Champ pulsing DC

[goldenGeek]

So I tried 470R stoppers, that didnt help. But now I changed the bias resistor to 470R and pulled one 6v6 out. Thing is that the oscillation is harder to acheive, but the other effects are more clear now. That is if I strum the guitar hard it farts out and the signar gets really low and distorted. Now it stays low for a long time even after turning down the volume, maybe like 10-20 seconds. Then its back to normal volume. But the thin I noticed is that the voltages is not dropping, its raising. I'm running with 4007s at the moment to rule out a bad 5y3, and theres for example about 350v on the plates before the incidents happen. When the volume drops and the distorting hits it raises to over 390v, and alters between when the pulsing is present. Pin 8 on 6v6 measures about 38V, but starts to DROP lower when things happen. Pin 5 is idle at a couple of volts, but starts to pull negative Volts whan things happen. I guess I should rebuild this whole mess.

I'm temped to use this whole thing as an anchor.

Anyway, I do have another OT I could use, thats a 125ESE which should be able to take more beating than the 125CSE.

[martin manning]

Pin 5 on the 6V6 should be at zero volts, which suggests the 0.022 coupling cap connected to V1b's plate is leaking.

[goldenGeek]

Pin 5 on the 6V6 should be at zero volts, which suggests the 0.022 coupling cap connected to V1b's plate is leaking.

I'll check, I think that's the last component I havent changed (plus the OT). I really hope that's the solution I'll post back when I have replaced the cap, I think I'll replace the one on V1a as well with a known good working cap, just to be sure.

[Firestorm]

Does the tone control work? If it's miswired, two 12AX7s in series will fart out. Gain of roughly 750. Too much.

[goldenGeek]

Does the tone control work? If it's miswired, two 12AX7s in series will fart out. Gain of roughly 750. Too much.

Checked the tone control, it seems to be wired right and working ok. I also replaced the coupling caps on V1a and V1b. No difference. I'm starting to think that the OT is my next target for testing. And there's also the possibility to rewire the whole damn thing. There has to be a flaw somewhere causing these problems.

One last note - when I hit the pre amp really hard for some seconds it farted completely out, hum and everything was gone - does that indicate anything?

[didit]

Have you tried unwiring NFB from V1b? Ensure the disconnected bit is well away from everything.

[Firestorm]

Does the tone control work? If it's miswired, two 12AX7s in series will fart out. Gain of roughly 750. Too much.

Checked the tone control, it seems to be wired right and working ok. I also replaced the coupling caps on V1a and V1b. No difference. I'm starting to think that the OT is my next target for testing. And there's also the possibility to rewire the whole damn thing. There has to be a flaw somewhere causing these problems.

One last note - when I hit the pre amp really hard for some seconds it farted completely out, hum and everything was gone - does that indicate anything?

Too much gain. Lead dress is critical in these. You have to redo the wires.

[goldenGeek]

Does the tone control work? If it's miswired, two 12AX7s in series will fart out. Gain of roughly 750. Too much.

Checked the tone control, it seems to be wired right and working ok. I also replaced the coupling caps on V1a and V1b. No difference. I'm starting to think that the OT is my next target for testing. And there's also the possibility to rewire the whole damn thing. There has to be a flaw somewhere causing these problems.

One last note - when I hit the pre amp really hard for some seconds it farted completely out, hum and everything was gone - does that indicate anything?

Too much gain. Lead dress is critical in these. You have to redo the wires.

So maybe the answer is to insert a, lets say, 470k to ground in between gain stages to tame the gain? Or maybe I should convert to a 5e3-type pre amp - my other SE-amp has a 5e3 (if I recall) pre amp and sounds great...

[martin manning]

One last note - when I hit the pre amp really hard for some seconds it farted completely out, hum and everything was gone - does that indicate anything?

I think this is a bias shift. Are you still getting +2V on the power tube grids? That should be a few mV at most.

[goldenGeek]

One last note - when I hit the pre amp really hard for some seconds it farted completely out, hum and everything was gone - does that indicate anything?

I think this is a bias shift. Are you still getting +2V on the power tube grids? That should be a few mV at most.

I didn't measure the last time I fired it up, but I will tomorrow morning. But I would guess that theres still a few volts + on the grids. If thats the case, what would you suggest I try?

[martin manning]

If there is a positive voltage there find out why. Are you using all new parts and board materials? There has to be some leakage path.

[Firestorm]

Got to be a lead dress problem. Pay attention to Martin. He's way smart.

[goldenGeek]

Yes, there has to be some serious leakage. I tried the other OT, no difference. However, I put the original OT back and measured some more. When it totally farts out its dead silent. Then I measure 6v6 pin 5, its pulsing between (about) +25V and -25V (AC???) the moment I put the multimeter on pin 5 the sound returns. Is that an indicator that something loses contact with ground but get pushed back when "grounded" through the meter? Anyway, Ill rebuild another board (I didn't bulid the board myself), and I'll redo all the wiring AND grounding.

[JazzGuitarGimp]

Do you have a solid ground reference at the bottom of the master volume pot?

[martin manning]

I agree make sure that the master pot is grounded. Does this problem occur when that pot is turned down (~12 o'clock)? If not that confirms that 1M to ground is too much. I would make that pot a 250k at most. You can try putting a 220k-270k across the outer lugs to test that theory.