.

[@DavidGatto]

@@UncleDoug I especially love those triad transformers😊

[@BluesPreacher89]

Hey Doug, first time caller, long time listener! Youre doin great job!! I was a little bit confused though, I thought black wire from AC outler is HOT and therefor have to go through the fuse, but in this video you did the other way round, being that the white wire is through the fuse. Am I missing something ? Thanks cheers for your channel man!

[@UncleDoug]

Many early circuits ran the hot wire through the power switch and the return wire through the fuse. I normally change this to the black wire running through BOTH fuse and switch. If I didn't in this case, it was an oversight.

[@100chuckjones]

great video Doug. thanks for the knowledge.

[@UncleDoug]

+chuck jones You're welcome, Chuck. Thanks for watching :)

[@donaldfilbert4832]

Great video - as always Uncle Doug !! One comment. It's hard to tell for sure - but in the video at 20:35; it sure looks like one end of that new grid stopper resistor is not soldered (just crimped on). If so - that could cause some intermittent noise in the future !!

[@UncleDoug]

+Donald Filbert Thanks for the sharp-eyed observation, Donald. I double checked and the resistor was indeed soldered at both ends, but I appreciate your helpful input.

[@explodingPSYCH]

I've been enjoying your videos. I'd love to see a break down on the Altec 1566a microphone preamp. It's a unit I've used a couple times in recordings and loved, but I've never been able to get one. I'm thinking of building a clone, but I'd love a better understanding of what makes it sound the way it does.

[@UncleDoug]

+explodingPSYCH Thanks, EP. I stick with vintage tube guitar amps, but the principles are the same. You could start off by downloading a schematic and analyzing it. Good tone is almost always a product of good design.

[@stanleychytla3786]

Thanks for another great video. You had mentioned checking the plate dissipation for the output tubes. Did the cathode bias resistor give appropriate plate current? What did the tube dissipation turn out to be? Thanks again. I enjoy these videos.

[@UncleDoug]

+Stanley Chytla You're welcome, Stanley. I added this as a note in the video. The output tubes ended up with a conservative bias (PD) of 9.3W each using the original CBR. They sounded anything but weak and sterile (indicative of under-biasing) so I left them alone.

[@stanleychytla3786]

Thanks for the update. I was curious about the effect of that unusual phase inverter method as it seems like it could be prone to poor balance between the two output tubes in a push-pull arrangement. However the sound seemed fine. I always look forward to your videos and I considered you my mentor.

[@UncleDoug]

You're welcome, Stanley. I am pleased and flattered by your nice comments.

[@PoJoWo]

Fantastic stuff. It would be interesting to hear your thoughts on the theory behind effect of the grid stopper. Is it reducing drive and compression hence allowing the high end a little more slack from the supply? You'd have thought the effect of the Miller capacitance would have reduced the highs by adding a stopper. I guess this shows how when other parameters are at limit we sometimes get the opposite of what we expect. I once had a later 70s two input hi watt that did not have a stopper on v1 and I experimented with it, yet ultimately there were many things about the sound that didn't not please me and it needed more modification to suit my ears. I was reluctant to do this as I had already tried to sell it once and had criticism from the hiwatt cognoscenti that there were repairs in the amp that did not have the hiwatt red varnish on the joints. I sometimes wish I'd kept it but my recent health scare has taught me, if nothing else, that you cant collect it all and take it with you. Ultimately someone else ends up with the pieces of the jigsaw to deal with. I say this with the most positive connotation, I must stress.

[@UncleDoug]

+PoJoWo I have a very simplistic viewpoint on the grid stopper.....it reduces the "strength" of the input signal to prevent over-driving the first pre-amp stage. What it's actually doing is a bit more complex, but this is its practical effect.

[@PoJoWo]

+Uncle Doug thanks for your speedy response. That surmises my feelings and experience. I guess it essentially is a pre volume if you consider the input circuit with the guitar from a thevenin equivalent sort of approach. Badger the tripod lurcher has been watching Rusty faithfully and has asked me to given him more soldering work so he can hone his skills and hope to impress his hero.

[@UncleDoug]

You're welcome, PJ. Rusty sends his regards to Badger :)

[@krang07]

Great video Uncle D. Its fun to watch how you do it all. Thanks for sharing. Now I must go to the garage and roll around on the concrete like a cat..... :-D

[@UncleDoug]

+August Lyons Thanks, August. By coincidence, I just got back from a good roll and am absolutely exhilarated :) Try it and see.

[@krang07]

+Uncle Doug LOL

[@TOMKATPEDALS]

Awesome video Doug! Big fan of Jack btw. Look forward to more videos!

[@UncleDoug]

+TOMKAT PEDALS Thanks, TK.....glad you like the videos and Jackie Boy.

[@WileECoyotey]

I have one of those sitting on a shelf .. Haven't quite figured out what to do with it

[@PeterWalkerHP16c]

Of course stolen cat food tastes better. He's a dog. :-) BTW, the amount of difference that grid stopper made was amazing, it's like V1a was blocking or overdriven.

[@matthewshaw2968]

Uncle Doug these videos are fantastic. This stuff is helping me immensely. Wish I was as proficient in reading and understanding diagrams. What is your background?

[@UncleDoug]

I began restoring vintage jukeboxes about 40 years ago, Matthew, and the skills I developed then were applicable to amp repair.

[@UncleDoug]

......and thanks for your nice comments :)

[@kingdomtone]

Uncle Doug... the guy I bought two of these amps from had the original paperwork and schematic with it. If you want it, I'd love to share it with the community. However, I'm not sure how to upload a scanned document to KZbin.

[@UncleDoug]

You could upload it to my Facebook page: Uncle Doug's Vintage Amps.

[@PrinceOfBodom]

Great video like always! First of all i very appreciate rustys effort to be content with jacks food in order to make things more easy for you. its a smart move from him, because the saved space from the no longer needed dog-food can be used to store more tubes! i am sure, that's what rusty intended. Could you make a video to share some more tips about safety in dealing with higher voltages? I build a little booster with a 12au7 running at 60V and i would love to feel more safe, so that my tubes would not always have to starve like this!

[@UncleDoug]

+PrinceOfBodom Thanks, PoB. Yes, Rusty is known for his cooperative, helpful work ethic :) I incorporate safety tips in many of my videos, but, other than my Shock Hazards video, have not yet consolidated them into a single production. I'll keep it in mind for the future. Meanwhile observe two simple rules: Avoid being grounded (i.e. no bare feet on concrete) and only use one hand when working directly on an energized circuit (to avoid completing a circuit through your body).

[@fenderbender909]

Hi Uncle Doug, I'm curious about how you wired up the AC power cord leads. I was under the impression it was better to run the AC hot (black) to the fuse and then to the power switch and then the power transformer with the neutral (white) going direct to the other end of the transformer. I'm not sure why, I just recall seeing that recommended. Are there different schools of thought on that maybe? In any case, great video, there are a lot of these old amps around that were used in schools and factories and such back in the day and waiting to be repurposed.

[@UncleDoug]

I agree, FB. The hot black lead goes to the on/off toggle, then the fuse, then the PT primary. The white lead goes directly to the other PT primary. If the wiring in this amp did not match this pattern, then it was an oversight.

[@chucknowakowski6676]

Great job, I know nothing about electronics but love watching and learning the little I comprehend! ( always Greek to me lol)

[@UncleDoug]

+Chuck Nowakowski Thanks, Chuck. Keep watching and it will start to make sense :)

[@EJP286CRSKW]

A grid stopper resistor is there to prevent oscillation. It would more normally be say10k, and it should be mounted right on the valve, as close to the grid pin as possible. By making it 68k you lowered the input level a touch, bearing in mind the output impedance of the guitar itself. I would say you are simply running with too high an input level and I would voltage-divide the input further until you get a clean sound, then back off until you have as much dirt as you want. It's unusual in a non-hifi amplifier of this vintage to see negative feedback applied all the way around the amplifier, and it really isn't correct given the tone control: the NFB will try to compensate for whatever the tone control is doing and possibly put the circuit into overdrive. The 82k resistor is curious. I'm wondering whether it isn't a schematic error: it would make more sense attached to the wiper rather than the bottom of the tone pot, acting as a law-faking resistor to make a log pot.

[@UncleDoug]

+QuadMaestro I used the 68K resistor in this position because it is a commonly seen value in guitar amps, particularly those by Fender. It works well for them, so I thought it might work well here.....and it did. The tone control was not particularly effective, and probably does need some work. I'll also check into the NFB loop. Thanks so much for your very knowledgeable and helpful input.

[@chrismahoney3553]

Thanks for the great video , I love mine and wish I had another. I would like an effects loop in mine but it is great the way it is.

[@UncleDoug]

You're welcome, Chris. Keep an eye on Ebay.....they appear from time to time.

[@jonnybeck6723]

Another winner, Uncle Doug. I wuz, however, hoping you would've addressed the National Football league loop by either simply lifting it or otherwise aiming brain energy towards it... as I invariably derive learning and entertainment from such endeavors. Also wuz wondering about the output transformer as to whether its larger size was due to the extra taps or if it had the capacity for more power, though I must confess I'm not the volume nut I wuz in me reckless and carefree youth... thanx and cheers

[@UncleDoug]

Thanks, Jonny. I guess you could make the NFB switchable or adjustable. Most PA amps have rather large, multi-tap OPT's to suit a wide variety of speakers. You're welcome.

[@bradleysmith4409]

Another great job UNC... thx!

[@UncleDoug]

You're welcome, Brad.

[@liumander]

Awesome! As interesting and entertaining as always

[@UncleDoug]

+Edu "Turupá" Sánchez Thanks, Edu :)

[@umajunkcollector]

nice little stack for bi-amp. simple fix, me like that! Don

[@UncleDoug]

+umajunkcollector Thanks, Don :)

[@utubehound69]

Thanks Uncle Doug!

[@UncleDoug]

+Watchman4u You're welcome, W4 :)

[@SiliconSet]

Great work, great video!!! Many thanks!!!

[@UncleDoug]

+SiliconSet You're welcome, SS :)