Koch Twintone I - issue in power section

[Tone Meister]

My Koch Twintone I (SN# H0674) produces a hideous noise from the speaker output. I can pull the phase inverter tube and the noise stops. It sounds a lot like DC on the output, but I only measure 2.5 mVDC at the speaker jack.

I took some voltages at various places and now have the amp loaded with an 8-ohm dummy load to protect my speaker. The correct schematic is attached.

Here are some measurements with only V4 phase inverter and V5, V6 power tubes installed:

V4 phase inverter
1 285 VDC
2 0.002 VDC
3 2.4 VDC
4 6.5 VAC
5 6.4 VAC
6 292 VDC
7 0.002 VDC
8 2.4 VDC
9 0.104 VAC

V5 power tube
1 0.038 VDC
2 3.24 VAC
3 436 VDC
4 434 VDC
5 -38.5 VDC
6 0.49 VDC & 0.321 VAC
7 3.35 VAC
8 0.0039 VDC

V6 power tube
1 0.038 VDC
2 3.24 VAC
3 435 VDC
4 433 VDC
5 -38.5 VDC
6 0.14 VDC & 0.6 VAC
7 3.17 VAC
8 0.04 VDC

IC1
1 0.00 VDC
2 0.00 VDC
3 0.00 VDC
4 -13.3 VDC
5 0.00 VDC
6 0.00 VDC
7 0.00 VDC
8 13.25 VDC

IC2
1 -14.15 VDC
2 0.00 VDC
3 0.002 VDC
4 -14.4 VDC
5 -14.25 VDC
6 0.001 VDC
7 14.4 VDC
8 0.002VDC

IC3
1 -14.16 VDC
2 -0.002 VDC
3 0.003 VDC
4 -14.19 VDC
5 -14.15 VDC
6 -0.00 VDC
7 14.15 VDC
8 0.002 VDC

[g1]

You don't have to worry about DC on the speaker with tube amps, the output transformer prevents any DC from getting to the speaker (unless there is a massive OT failure).
So what ever you are hearing is not DC.
To isolate the problem, with all tubes installed, plug a cord into the FX return jack. If the noise stops, it is coming from somewhere before the FX return jack (preamp), if the noise is still there it is coming from after the FX return (power amp).
You seem to be off with the pin numbering on the IC readings, IC1 should have +Volts on pin 8, not pin 5: IC's 2&3 should have +volts on pin7, not pin 6. Recheck your numbering.

[Tone Meister]

Yep, I did have the right side of the IC pins wrong from bottom to top - the first post has been corrected. The noise is still there with all tubes installed and a cable patched into the FX return jack.

[g1]

That patch cable is plugged into the FX return and the other end is left disconnected, yes?
In this case the problem is after the return jack. As you also got rid of the noise by removing the phase splitter, the power tubes should be ok.
Have you tried another tube in the phase splitter spot? Is the tube socket mounted on the circuit board?

[Tone Meister]

Yes, the other end of the patch cable is disconnected. Yes, I tried a couple of known good preamp tubes in the PI slot. Yes, the socket is mounted to the circuit board.

[g1]

Try connecting a small cap like a .1 or .047 or similar between ground and pin7 of the phase splitter. Does it kill the noise or not?

[Tone Meister]

Try connecting a small cap like a .1 or .047 or similar between ground and pin7 of the phase splitter. Does it kill the noise or not?

It kills it for the most part, yes. But there is still a hum/buzz, but not nearly as scary as it was before I used the cap to shunt pin 7 of the phase inverter.

That noise was/is so loud that it seems imminent that it will destroy my speaker in very short order. Almost like major out-of-control oscillation ...

[g1]

So the problem is from before the phase splitter, but after the FX return jack. So you can concentrate on the FX loop schematic.
I'll assume you had the reverb pot turned down so that leaves IC1B and relay 2, and all associated components and solder connections.
When that master P11 is engaged by relay 2, does it control the noise?

[Tone Meister]

...I'll assume you had the reverb pot turned down so that leaves IC1B and relay 2, and all associated components and solder connections.
When that master P11 is engaged by relay 2, does it control the noise?

All potentiometers on the amp are turned down. When P11 is turned all the way down the noise is a high-pitched whistle. When P11 is turned up any at all the noise gets much more aggressive and "dangerous" sounding.

P11 pokes through the back panel and is labeled "Rhythm Volume" on the back panel. It is actually the reverse of a boost, more like a master volume. It allows me to preset attenuation that can be "let up" from the footswitch to boost overall volume when needed. So by playing the amp at a preset attenuated level, the amp is effectively "boosted" when the "Solo/Rhythm switch is engaged.

[Tone Meister]

The noise is right at 4.2 kHz, and it makes no difference whether I engage relay 2 with the footswitch or not. However, if I turn up P11 then the noise gets much more aggressive and I'm scared my speaker's voice coil will melt.

[g1]

Well, if P11 is having effect on the noise that pretty much narrows it down to IC1. I would be hitting it with freeze spray or just replacing it.
But just to be sure it's the IC, try your cap to ground at pin 3 and then pin 1. Pin3 probably won't have any effect, pin1 should reduce the noise substantially. If so, replace IC1. If grounding pin3 with the cap kills the noise, the IC should be ok.

[Tone Meister]

Grounding pin 1 and pin 3 with the cap has no effect. I just swapped in another OP249 and nothing changed

[g1]

Grounding pin1 of IC1 no effect, grounding pin7 of phase splitter has major effect, the only things between are Relay 2 and R60. and associated connections/solder joints. If you have good continuity for the connections, try bypassing the relay with a jumper.

[g1]

Sorry, I had forgot that P11 was affecting the noise, yet grounding pin1 of IC1 made no difference. There may be a problem with connections between the IC and the pot, or the pot itself.
But if that were the case, when the relay bypasses the pot, the problem would go away, so I'm a bit confused about that.

[Tone Meister]

I can pull IC1 and the noise remains the same. The ICs are socketed, which is helping me keep up with you.