A Very Unusual Tube Amp Problem......and Repair

While working on a Davis 106 P.A. amp, I encountered a serious problem which is quite uncommon in guitar amps. In addition, the cause of the problem.....and its resolution.....were equally unusual. It's a fairly short video, and I think/hope you will enjoy the progressive diagnosis and repair. See if you can predict the cause of the problem.
Please subscribe if you have not already done so. Rusty and I sincerely appreciate your time and interest.

[@2right4words]

I love the Precision and Clarity of your videos!

[@UncleDoug]

Thanks, 2R :)

[@randolphpatterson5061]

I love the clarity of his videos, too, but I prefer the Jazz to the Precision..

[@Finom1]

This is the best way to spend a Saturday watch Uncle Doug & Rusty work their magic. Thank you for teaching us!!!

[@UncleDoug]

+Finom1 You're welcome, F1.....glad you enjoyed the video :)

[@ionizerusa]

Dear Uncle Doug, you bring back a multiple dose of memories for me. Many thanks!

[@UncleDoug]

I hope they are pleasant ones, Tim. You're welcome.

[@ix-Xafra]

This gem came up in my Ewetube feed at 4am. Love your work, Doug Regards from Vinny in Brisbane

[@UncleDoug]

Thank heavens you were up to watch it, Vinny. Thanks for the nice comments :)

[@kardRatzinger]

Usually I hear the term "motorboating" used to describe something completely different than an amp failure :) Congratulations on the repair.

[@UncleDoug]

+kardRatzinger Thanks, Kard. I understand there are several other meanings for the term, but I think you can see why it's particularly appropriate in this case :)

[@fortj3]

Absolutely. Usually when I hear motorboating of an amp, it reminds me of a thumper. Sounds a lot like my Yamaha SR400 idling.

[@vminator]

You sir have been a huge source of information for us all. Because of watching all your videos over and over again, I am now on my third scratch built tube amplifier! I love the sound so much I sold all my other amps! I hope your shop assistants and yourself are doing great. Keep the videos coming!!

[@UncleDoug]

That's great to hear, Miguel. Congratulations on your scratch-building success. We will :)

[@vminator]

@@UncleDoug Had a question for you, If I'm using a PT with 120mA HV secondary. Do I need to use a filter choke thats rated for 120mA? or does the filter choke only need to be rated for the current your actually drawing from that secondary? I have a 100mA filter choke I want to use but the PT secondary is rated at 120mA. Would it blow the filter choke?

[@audiotechlabs4650]

Wow Back to Back gems! These videos give me such encouragement. Thankz

[@UncleDoug]

+Audio Tech Labs You're welcome, ATL :)

[@edwardhannigan6324]

Hi Doug, still trawling through your videos in my quest for electronic knowledge. Unusual noise and great fix. It's amazing all the things that can go wrong in an amp circuit. I know this is one of your older videos, but enlightening non the less. Good to see Rusty' enthusiasm brimming over. Look forward to anything new in the pipeline..Thanks again for all this knowledge..Ed..U.K..😀

[@UncleDoug]

Thanks, Ed. It's always good to hear from you.

[@davida1hiwaaynet]

Excellent repair. Never give up on anything - locating and repairing the transformer problem is always so satisfying.

[@UncleDoug]

Thanks, David :)

[@jimlabos]

Thanks for taking us on this journey with you. So much to learn from you.

[@UncleDoug]

Thanks, Jim, for joining us and for posting a nice comment.

[@supermist100]

Nice to see Rusty is still with us! Thanks 4 your videos!

[@UncleDoug]

+Haroslav Drastic Like death and taxes, Haroslav, Rusty and I are here to stay :)

[@Zone1242]

Nice one Doug. I've never been lucky enough to find the broken end of the winding - usually buried down inside somewhere. Thanks for another great video. Cheers, John

[@UncleDoug]

You're welcome, John. I figure there is nothing to lose, so I root around looking for the broken lead and am sometimes able to find it.....but not always :)

[@waynekozak1462]

Hey Uncle Doug! Great video you are a superb diagnostician Thanx I am glad to see that Rusty has everything under control, as always !! LOL

[@UncleDoug]

You're welcome, Wayne. We appreciate it :)

[@jasonbrindamour903]

Cheers to Rusty and yourself sir...A big thanks for not being afraid to share this information with us in a video format. I greatly appreciate your knowledge.

[@UncleDoug]

You're welcome, Jason :)

[@daveb9370]

Hey Uncle Doug, great video and, once again, glad Rusty solved the mystery! Very educational, I learn alot from watching you guys work. I hope Rusty isn't pushing you too hard. My Husky Igloo pushes me hard sometimes, but he says it's for my own good, and I guess he's right. Have a great day and keep the great videos coming!

[@UncleDoug]

+Dave Bogusky Thanks, Dave, and regards to Igloo :)

[@harrisonhein1287]

So happy to see you and Rusty making videos again. You've helped me heaps with learning and fixing amps

[@UncleDoug]

+Harrison Hein That's good news, Harrison.....keep up the good work. Old amps need love too :)

[@harrisonhein1287]

Will do

[@andrewdarnley4608]

Another great demo of fault finding and selection of suitable repair materials. After watching Rusty's reaction over a number of videos i don't think he's camera shy. I reckon he's waiting for a proper remuneration deal before he'll stay in front of the camera. Maybe get your people to talk to his people ?

[@UncleDoug]

Thanks, Andrew. Rusty is a tough negotiator, but his agent is even worse.....we're still working on his new contract :)

[@aaronjohnson8211]

You're truly the king of amp repair

[@UncleDoug]

If it weren't for Rusty's guidance, I would just be an old hack :)

[@JoelzombieThomas]

This is one of the most brilliant repairs I've seen you do. I've mentioned it before, but that emphatic tapping on amplifier components you do has brought so much joy to my life. My new wife and I laugh a lot when doing the Uncle Doug Tap. You should patent it. We are big fans and hope to meet you someday.

[@UncleDoug]

Thanks so much, Joel. It took many years to perfect "the tap" and I'm glad someone out there appreciates it :)

[@saturn5tony]

Awesome repair on this cool PA Doug. Seems Rusty's language skills are improving as well. Looking forward to your next vid, thanks4sharing.

[@UncleDoug]

+saturn5tony You're welcome, Tony....glad you liked it.

[@RandySchartiger]

Great video Doug! And a very unusual problem. Rusty was smart to leave it to you, tell Rusty Q-ball says hey! :)

[@UncleDoug]

+Randy Schartiger Thanks, Randy.....glad you liked it. Regards to Q-Ball :)

[@ThomasACarlos]

Permatex High-Temp Silicone Gasket, who knew? I learned something today. Thanks Uncle Doug.

[@UncleDoug]

You're welcome, Thomas.

[@breth8159]

Rusty the Wonder Dog awesome. the little mouse that chewed on your Transformer not so is awesome I find in vintage well-lit magnification in close-up inspection saves me a lot of call backs thanks another great video

[@UncleDoug]

+Bret H I agree with your analysis, Bret. Thanks for watching and commenting :)

[@kendavis8046]

My dog Tater says Rusty is one cool dog! These are really interesting videos, and thanks so much for posting.

[@UncleDoug]

+Ken Davis You're welcome, Ken, and best regards to Tater :)

[@kevinbeckenham3872]

Thanks for a great documentary on motor boating problem that happen from to time on valve amplifier. Keep up the good works you do amplifiers.

[@UncleDoug]

You're welcome, Kevin. Thanks for your nice comments. We will do our best :)

[@GregsGarage]

I always enjoy your vids UD and Rusty.

[@UncleDoug]

+Greg's Garage Thanks, Greg :)

[@ThaUltimateDragon]

+Greg's Garage Hey Greg! Cool to see you here.

[@GregsGarage]

I love this channel... Sometimes you'll see me post as SkyScraper Guitars too!!! UD &R rock.

[@UncleDoug]

Thanks, Greg. Rusty and I have come to really like and appreciate our viewers....like you. What a great extended family we all belong to :)

[@jwl9286]

wonderfully done. loved it, thanks

[@UncleDoug]

You're welcome. Thanks for the nice comments.

[@vhm14u2c]

Amazing TS work on finding the issue with the open winding on the output xformer. Thanks for sharing!

[@UncleDoug]

Thanks, Vince :)

[@StonyRC]

Hi Uncle Doug (and Rusty) - That was a Masterful diagnostic procedure and your repair was positively surgical - a job equal to any Vascular Surgeon!. Thoroughly entertaining and (as always) extremely interesting. Please keep 'em coming.

[@UncleDoug]

+StonyRC Thanks, Stony. Now, if I could only get PAID like a vascular surgeon, Rusty's (and my) menu would definitely improve :)

[@leftyfusion88]

Great videos for someone like me who is trying to absorb how this stuff all works. Thanks again.

[@UncleDoug]

You're welcome, Lefty.

[@vonzigle]

Nicely done! 👍

[@UncleDoug]

Thanks, VonZ :)

[@budandbean1]

Wow, that's very interesting, congrats for finding the fault Uncle Doug. I actually didn't know that you could find and much less correct a break in a transformer winding, I really learned several things today. Thank you Uncle Doug...

[@UncleDoug]

+Buddy Martin You're welcome, Buddy. The trick is to find the other end of the broken lead. Most of the time, I can, and if so then it's generally fairly easy (not on the eyes) to mend the break. Once repaired, the OPT is generally as good as new.

[@supyrow]

Hey Uncle Doug, Hey Rusty! Great Video, Love it! 73!

[@UncleDoug]

+su pyrow Thanks, SP :)

[@supyrow]

You got it good buddy! 73!

[@aaronjohnmaughan]

Some fine troubleshooting here. Well executed repair, too.

[@UncleDoug]

Thanks, Aaron.

[@djvycious]

Uncle Doug! Glad to have you back! Would love to see a video about some vintage rock amps and getting the perfect overdrive sound.

[@UncleDoug]

+Cameron Walker Thanks, Cameron. It's good to be back. Hopefully, some vintage rock amps will show up for sale or repair. If they do, I will definitely feature them in videos.....so stay tuned :)

[@fredbloggs4829]

Great video. Loved the commentary, especially the part about grabbing a beer and pulling up a comfortable chair.

[@UncleDoug]

It's always a good way to begin any conversation :)

[@alanpassat6759]

Jack Darr mentioned this in his book, and I had always wondered what it sounded like. Thanks for posting!

[@UncleDoug]

+Paul Passat You're welcome, Paul. I have heard it before, when working on radios, but have never encountered the problem with a guitar or PA amp. It generally "putts" at a faster rate, but the sound is unmistakable.

[@zbdd8]

So helpful and detailed. Thank you!

[@UncleDoug]

You're welcome, ZB :)

[@cawfeedawg]

Missed your videos.. welcome back Doug.

[@UncleDoug]

+Cawfee Dawg Thanks, CD. It's good to be back :)

[@T25de]

Interesting repair

[@florascent9ts]

love to hear troubleshooting thought processes!

[@UncleDoug]

It's our pleasure to share them with you, CC.

[@toxicgraphix]

I love your videos! Thank you so much for passing down your knowledge to us all... and future generations. I noticed this video has one thumbs down, somebody must have hit the down button by mistake... I can't believe anyone would dislike any of your videos.

[@UncleDoug]

+toxicgraphix You're welcome, TG, and thanks for your very nice comments. Rusty and I really appreciate it :)

[@deluxetone]

Thanks for another fantastic and informative video. Excellent content and very entertaining .

[@UncleDoug]

You're welcome, DT.....glad you liked it :)

[@kennethjanczak4900]

Thanks for another good video, really like the way u explain things Keep the videos comming😊😊😊

[@michaelpukmel9175]

Oh my gosh LOOK at that transformer! That's a keeper, have to frame it! :) You're a master, Doug.

[@UncleDoug]

Thanks, Michael.

[@marviosantos]

HA! I actually "guessed" the problem before you got to the conclusion, and I only even mention it because this is a credit to you sir, for teaching me all I know about tube amps! Thanks for another great video!

[@UncleDoug]

+Marvio Botticelli You're welcome, Marvio. It looks like the student has progressed beyond the teacher :) Rusty and I are glad the videos were helpful. Thanks for watching.

[@redfishbum]

Simply AMAZING!!

[@UncleDoug]

+redfishbum Thanks, Bart :)

[@nikoladivic2786]

Very nice explanations like ever.Thank you !!!

[@UncleDoug]

You're welcome, Nik :)

[@49VinCom]

Great troubleshooting. Thanks for sharing.

[@UncleDoug]

Thanks. Glad you enjoyed it :)

[@RossDuClair]

Doug: Nicely done. Great piece of deductive reasoning and trouble shooting. This "eaten" transformer would be rare to most people, but with more than three decades of repairing broadcast transmitters and other remote location electronics, I have replaced many chewed on components, removed at least three snakes from blower inputs and scrubbed more than my share of rodent urine from circuit boards.

[@UncleDoug]

Thanks, Ross. One of my favorite "de-animalizations" was the removal of a putrid rat's nest, constructed mainly of tin foil, from the chassis of a really nice early tweed Gretsch amp......I shudder to think about the "light show" that might have occurred if I had simply plugged it in, as-is.

[@klokibril]

Great explanation and nice repair job. Thanks for the upload!

[@UncleDoug]

+Joop S You're welcome, JS :)

[@CC-te5zf]

Wonderful demo of preservation, v/s t/s by component replacement. This is great!

[@UncleDoug]

Thanks, Chad :)

[@treadmillrepair754]

Nice job, I love your dog.

[@UncleDoug]

Thanks twice, Ricardo :)

[@TheGuitologist]

Nice job on a successful repair. I really like the look of these amps. Unusual and cool. I guess they'll join Alamo Amps as the only vintage amp companies I'm aware of from Texas.

[@UncleDoug]

+The Guitologist Thanks, Brad. Yes, they are very well-made, attractive, compact, and functional. I would say they are one of the best candidates I can think of for guitar conversion.....and, as you said, one of only two brands I can think of from Texas :)

[@timothybedwards5440]

Awesome repair

[@UncleDoug]

Thanks, Tim :)

[@bluesingmusic3443]

Good find Uncle Doug. I love the old Simpson 260, that one looks just like the ones I used in the Navy. (Back when I still remembered most of the stuff they taught me during my year & a half of Aviation Electricians training. We went in depth on vacuum tubes, etc. Unfortunately I rarely used any of that knowledge in the decades following. I'd even forgotten the resistor "code" about "Bad boys....etc". Perhaps that's why I enjoy these videos so much, I'm forever thinking,"wow, now I remember that.) Youth is indeed wasted on the young.(at least in my case.)

[@UncleDoug]

Thanks, BiG........glad you enjoyed it. We do tend to lose that which we cease to use.

[@otteronbass]

thanks for another great video

[@UncleDoug]

+otteronbass You're welcome, Otter.

[@charliebrowncbiii]

I've only watched a handfull of your videos and i got to say I'm hooked....keep em coming uncle doug and say hi to rusty for me!!!!

[@UncleDoug]

+charlie brown We will, Charlie......and Rusty says "hi" to you all.

[@zanebullard8665]

Hey there Doug! I'm new to your channel, and I love your vids! I'm a guitar tech looking to get into amp repair and your videos have been a great insight into it! Cheers!

[@UncleDoug]

Greetings, Zane. That's great to hear. Best of luck with your new career :)

[@aperioguitar]

Awesome Uncle Doug!

[@UncleDoug]

+aperioguitar Thanks, AG :)

[@PLINKER]

I would not know a wizard with more knowledge than you Uncle Doug!

[@UncleDoug]

There are people out there who have forgotten more than I know, Tom.......but thanks !!

[@Barefeet200]

Wow a video to go with my morning tea, and then one to serve as a nightcap ! Great feeling when you can save a transformer, nicely done Uncle Doug.

[@UncleDoug]

+Barefeet200 Thanks, BF :)

[@jackallen6261]

I LOVE these old Davis amps, so cool. I have a pair of Infinity speakers to "re-foam" (replace the foam surrounds on the cones) tomorrow, baby steps, lol. Maybe one day I can work my way up to re-coning a speaker...maybe. As always very informative and helpful video! Thanks

[@UncleDoug]

You're welcome, Jack. Good luck with your project.

[@theabsolutetruth3364]

I bet your fun guy to hang out with ... lol ... I love your sarcasm ... Priceless ... ))) ... Thx for the Knowledge Uncle Doug ... ))) ... Also , Priceless ... ))) ... Blessings to you and your other half ... Say hey to Rusty for me , oh Yea ... Kitty ... )))

[@UncleDoug]

Thanks, TA. Sarcasm is like aftershave lotion......a little can go a long way :) But when it's directed at objects and deeds, rather than specific individuals, I tend to like it too......as you can probably tell ;) The pets all say "Hi".

[@repairfreak]

Thanks for the video Uncle Doug, give Rusty a hug and a treat from me too. Best wishes always.

[@UncleDoug]

You're welcome, Mike. Sadly, Rusty passed away several years ago, but I'm sure he would have appreciated your kindness.

[@repairfreak]

Uncle Doug Im sorry to hear this Uncle Doug, he looked like such a nice boy. I love our dogs like children, sometimes I think even more because my dogs don’t disrespect me, lol. All the wonderful creatures need is love, food, water and a warm bed and they are totally content and give back love in such great magnitude. Keep up the great work with your very helpful videos, you have furthered my knowledge considerably, and for this I thank you sir, God bless 🙂❤️

[@guitardavidc]

Uncle Doug, definitely enjoy your videos. Rusty Too!! 🎸

[@UncleDoug]

That's good to hear, GM :)

[@cassvirgillo3395]

Hey Uncle Doug & Rusty, Good to see you boy's up to your old trick's. Good diagnosis and repair, you are good. I am learning so much from your amp video's I almost feel qualified, not. I hope all is well with you and your's. 80's comin' here in Oregon and looking green with the leave's coming on. Take care, C.

[@cassvirgillo3395]

***** Hey Sceaming deal, Yes, very nice . I'm just outside Willamina, so not to far from Lincoln City. I've seen some nice day's at LC. Will take the winter's for the summer's. Thanks for commenting. Be good, C.

[@BrianOfAteionas]

+Cass Virgillo Hey fellow Oregonian! The weather was perfect here in Portland today. Good day for beers, music, and top it all of with a super cool Uncle Doug video. What could be better?

[@cassvirgillo3395]

BrianOfAteionas Hey Brian, I couldn't agree more. Happy 1st. Uncle Doug is my favorite channel. I'm getting part's together to build a Fender 5F1 Champ as seen in his 2 part video series. Take care, C.

[@UncleDoug]

+Cass Virgillo It looks like you Northwesterner's are having a nice chat. Thanks for all the nice comments.....Rusty and I really appreciate hearing from you all.

[@cassvirgillo3395]

Uncle Doug Hi Uncle Doug, I've never had so many reply's to one comment. Be good, C.

[@Pwaak]

Great video and a very good repair! Got a good laugh at video beginning, sounded like someone knocking on the door! Hi to Rusty!

[@UncleDoug]

Thanks, Tony.

[@Barnekkid]

Another good one from Uncle Doug and his enthusiastic pet. Just wanted to say thank you for the tip about the analog meter. I have a Simpson 260 also, but I largely kept it for posterity. Now I have a good reason to use it, maybe.

[@UncleDoug]

+Barnekkid Analog meters can perform tasks that digital meters cannot. They are both very valuable and useful tools. By all means, dust it off and begin using it, BK. You will wonder how you ever did without it :)

[@ziggfreud9820]

excellent video. really enjoy your videos.

[@UncleDoug]

+John Miner Thanks, John. Rusty and I appreciate the nice comments.

[@johncunningham5435]

I have serviced countless amps over at least 35 years or more and never run into this type of problem. It has always been a bad capacitor in most cases that caused the motor boating, but in the open winding of one side of the tap the sound was distinctively different. Your analyzes of half the active tap does suggest the capacitor is charging and discharging creating that most unusual sound, and after careful thought I have come to the same conclusion you have, and agree with your findings. "Well Done"

[@UncleDoug]

+John Cunningham Thanks, John. I've never seen it before in a guitar amp, and it's good to know that I'm not alone. It now seems to perform and behave quite well, so the problem appears to be fixed.

[@JesusIsLordofMybeing]

Nice save Uncle Doug!!!

[@UncleDoug]

Thanks, JT :)

[@benmorris4589]

Great, as usual. It struck an immediate chord with me, as I have a transistor P.A. amp, which developed the same problem. One minute it was working, with my Fender going through it - the next thing, all hell broke loose, and at a high rate of level! I'm sure you said that the problem on yours was caps, in the power supply? Maybe I'll climb in there and see what I can do, otherwise I'll have to spend some (a lot of) pennies! Keep 'em coming! Ben.

[@UncleDoug]

+Ben Morris Thanks, Ben. In my case, it was caused by an open primary winding in the output transformer, but filter caps are a more common cause of the problem.....usually accompanied by a hum. Good luck with the repair.

[@benmorris4589]

Thanks, Doug. (When I get round to it)!

[@SiliconSet]

Great video, many thanks! Rusty thinks you covered this so well he had no need to contribute.

[@UncleDoug]

+SiliconSet Thanks, SS. I guess he always thinks that way :)

[@denniswalsh8476]

Great surgery on the output transformer and finding the primary coil end (without making the patient any worse). Best ever for me was managing to (accidently?) find, clear and re-insulate a shorted pair of leads on the core of a Twin Reverb II power transformer. Good luck too, an "in kind" rewind by Mercury Magnetics was going to be over $300 (for a $500-600 amp.).

[@UncleDoug]

+dennis walsh Thanks, Dennis, and congratulations on your successful (and money saving) surgery :)

[@stevenwilliamdewitt]

impressive! You are truly the Man

[@UncleDoug]

+Steve Dewitt Thanks, Steve :)

[@krang07]

Enjoy all your videos Uncle D. Keep um comin`! Cheers!

[@UncleDoug]

+August Lyons Thanks, August....we will :)

[@raymondsmith6494]

Love this video!!! Class dog.

[@UncleDoug]

Thanks, Ray :)

[@RickGreenPhoto]

great video!

[@paulhammersley4562]

another good one, thank's doug,

[@UncleDoug]

+PAUL HAMMERSLEY You're welcome, Paul :)

[@dwightbehm4747]

As usual another fine vidio Uncle Doug I like what Ypu did for the output transformer for the amp very logicle choice of methods I was a service technician at a chevy dealership and used high temp silecone Sealers and twopart Fast or slow hardening epoxies For some repairs on things I think I would do the same as you. Wish I had a shop like yours live long and Prosper. Dwight from Detroit,Mi.

[@UncleDoug]

Thanks so much, Dwight. Likewise :)

[@peterbustin8604]

I love Rusty !!

[@UncleDoug]

It's hard not to, Peter. He's a very special guy :)

[@troynorris7666]

Uncle Doug, I appreciate your videos and technical expertise. I had a volume fade problem with my Marshall JCM 2000 DSL 50 a few months ago. I am a guitarist,not an amp repair technician so I took my amp to a man referred to me by a friend. After spending 282.00 I brought the amp home and had the same problem. Volume fade and then after a few minutes it may or may not work "correctly". After searching and reading many forums I came across someone who had the exact same problems I was having. I ordered a new $2.95 bridge rectifier and installed it myself. Then I learned that after making such a repair it is necessary to check the bias voltage. I nearly burned up my power tubes. They looked like light bulbs. Max voltage should be 45 ma and mine were around 96ma. A readjustment and my amp has not sounded so good in years. Now if I could only figure out a way to add a separate EQ to the amp and lower the high gain white noise it would truly be what an amp should be. Being able to EQ the clean channel and the dirty channel separately is no brainer and I have often wondered why Marshall didn't include this in the original design. Thanks again for your videos sir .

[@UncleDoug]

You're welcome, Troy, and congratulations on a successful repair of your amp. That was a close call on the plate current. I shudder to think what your plate dissipation was at 96mA for each output tube. To reduce your gain and mellow things out a little, you might try experimenting by substituting lower gain duo-triodes (12AT7, 12AU7, 12AY7, etc.) for one of the 12AX7's in your pre-amp.

[@troynorris7666]

Ahhh, I was under the impression that my power tubes were burning so hot due to the failing Bridge rectifier that was in the amp originally. After installing a new one that is beefed-up according to the specifications I assumed that was the reason why the voltage changed on my power tubes. 45 milliamps is maximum for my amplifier readjusting the bias after replacing the bridge rectifier has put the amplifier back in correct operating condition. I am an automotive technician by trade so I do understand alternating current and direct current. I am slowly learning the current flow in a guitar amplifier and why components do what they do.

[@UncleDoug]

Tubes overheat when the plate voltage and plate current within them is too high. This would not be the result of a failing rectifier or a new rectifier, but because the bias of the tubes is set incorrectly. Biasing involves more than simply adjusting the plate current. You should watch my video on biasing of double-ended amplifiers to see and understand the full process.

[@VegasCyclingFreak]

Very interesting problem, never seen that one before. Nice fix on that OT

[@UncleDoug]

Thanks, VC :)

[@donaldfilbert4832]

Great Video Uncle Doug !! I was surprised to see how small that wire was on the primary / High voltage side of that output transformer. Nice save !!

[@tubical71]

+Donald Filbert The diameter of a tranny coil wire only depends on the current flowing through at first glances. As being this one the wire of the primary coil the current through it is rather small.

[@UncleDoug]

+TubiCal Good answer, TC. Thanks.

[@gerrabath]

Just repaired a 30w double ended amp with a 250mS motorboating fault. Figured it would be a big cap problem what with the low frequency repeat and changed both B+ filter caps to no avail. After much head scratching decided the best option would be to inject and trace a signal. Turned out to be a dodgy phase inverting tube that was causing the problem. Note to self and anyone else who may encounter similar :- Always start with the easiest option when amp fault finding as assumption will almost invariably lead you down the wrong path costing time and possibly money.

[@UncleDoug]

Excellent advice, GB. Thanks for sharing it with us.

[@michaeljohnaustin6745]

Great video ,

[@UncleDoug]

+Michael Austin Thanks, Michael.

[@tectalabyss]

Hello Doug. What a wonderful treat, to find another awesome video from you, waiting in my email. :) Though Molly is laying on her back sound asleep She sends her regards to You Rusty and handsome Jack. Has always very well done,in both fun and very Educational . I really hope to see more videos from you. All my best. Bobby

[@UncleDoug]

+Bobby Tectalabyss Thanks, Bobby, and best regards to Molly :)

[@is1amizationbyimmigration262]

my first guess, was that a capacitor was installed backwards. Your videos are always enjoyable another 10 out of 10

[@UncleDoug]

+Is1amization by Immigration Thanks, I by I. I too initially suspected a filter cap as the likely culprit.

[@johnzook7533]

Oh man, do I wish I still had that old Sunn Sentura II. After doing a bit of work on it ...tone stack and adding a bypass cap to brighten it up after a bit it developed a ticking noise in the audio. As I say if I still had that amp, it would be interesting to see just hwat caused it. The amp worked but it had that tick motor boating instead of a lower frequency thump. As always another fine video and Rusty as usual makes a great producer.

[@UncleDoug]

Thanks, John. Rusty and I appreciate the kind words :)

[@benmorris4589]

Well, of course, apart from the broken winding! It took me back to the sixties, when we used to use radios as guitar amps, connecting up to the cap of the first valve (detector?).

[@nor4277]

Thanks for the transformer lessen I no very little about them nice save.

[@UncleDoug]

You're welcome, Ron :)

[@dancraver3815]

Rusty could be like the RCA Dog looking into a old record player speaker

[@UncleDoug]

I never thought of that, Dan. What a great idea :)

[@henryedwards4116]

I wish dogs could live forever, R.I.P. Rusty. ( I also love your videos)

[@UncleDoug]

Me too, Henry. Thanks :)

[@lavejim11]

Great video...that amp does look like it might have spent some time in an attic or a barn...good troubleshooting. My regards to Rusty.

[@UncleDoug]

+Jim Lavespere Thanks, Jim. With so much rust, it must have been in a damp location....yet the wooden cabinet is in great shape. Very mysterious.

[@christopherweber8047]

I love your videos! They make it very easy to understand how a amp and the components work! At the moment my diy amp (JCA20HV like amp) make a single "knock" when I get over 10 o'clock, 1 o'clock and 4 o'clock with the master volume. When I pull one El84 out, the amp starts with motorboating but only when I set the master exactly at this 3 points... Very strange. Never had a problem like this.

[@UncleDoug]

Have you checked all your electrolytic caps, including the cathode bypass caps, and your output transformer ??

[@christopherweber8047]

Uncle Doug thanks for answer! :) Yes I checked them all twice....all seems to be fine. I cut of the negative feedback; the motorboating is gone now. Maybe I should take a closer look at die PI/whole PA. Thanks you With best regards from Germany

[@UncleDoug]

You're welcome, Christopher. Let us know what was causing the problem, once you find out what it was.

[@christopherweber8047]

Sure! I Let you know! Thanks

[@christopherweber8047]

Hi, I can't find the main issue... I believe there is a much bigger issue in my amp. Do you ever had an issue like, that the plate voltage go down by turning the bias Poti? With best regards

[@tubehead2002]

Good job.

[@UncleDoug]

Thanks, TH :)

[@juddalexander5642]

Awesome!

[@UncleDoug]

+Judd Alexander Thanks, Judd :)

[@denniscochran1747]

Great Stuff

[@UncleDoug]

+Dennis Cochran Thanks, Dennis.

[@MadScientistGuitarLab]

I picked up a Simpson 260 just like that at a flea market 2 weeks ago. I had to replace a corroded battery terminal and it’s ready for another 60 years of service. 😁

[@UncleDoug]

Mine was the same, Bill. A leaky battery had made a mess of the battery compartment, but after an arduous cleanup, it worked perfectly.