Need help with noisy Ampeg J-20

[mfc_pj]

I need help with this Amp. It's really farting out when pushed with some modulation and crackling noises. I tried new tubes, different speakers, tighten up the screws, nothing helped. Then I tried some filtering in the poweramp section and changed all the caps with no success. My guess is it's something in the preamp section.
Maybe someone has an idea?

[Leevi]

Take a wooden or plastic soup stick and touch different part of the circuit and the wires
with the stick when the amp is operating. The crackling noise is probably caused by a bad contact
somewhere.
/Leevi

[rafe]

Is that a new amp? If so, I went over one for a friend and chop-sticking revealed nothing. It was under warranty so I didn't go in any deeper. Although someone suggested shielding the transformer might help or actually turning one 90 degrees might help....if this IS the Chinese Jet there is a lot of interest in those
and I'm sure answers can be found......

[mfc_pj]

I did chopstick it with no result. I also replaced all the caps with better quality ones and renewd every single solder joint. Disconected the trem cirquit, switched to a solid state rectifier and tried higher value filter caps (up to 100uf), no change.

Here's a video where you could hear the same "fart out" when he turns the amp up:

Oh, it's the korean version.

[Shrapnel]

I did chopstick it with no result. I also replaced all the caps with better quality ones and renewd every single solder joint. Disconected the trem cirquit, switched to a solid state rectifier and tried higher value filter caps (up to 100uf), no change.

Here's a video where you could hear the same "fart out" when he turns the amp up:

Oh, it's the korean version.

Too bad "The Tone King" isn't as knowledgeable as he thinks he is. Looking at his various videos there are errors in his identification of certain parts, (Egnator Rebel: He Identified it as having EL34 and 6V6 output tubes

&feature=channel ) He also tried to pass of an '80 Marshall JCM800 (4-input) as a Plexi.

&feature=related  His knowledge on this one... WOW...

&feature=related As a side note on the You-Tube video mfc_pj listed, he doesn't mention anything about the fart-out.

[mfc_pj]

I upoaded this video just for the noise issue. I think I don't have to mention it, it's very audible. Here are some mp3's I recorded, trying to bring the noise to front:
http://www.upload-mp3.com/files/188102_evi8s/05.05.10%2023_01.mp3
http://www.upload-mp3.com/files/188101_9s0ks/05.05.10%2022_53.mp3
http://www.upload-mp3.com/files/185826_m3pit/29.04.10%2021_47.mp3
http://www.upload-mp3.com/files/181786_9z7oe/20.04.10%2020_40.mp3

The first one is just the Amp turned up to full Volume. You could hear this background modulation with that frying bacon crackle.

I also thought about the plate resistors as they are carbon comp but the amp is pretty new.

[mfc_pj]

Maybe some more ideas?

[mfc_pj]

desperate bump

[rafe]

Similar sounding to one I've heard. The first MP3 anyway. The other three were not as bad! the one I looked at was totally unacceptable.  Resistors are pretty cheap to replace, did you try shielding the transformers from each other? Perhaps a piece of aluminum? There may be an old thread here that addresses some of the problems with that amp....
http://www.el34world.com/Forum/index.php?topic=7330.msg64808#msg64808

[PRR]

http://thetubestore.com/bmsufra.html

"Mod Kit for the Jet City JA 20 Amp
"... the mods solve some issues and add very useful features ... I have heard complaints that the amp lacks clean headroom and punch, and can get a bit fizzy."

[Rich]

http://www.upload-mp3.com/files/188102_evi8s/05.05.10%2023_01.mp3

You did mention you tried new tubes, but I had an amp making that sound when the V1 tube was dying (but not all the time). One side of it finally died while I was double checking it in my tube tester (EICO 666).  Up to that point it seemed good to go - most of the time.