I have an odd distortion problem. Please help if you can. Here's the scoop.
The amp plays very well up to about 8 on the volumes. It just begins to distort as expected (nicely distort) at the 6-7 volume settings with moderate strumming. If I strum hard at volumes set around 8 it will make an inconsistent "popping" type of distortion. There is also sometimes a very brief chirp or two with the pops. The pops are irregular and not frequent. It gets worse with volume settings above 8, but still requires moderate to heavy strumming to occur.
Please read the following if you want to reply. This is important history and data.
1) The schematic and layout are based on the current data in the download area for the 36 watter. I applied all of Richie's tweaks, except the 91K resistor on pin 1 of V2.
2) I am also using a 5U4GB Electro Harmonetics for the rectifier tube. (See history below.)
3) See my voltages in the pic on this post. Look at V3. Are the voltages way off (too far off) for pins 2, 3, 7 & 8?
4) I am using 150 ohm resistors on the cathodes of the output tubes.
History:
When I first tested the amp I had similar issues as described above at lower volumes around 5 and 6. I re-soldered all the wiring in my speaker cabinet because it was suspect. I also replaced the jack and re-wired the outputs in the amp. This eliminated the issues at volumes below 8, and I did not test above 8, so I thought I had eliminated all the issues. Afterwards I started tweaking carefully. I applied Richie's tweaks, replaced 150 series mallory caps with polypropylenes, found 68K input resistors were making some noise at higher volumes (with no signal applied) and replaced them, using chop-sticks moved a few wires to get even cleaner. All these tweaks were to my liking and generally reduced noise so that the amp is almost totally quiet up to 8 on the volumes. It was initially that way up to 7. So the changes were an improvement in tones and reduced noise. Then I changed the rectifier tube from a GZ34 to the 5U4GB. This I also liked. It changed the tone response and I found it reduced voltages slightly as well. Being very happy I decided to see what she would do. So I cranked the volumes up to 9, and found the "poping, chirping" problem again. However it is not as severe as before resoldering the speaker cabinet, etc.
Here are some more characteristics to be aware of.
1) It happens on both channels.
2) Feedback on definitely reduces the issue.
3) It does not happen with light strumming (but I have not cranked to 10 yet).
4) It is worse using the GZ34, (which also increases voltages).
5) It never happens at volumes 7 and below, no matter how hard I strum. A humbucker did not seem to make it any worse.
6) Lower cords, especially the E and A power cords make it worse.
7) I'm not seeing any issues with red tubes, arcing, etc. They seem to be fine and my output tube voltages indicate moderate idle tube current, well within specs for the EL84 JJs I am using.
Reducing volume on the guitar eliminates the issue.9) I've checked tube seating and they seem to be fine.
10) I was thinking loose connection, but can't find one.
Here are some questions.
1) Does this sound like "Blocking Distortion". I read up on that where they say it's a "farting" sound. It dosen't sound like a fart to me and the only gas in the room came from either me or the dog, I can't tell which!
2) Could it be my V3 voltages?
3) What about the volatges on the output tubes? Aren't those high? JJ specs say 300 max, but other posts say it's OK to go over 300v on them.
4) Has anyone ever seen this happen, caused by an impedance selector switch?
5) Could it it be a bad pot, like maybe the main volume?
The amp is certaintly playable, and very nice around 7. However, I'd like it to play properly all the way up to full volumes. I'm stumped.
Thanks for any ideas...
) is to get all of the bugs out of it, and then start making mods/tweaks to it one at a time. I typically try EVERYTHING out like master vol, treb, etc. I max all of them and try a bunch of different settings just to see what I get. That's just me.. others might call me crazy. lol (but that's what guitars and that dang devil music will do to ya! lol)
Here's what I did and I am in sonic bliss.
My trouble is I drilled the origional chassis so that the tubes are rotated according to the heat disipation theory someone told me. And I placed the OT as far away from the PT as reasonably possible according to more info that may not have been the best. I probably should have positioned the OT so that the speaker wires are clear of the EL84 inputs, and positioned the sockets as shown in the layout. 