Very strange problem

[glasman]

Based on your 4X20 uf cap. I would try isolating the screens and find a separate ground close to the output tubes.

Regards,
Gary

[Bob-I]

Based on your 4X20 uf cap. I would try isolating the screens and find a separate ground close to the output tubes.

Regards,
Gary

I think you're onto something here. I isolated the screens and the bleed through was reduced dramatically. I'm thinking that the 4x20uF isn't isolating properly. I'll try 4 22uF caps when I get more time. It just might be a combination of stages bleeding.

[Bob-I]

Damn.... I put in 4 - 22uF caps. I still have bleeding. I returned the 4x20uF cap and it's about the same. I'm completley puzzled.

[BobW]

Bob, now that you have discrete caps, try connecting each respective preamp filter cap grounds B+4 and B+5 directly to their respective cathode grounds. Then run individual ground wires from each stage ground to the common chassis ground. The purpose of this is to provide a lower impedance ground from each filter cap to their stages. Allowing a lower impedance return through a different stage can cause interaction of return currents. This may be the problem you are having. hth BobW

[Bob-I]

Bob, now that you have discrete caps, try connecting each respective preamp filter cap grounds B+4 and B+5 directly to their respective cathode grounds. Then run individual ground wires from each stage ground to the common chassis ground. The purpose of this is to provide a lower impedance ground from each filter cap to their stages. Allowing a lower impedance return through a different stage can cause interaction of return currents. This may be the problem you are having. hth BobW

Thx, but I really can't do that with this layout. All the cathodes go to a common turret.

I'm at a point where I'm gonna hafta accept this little bit of bleed. I have other things to troubleshoot, like an oscillation when everything is dimed.

[BobW]

Bob, Chances are you have already tried these, but since you don't have a lot of real estate to move components or wiring, around in a princeton chassis, maybe these will help?
shielded cable
Snubbers
~250pf @ grid to act as a LP filter?

[Bob-I]

Bob, Chances are you have already tried these, but since you don't have a lot of real estate to move components or wiring, around in a princeton chassis, maybe these will help?
shielded cable
Snubbers

I have some shielded cable to combat some HF oscillation, there are snubbers on OD1 and OD2.

~250pf @ grid to act as a LP filter?

Which grids, and what's an LP filter?

[BobW]

Bob, Have been TDY till now, sorry about the late reply. A cap from grid to ground acts as a Low Pass filter with the grid resistor.
You can calculate at what frequency the attenuation begins to occurs, if you know at what frequency you want to start the attenuation using the formula F(Hz) = 1 / (6.28 * Rg * C). It just as easy to try a few values. I initially suggested starting with 250pF, but a 500pF may be better look at the chart. The attenuation frequency is at the 3 dB point and this assumes a 68k grid. go smaller if it cuts the highs too much, go larger if it's not enough. I'd start at OD2 first, then try it at OD1. I like to use this circuit sometimes rather than a snubber. hth

[Bob-I]

Thx Bob, that's a great explaination. I understand the math and the theory, now I'll have to put it into practice. I'll let you know next weekend.

As far as the bleed through, I've got it minimzed by re-dressing some leads by the master volume. It's at an acceptable level now, but I feel that it should be eleminated completley.

[tonelab2]

I had a similar experience with clean channel bleeding in overdrive channel. Only noticed it at very low volumes with the OD channel almost turned completely off and CL gain above 6-7. I didn't know how to fix, replaced relay, and redressed wires around relay and Master ( MV is under chassis between PI and output tubes on my build). Fine now.