Strange Trim Knob Issue

Well it is strange to me anyway.
Ok so here is the problem:
I have a basic tube preamp circuit running some transformers in and out (typical setup), two 12ax7 gainstages, normal pad switch, gain knob in between the first and second gain stages, and a trim knob after the second gain stage going directly to the output transformer (10k:500R).

With this setup the preamp has worked fine. However, for some reason, as I was building another one I ran into an issue where the trim knob acts strangely. When all the way up, everything sounds great (using the gain knob to control volume) but when I bring the trim knob down, it very quickly starts to add this bad distortion, almost like the tubes are near cutoff or something.  This bad distortion continues until the trim is all the way down and the audio is gone. Note that the trim does indeed control the volume like normal even while causing the distortion problem.

One other side note, the 12ax7 is obviously a little dark in this configuration so I tried adding a 22n bypass cap on the 1k5 cathode of the second gainstage (first and second gainstages use 100k plate and 1k5 cathode resistors and no negative feedback loops).

Also, the resistance to ground after each coupling cap is in the form of the gain or trim pots (100k). From there the signal goes out to the next gainstage or transformer.

When I disconnect the 22n bypass cap the trim knob acts normal with no distortion.


Any ideas?  I'm using a star ground setup, but maybe it is still a grounding issue?  The trim knob does alter the voltage present on the second gainstage cathode from about 1k7v to 2k8v, if that is relevant.

Thanks!

I'm not a tube guy but perhaps an oscillation?  At high gain the NF is more attenuated so amp is more stable at high gain.

Maybe check lead dress for high impedance paths.

JR

This sounds like a plate loading problem.Can you post a schematic please.

Cheers

Ian

JohnRoberts said:

I'm not a tube guy but perhaps an oscillation?  At high gain the NF is more attenuated so amp is more stable at high gain.

Maybe check lead dress for high impedance paths.

JR

Click to expand...

JohnRoberts said:

I'm not a tube guy but perhaps an oscillation?  At high gain the NF is more attenuated so amp is more stable at high gain.

Maybe check lead dress for high impedance paths.

JR

Click to expand...

Well for not being a tube guy you nailed it. I went back into my rats nest of a prototyping unit and figured out that I, for some reason, left a wire connected to the trim knob input (from the plate coupling cap) hanging out under the turret board going nowhere. I think I wired it and left it there assuming I was going to follow an early layout but when it changed I forgot about the wire and went another direction later from the top. 

Yes I am often careless and dumb.

All that to be said, I clipped the extra wire and the trim knob works great now.

I don't actually understand how this phenomenon is caused by a stray wire but I'm at least happy it is all good now.

Thank you!

Glad it worked out.

My first hifi amp was a small 4 tube Lafayette radio model... and I did build one kit short wave radio (communications receiver) that used several tubes, but by the time I was seriously doing electronic technician work, tubes were pretty much passe for cutting edge design (with the exception of high power radio, and now maybe even them?).

That said my mixer design group shared a common lab space with the guitar amp engineering group so there was cross pollination  and I picked up a little from BS sessions with modern tube guitar amp engineers (not to mention a few studio tube designs done for the AMR division.)

One tidbit I picked up from the guitar amp designers is how much crosstalk goes on inside typical guitar amps and how this can dominate the characteristic sound of some classic amps. For better or worse, tube amps on PCB will be far more repeatable than hand wired amps that could vary quite a bit between builds if the point to point wiring isn't consistent from unit to unit.  Probably lots of amps with identical schematics that sounded different. 

JR

PS. Of course there are many factors affecting tube guitar amp sound, crosstalk is just one.