Really buzzy preamp sound. Ideas?

[ECV02]

Hi all,
I've been building/tweaking my first real tube amp. I've pretty much got the power section nailed (6BM8's in push pull), but this preamp has had me tearing my hair out for days on end now. I took some inspiration from the Fender Champ, I'll attach a schem below. It sounds nice, rich and warm with the gain turned down, but the instant I start cranking it it sounds super buzzy and nasty. Reminds me of the fuzz used in Muse's track Hysteria. Then, all of a sudden, if I push it even harder with an overdrive, it sounds absolutely killer again. I just can't dial in that nice crunch sound.


By the way, I'm using a JJ 12AX7/ECC83. S1 is an optional overdrive switch (amp sounds fine with it off, but it stays really clean. Lookin' for some crunch). Pots are both 500k audio taper.

Thanks a bunch,
E

[sluckey]

Preamp looks fine. Can we see the schematic for the entire amp?

[tubenit]

Your preamp uses 100uf cathode caps.  I rarely use over 5uf.   Fender uses 22uf or 25uf. 

I would try a 10uf in the first gain stage and 5uf in the second gain stage and see how you like that?   You can lift the ground end of the 100uf caps and then safely with insulated alligator clipped wires connect a cathode cap that is MUCH smaller in value.

You could also try a 12DW7 or reverse 12DW7.

With respect, Tubenit

[ECV02]

Hey, sorry this took so long. My house lost power all of a sudden, along with my PC.
I've got the full amp schem here. The tone stack isn't designed yet, so I'm just shorting across it (there will be a switch to bypass it anyways). Also, please treat those final output "triodes" as pentodes with 2k2 screen grid resistors fed from 250v. My stupid sim won't let me draw in pentodes.

Tubenit, I just switched those cathode caps (10u and 5u, as you suggested) and the buzzing is better, but not much. I'm starting to suspect the PI or maybe even the output xformer (HMD125D).


Thanks,
E

[tubenit]

I'd change the .1 cap going into the phase invertor to .0047 or .01          I'd change the 500k resistors to ground to 220k.   There may be some useful info in this thread (see schematic on Reply #21):

http://el34world.com/Forum/index.php?topic=23013.0


With respect,  Tubenit