Yet another bias circuit question

[Shermock]

Hello everyone!

What a great group! I have learned so much from this forum I can't even tell ya--everyone's work looks so good, and I'm sure sounds even better!

I have been toying with amps, circuits, and repairs for about 10 years but I've never tackled anything quite like a Twreck. So I'm relatively new to this particular thang, but I feel I've got a bit of experience to go on. That said:

I have an A1a (well, not so A1a anymore) that I did the suggested bias circuit change on, namely replacing the 220K resistor with 100k (actually now it's 68K) to give me more "room on the dial," along with changing the tap from the pot and the resistor to ground. I can get from -5 to -52 volts.

However, my problem is this: as I dial in the bias for the OP tubes, at one point along the pot's travel the idle current wil jump from relatively small, like 19 or 20 mA, to 65-70 (the reading for one tube, not both). This happens suddenly, and if I tweak the pot just right I can get it to jump back and forth between too little and too much without touching the pot. The pot is fine, and reads as it should through the entire range of motion. It happens to be 50K, which is what I had to work with. I would like to get a nicer pot but honestly it wasn't bothering me or doing anything wierd.

This same problem presented itself about 2 weeks ago, and I found a broken 82K plate load resistor in the PI, which upon replacement seemed to alleviate it. Now I can't find any broken leads or such that would cause this phenomenon.

The amp was working very well this morning, and then this problem started after I powered down, tried a different cap idea, and powered back on. I have checked all leads and wires, hoping to find something else broken or shorting but can't find anything.

Is there some tube "thing" going on that I'm just not educated enough to understand, and therefore find, or...? Has anyone else had this type of problem?

Hope someone can help--all the best!

Shermock

[RossH]

However, my problem is this: as I dial in the bias for the OP tubes, at one point along the pot's travel the idle current wil jump from relatively small, like 19 or 20 mA, to 65-70 (the reading for one tube, not both). This happens suddenly, and if I tweak the pot just right I can get it to jump back and forth between too little and too much without touching the pot.

This says "bad pot" to me. Dirty or something. It makes me think of the treble pot I had where it measured fine with a meter (but then again I didn't know where the bad spot was) but when I had it in the amp it would cut out and get loud/soft in one particular spot.

~ Ross

[Funkalicousgroove]

Could also be a funky tube, if one is shorting internally it could draw massive current in spurts until it goes, have you tried a different pair? You can also look at them in the dark and see if there are any teeny lightning storms, but if that is the issue turn it off QUICK!

[Shermock]

Hey thanks for the replies....

I have tried different tubes to no avail. I have tried replacing the PI caps just in case one or both were leaking, but nada.

The idle current, when measured on the cathode resistor method, causes a slow putt-putt right around 27 mA; it won't happen at lower or higher current settings. The tubes will still jump, though, from drawing no current to ~65 mA without me touching the bias at all.

If I turn the amp on, set the idle current at ~15mA and start playing, the current will jump to ~50 mA. Then I dial it back toward 30-ish and the putt-putt starts happening.

I have done the cursory check of the bias circuit, and have tried a couple other circuits to make sure the supply is accurate and steady. It has all checked out.

So now I am looking elsewhere for the problem- my difficulty lies in that I'm no EE, I'm a musician who happens to build amps for fun, so I don't necessarily have the technical tube theory under my belt to draw upon. I know I'm missing something!

Thanks again for the help!

Shermock

p.s. Oh yeah--the bias voltage remains steady when the tubes start drawing more current than they should. I measured -42 volts constant when the tubes were set at .001 mA and when they would jump to 65 mA. That's why I'm thinking it has to be something else.

[Shermock]

Ok, ok, I'll keep posting what I find....Most perplexing.

The bias voltage remains constant, but the idle current changes when:

The mid control is up
The presence is up
And when I turn up the volume pot past 9 o'clock

...with no guitar plugged in.

The reading will jump from 13 mA, which is the lowest setting I can have right now, to 80 mA when I turn up the volume past a certain point.

The reading will stay at 13 mA if the presence and mids are turned all the way down. Then I can turn the volume pot up and no increase in idle current.

Bass and treble pots don't affect this.

I also noticed that certain components on the board are more microphonic than others...I'm assuming this is a known phenomenon?

Thanks for any and all help!

Shermock

[Dai H.]

maybe you have an oscillation. The bias is a static setting (no signal).

[Shermock]

Y'know, that could be....I'll check that--it's gotta be ultrasonic, 'cuz I'm not hearing anything, but it very well could be....Thanks!

[Shermock]

OK, so it was an oscillation that reacted with the mid setting most of all--quite interesting. I finally had to take off the bright cap, but that's ok--it cleared up the problem by doing that.

I'm not sure the oscillation has gone away completely. I don't have a scope so I'm trying other things to see if the amp starts drawing current just from turning knobs with no signal input. My suspicion originates from hearing a high-frequency buzz on top of the distortion tone, especially on single notes.

Thinking out loud, I suppose a change of caps to perhaps "darker" ones that don't pass as much high end would be a good experiment.

I've never worked on an amp project as touchy as this! I certainly am feeling "schooled!"

Cheers

Shermock