18 Watt gremlins - Need help (with audio clip)

[tjk3052]

I just joined here and would really appreciate some help with a recent build of mine.

- sorry if this is long -

It's based on a design posted on the 18watt forum; basically a plexi preamp (3 12AX7s) and a normal 18 watt PI and PA (2 EL84, cathode bias). I added a post PI master volume and VVR on the power tubes. I built it a few months ago with no problems, but recently I cleaned up some lead dress and when I turned it on again, there was arcing inside of the EZ81 rectifier tube and the amp's breaker tripped.

I checked and measured everything I could think of, didn't see anything wrong, so I removed the EZ81 and put in a couple 1N4007s to get it going again. The voltage after the diodes was higher than the EZ81, but okay, and I turned the VVR down to lower the voltage on the EL84s. When I flipped the standby switch, there was arcing inside of one of the EL84 tubes. I shut it all down, checked everything over, and once again could not find anything wrong. I got correct voltages with no tubes in. I replaced ALL the tubes with new ones and tried again.

It now works, I get correct voltages at every tube pin. I checked the bias on the EL84s. Everything looks and acts like it used to. Except... There is a crazy sizzle that is on top of any note being played. It's there for a while, then goes away before the note is done ringing. It's not present at all amp settings, and strangely enough, when I turn the treble control all the way up OR all the way down it stops - and stops abruptly.

Here is a rough recording of it. At the end, I am turning the treble knob and you can hear it almost acts like an on/off for the sizzle. Any ideas what might be going on?

< LISTEN sizzling sound clip >

[Bruce / Mission Amps]

Assuming your VVR circuit is working correctly... I can't tell you why or where, but it sounds like a classic preamp or phase inverter oscillation to me... probably your "cleaned up wiring" has created a positive feedback to a preamp stage... I'd move some wires around like they use to be just to see if anything changes.. and I mean the frequency it buzzes or the intensity... etc.
As it is now.... that is a BUTT load of gain for couple EL84s!!
Is it the same with the low gain setup as the high gain, or worse with one?

[Chuck H]

Yup. With the treble down the oscillation goes away and with the treble up the oscillation becomes ultrasonic. I've heard it before.

Check to see that your 47pf shunt cap is properly in place on the PI. Check to see that your 8.2k grid stoppers are in place. Check to see that your C17 shunt cap is in place.

It's also possible that the problem always existed, but you couldn't hear it because it was happening full time the way it does with now with the treble at extreme settings. I don't know what you did to "clean up" the lead dress. That is, did you try to make it look neater or did you try to route it in a more functional way to avoid oscillation? The latter is the only important criteria. some gut shots might be helpful.

I might remove the effects loop. It's not a good place for it anyway and it would reduce lead wire length considerably. How and where leads are routed is of paramount importance. As I said, some gut shots could help.

FWIW, WRT the "plexi" type topography, using two 1M volume pots with a channel stack is just too much gain building too quickly. With that much gain in only a couple of triode stages there are lots of opportunities for gremlins. When I do a channel stack to this type of amp I usually have a signal pad and an additional bleeder cap included somewhere.