amp help

[fen1der]

I have a boutique marshall clone jtm45 type 25 watts, it developed motor boating, so i got in touch with the builder. he suggested i replace the tubes which i did output tubes (6v6) first then the rest of them 12ax7s and gz34 rectifier. This didnt help and so the builder suggested i ship it back and it could be fixed. it is very expensive to do so, and so i am looking for some advice. i was thinking of checking the coupling capacitors can i do that with them in the circuit or do i have to unsolder one leg, any help or ideas would be much appreciated.



I am a newbie to this forum, thanks in advance for any help/ideas

[drewl]

If this is a new build I would check for a loose wire or connection, most likely a ground connection.

If you have never worked on tube amps you should read up on safety procedures like make sure amp is unplugged and high voltage dissipated.

[fen1der]

Thanks for the safety warning, i am always afraid i will forget to discharge something. this is not a new build it is 9 years old, never had a problem with it til recently. I also forgot to include in my post that it doesnt motorboatwhen i first turn it on its only after i begin to play that i begin to notice it, should i say when i stop playing. it will go away with a poweronreset, but returns as soon as i play for a minute or so.

[fen1der]

I also forgot to mention that i took the chassis out and after discharging and such did some easter egging to look for something obvious loose burned or otherwise but no clues there.

[drewl]

Bad filter cap maybe.

Look at the big round filter caps and see if any have gunk coming out by the leads or bumps.

[Justin Thomas]

Fen1der,

Go to R.G. Keen's tube amp debugging page. He's a frequent visitor here, and an engineer of many years. He'll get you read up on safety, and it's fairly comprehensive. You won't get steered wrong, because it's methodical. And he's not just a self-proclaimed internet amp guru. We don't try to dissuade anyone new from learning, but we'd much rather sacrifice an amp than a life. Since you are a self-professed noob, and since motorboating could very well involve working with the highest voltages in the amp...

I'd think filter caps.

Justin

[Chuck H]

+1 (or two?) to both the filter caps and referencing R.G.'s faq.

Motor boating is a by product of positive feedback. Filter caps age and lose effectiveness. Those caps decouple signal that would interact (potential for positive feedback) within the power supply. So, like drewl said. That or possibly a bad ground. Any tech could diagnose and repair that amp easily on the bench. And it's a bad idea for a novice to poke around in a tube amp. Let alone doing repairs to the power supply. I think most people could handle it but I'm amazed by how common it goes wrong just based on threads started here. IMHO, unless you actually WANT to learn about amplifiers you're best money and effort is in taking the amp to a local tech.

[fen1der]

Thank you all for the very helpful posts. I havent worked on amps but have worked around hi voltage. I want to learn so i will do some investigating into some theory befroe i attempt any hands on.. Thanks for your concern great group of guys on here from the responses I received and in reading through other posts.

[Jazz P Bass]

My vote is you have a failing decoupling capacitor on the preamp node.

A 10 year old capacitor puts it at the end of the "capacitor plague' timeline.

Further reading:

Motorboating (electronics) - Wikipedia, the free encyclopedia

Tube Amplifier Debugging Page