Conrad Johnson PV10 problem

[Kayjay73]

Hello
I'm a New Zealand Electronics Tech presently working on a Conrad Johnson PV10 Preamp.
The Original Problems were 1; an Open Circuit HT Supply Resistor, and 2; an Open Circuit 12v Tube Heater Supply regulator device.
After replacement of these, the only problem now remaining is a very slow oscillation - about 1 cycle per 5 seconds - of the Plate Voltages in all of the Front End amplifier stages. If You remove any tube from the front end, the problem vanishes. Both Line Amp Stages are perfectly stable.
I had replaced a couple of the Coupling Capacitors (with Good Quality ones), as the originals showed some signs of very slight DC leakage.
This problem was there before and after doing this. I have tried replacing some or all of the tubes in the Front End, but the problem persists.
The positive supply voltage shown as 'A' on the Conrad Johnson schematic is +317 Volts.
I have closely measured all of the Resistor Values, and all look to be correct.
Has anyone encountered this problem before?
Thanks, and Kindest Regards
Kayjay73

[admin]

Not sure what to recommend but does this influence the sound in any way, ie hum, distortion, etc?

[AnotherJohnson]

You’ll never hear .2Hz ... but it shouldn’t be there.

[admin]

This is beyond my expertise. However, any chance it could be one of the caps? You mentioned you replaced "a couple" of them. Could it be a cap that is discharging improperly?

[AnotherJohnson]

It’s so close to DC, that filter caps, which pass DC, would seem like low hanging fruit. But why are the line stages stable? And why does removal of any front end tube stabilize it?

[Kayjay73]

Hello
Thanks to everybody for the help.
The Capacitors I replaced were the 4 coupling ones marked as C12 and C14 on the PV10 Schematic. This problem was there before (the reason I replaced them), and now after. If I didn't know better, it almost seems to be a Very Low Frequency Oscillation. I have not so far run any Audio Signal through it, as I just wanted to make sure all was making sense. I did try lowering the Main Positive Supply voltage to about half, and the problem ceased to exist, which made me ask the question about the recommended HT Voltage. I had suspected the HT Voltage Regulator, but as no-one had made a comment about the present +317v High Voltage, I assumed that this was correct/okay. In all my work over the years on Vacuum Tube powered gear, I have never come across a problem like this, and that is why I joined this forum in the hope of finding an answer. I will try looking at the ESRs of the Main HT Filter Capacitors, and see what I can find --
Thanks to all, and Kindest Regards

[goonybird]

I had one of the output valves arc. This resulted in lower audio combined with hum and motorboating
One of teh caps appeared faulty, but out of circuit measured ok. I think this was a result of the trannies going down.
I found the two HT transistors had gone.
I think I replaced Q2 Q3 and the diodes D21 D22
For what the parts cost is is it worth rebuilding the HT supply section?

[roberto]

Hola. I think that still have a faulty circuit joint somewhere. A bad solder joint.
Also leaking cap can produce that oscillation.

Please keep us posted.

Happy listening!

[bpasok]

I think you may be right as far as the voltage regulation... I think that with the zener string, you should have approx. 340v output. make sure your input is adequately high to support this (not too much loss from rectifiers) and possibly a shorted diode.... motorboating at these low frequencies is often a problem with being able to manage the load.