ax84 p1 amp build problems

[wsaraceni]

im having a problem with my ax84 p1 build. when all the kobs are all the way up, and i play guitar through it, i get a weird sound as if its cutting out. placing a small vlan .01uf cap in the 2nd gain stage stops this noise, which some say is oscillation.

ive heard 2 terms of what it could be. oscillation and blocking distortion. i had a microphonic preamp tube which was adding to the problem but at least thats fixed.

any ideas.

here is a pic of the messy, uncleaned build. sorry for the large pic

http://temp.corvetteforum.net/c5/dngrzne//dsc00517.jpg

[Bob-I]

I'd doubt if it's blocking distortion. The P1 is a well engineered amp so my bet is on oscillation.

Looking over your picture I see some solder joints that may need cleaning up. There are some with globs of solder that apparently didn't flow well. I'd start with removing the solder with solder wick and reflowing these joints.

Next I'd get a chopstick and while this is happening, move some of the wires around listening for a change in sound. When you hear it you'll have located the source of the oscillation. You can then adjust the lead dress to remove the issue.

Good luck.

[wsaraceni]

the only wire that moving changed the problem was the blue wire going to the el84. since then, we sheilded it with the black wire and still does not fix it. ill try and clean up the solder joints and the board in general.

[Bob-I]

the only wire that moving changed the problem was the blue wire going to the el84. since then, we sheilded it with the black wire and still does not fix it. ill try and clean up the solder joints and the board in general.

It's more on the input jack and pots where I see solder joints that need reflowing.

I can't see the output jacks on the pic, is the jack grounded? If yes it should be grounded close to the jack. You really don't want all the stray voltages running through the amp.

I'm not a huge fan of these star grounding schemes. You did it well but I've had problems with these. For example I'd connect all the filter caps - side together, daisy chain style, then have a single ground all the way to the right of the pic, away from the input jack. I'd then connect the cathodes together and ground them to a point near the input. I'd also daisy chain all the grounds from the pots and input jack together and ground them near the input, possibly the same point. Personally I like to use a piece of 3/64" brass rod as a buss. I hope you can see the bar along the board in this pic.

I might also be tempted to move the cathode connections on the EL84 closer to the tube.

Good luck.

[wsaraceni]

well it seems you totally diagnosed the problem, along with some help from others on the gear page.


resoldered some bad solder joints on the pots in the tone stack. the oscillation went away, but had a really bad hum. louder than the guitar even.

changed the ground to this final shot here and no hum (well not any more than a normal amp would) and no oscillations. 1st amp build. done. now to change it to a high octane design

http://temp.corvetteforum.net/c5/dngrzne//dsc00520.jpg

thanks
bill

[Bob-I]

well it seems you totally diagnosed the problem, along with some help from others on the gear page.


resoldered some bad solder joints on the pots in the tone stack. the oscillation went away, but had a really bad hum. louder than the guitar even.

changed the ground to this final shot here and no hum (well not any more than a normal amp would) and no oscillations. 1st amp build. done. now to change it to a high octane design

http://temp.corvetteforum.net/c5/dngrzne//dsc00520.jpg

thanks
bill

Great news. I've had great success with this buss ground scheme. Glad it worked out for you.