Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on August 19, 2012, 05:32:10 am

Title: Weird tone in VOX AC30
Post by: Leevi on August 19, 2012, 05:32:10 am

I'm repairing an old VOX AC30 which produces a weird tone/note after the amp has warmed-up.
The tone comes up when picking harder with higher volume and it's quite sustainable and it sounds
like a siren. I get it out easier with bass notes. It occurs in each channel.
/Leevi

Title: Re: Weird tone in VOX AC30
Post by: bnwitt on August 19, 2012, 07:25:05 pm

Leevi,
The first thing is always swap our existing tubes with known good ones.  After that, how old are the electrolytic caps in the amp?  If too old recapping is a good idea to make sure the power supply is doing its job properly  Then plug the amp into an extension speaker to see if it goes away (cone cry) and another possibility is a bad ground connection in the amp wiring.  Now when you say and old Vox AC30 that could mean alot of things.  If the amp is really old, check all CC resistors for proper value in case some of them have drifted way up in value.

By the way, your dog is cute as hell. :grin:

Title: Re: Weird tone in VOX AC30
Post by: Leevi on August 20, 2012, 09:51:46 am

Thanks for the tips bnwitt. However I cannot go forward since I don't get the tone reproduced anymore.

Thanks for the comments regarding my dog. He is now 15.5 years old.

/Leevi

Title: Re: Weird tone in VOX AC30
Post by: Leevi on August 20, 2012, 11:25:50 am

I probably found the reason for the problem. One of the 220K carbon resistors in the power amp
was partly broken and by touching it became totally dis-conductive.
/Leevi

Title: Re: Weird tone in VOX AC30
Post by: bnwitt on August 20, 2012, 01:35:48 pm

Leevi,
This is why I always chopstick the heck out of and amp.  Bad connections and broken components will show up when that is done.  Congrats on finding the problem.  Never give up never surrender