large voltage drop on 12AX7 issue

[luthierwnc]

Hello everyone,

I'm trying to finish a preamp project with one more mystery to solve. This is basically a Dumble-based preamp with a clean tube feeding either a normal overdrive, an HRM overdrive or the normal into the HRM. The clean and the normal work as they should. The HRM preamp is behaving very strangely and has me stumped. The first half of the tube is fine. A sine wave going through is slightly clipped but you would expect that after three gain stages. It looks OK on the grid of the second half as well but going into the second triode tone stack the signal is weak and clips very sharply on both ends of the waveform. The tone is weak and nasal as well. BTW, I've taken lots of pictures of the scope and am glad to throw them up if it clarifies the discussion.

For more detail, the second triode has about 220 VDC on the plate when there is no signal going through the HRM circuit. It drops to about 160 VDC when taking a signal. I suspected the power supply but the first half of the tube only sees about an 8 volt drop so I think the rail is stiff enough. Quiescent, the cathode voltage is 1.9 VDC. Engaged it is 3.5 volts. All the relays are working fine. The caps are new and aren't leaking. I'm using RN metal film resistors and they are right on spec. I've tried it with three known-good tubes and the results all have the same percentage drop.

So I guess my questions could be put in two parts; what would cause a 60 volt drop on a preamp tube that isn't being used as a cathode follower and should I be looking beyond the tone-stack? Thanks for any ideas. BTW, this is the same gizmo of the footswitch question I posted last week. Skip

[g1]

I'm not sure how well your meter could measure the DC while there is AC riding on it. As the signal level would probably be much higher at the second stage, maybe that is why it is measuring differently. Or are you looking at the DC level on a scope?

[Mick Bailey]

I'd disconnect the output off the wiper of the treble pot and then see if everything then measures ok. If not, you may have some parasitic HF oscillation, especially if the tone is being affected so drastically. You won't see this ordinarily if you're scoping for audio - you need to go looking for it.

[luthierwnc]

Thanks gents. Turns out is was the supply. The parallel overdrive rails were supposed to create more-or-less equal voltages but didn't. I rewired it in series with 4k7, 2k7 and 2k2 resistors to B+4, B+5 and B+6 respectively and it holds up much better. Now that I have the volts right, I need to tweak the entrance from the normal OD to the HRM. I'd thought (and this is uncharted territory here) that I'd need to damp the signal from the one to the other with the cap/trimmer circuit. Turns out I need to increase the signal so this afternoon's project will be to rewire the relay board so the 500k trimmer reduces the entrance resistance (chart attached). The tail resistor on the HRM stack bypass is also much too large so I'm reducing that to 68k.

Again, thanks for looking. sh