Really need help with my 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. I did chopstick it with no result. I also replaced all the caps with better quality ones and renewed 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: YouTube - The Tone King Presents - Ampeg J-20 Point-2-Point All Tube Amp Review

Some mp3's I uploaded:
http://www.upload-mp3.com/files/1881...10%2023_01.mp3
http://www.upload-mp3.com/files/1881...10%2022_53.mp3
http://www.upload-mp3.com/files/1858...10%2021_47.mp3
http://www.upload-mp3.com/files/1817...10%2020_40.mp3

Here's a schematic: Prowess Amplifiers - Ampeg - Schematics - Ampeg J20 Jet Current Production - fender 6g3 brown deluxe Clone hand wired J 20 Jet

My guess is it's something in the preamp section or a transformer.

Maybe someone has an idea?

[mfc_pj]

no one?

[Steve Conner]

Yep, that sounds pretty bad.

What guitar are you using, what pickups, did you change your strings recently? I've heard that kind of tone before from overwound pickups and old strings.

[mfc_pj]

I think I can exclude the guitar as the Problem. Tried about 5 different guitars and a lot of different cables. All the Guitars are low to moderate output with good stringes. I also tried a lot of different tubes, speakers, extension cabs. It must come from inside the amp.

[Bruce / Mission Amps]

Wow!! I hate that, no talent, ah bop bop bop... boppy boppy bop, cliche' pop rhythm guitar lick... (insert puking here) .. yeah I'm getting jadded in my old age... but "really!"
anyhow, sounds like too much preamp gain and phase inverter over drive to me...
A simple PI mod I'd try for a start:
Pop that 470 biasing R20 resistor out and use a 1K and pull the 10K tail R19 resistor out and replace it with a 27K-33K... just a thought.


Regardless of swapping out the tubes, the other preamp noise sounds like bad tubes or bad resistors in the plate leads somewhere but without having it in front of me it's really hard to figure that one out.

Added... the $800-$900 is actually a good price for this if you want to mess around with some tweaking.

[mfc_pj]

Wow!! I hate that, no talent, ah bop bop bop... boppy boppy bop, cliche' pop rhythm guitar lick... (insert puking here) .. yeah I'm getting jadded in my old age... but "really!"
anyhow, sounds like too much preamp gain and phase inverter over drive to me...
A simple PI mod I'd try for a start:
Pop that 470 biasing R20 resistor out and use a 1K and pull the 10K tail R19 resistor out and replace it with a 27K-33K... just a thought.


Regardless of swapping out the tubes, the other preamp noise sounds like bad tubes or bad resistors in the plate leads somewhere but without having it in front of me it's really hard to figure that one out.

Added... the $800-$900 is actually a good price for this if you want to mess around with some tweaking.

Thanks for that Idea, I'll try that with the PI.

I tried nearly every tube I have lying around here, preamp, power amp and rectifier. And I changed every single resistor and capacitor in the amp.
One more thing I have in mind are the transformers but I have no Idea if they could cause that.