No output

[Phil_S]

I'm late coming into this thread and not as expert as many of the others. What makes you think the 90v drop is a problem? Yes, I understand you want more plate voltage, but there is no way to get into the 400's when you are starting at 390. The AA864 schematic I'm looking at says Va=420.

What is the PT secondary rated for? Or is it unknown? A soft PT could easily drop this kind of voltage. Did you measure and record the secondary DCR before you installed the OT? If you know the DCR, you can figure the correct current draw by simply applying Ohm's Law to the voltage readings; compare the input at the CT to each plate. V(drop across the OT secondary)=IR. If you know R, you can solve for I. Worth knowing when you are working with a problem, as it is a direct reading, but I diverge here a bit.

You say 30mA at pin 8. I assume you mean across a 1 ohm resistor you have 30mV. There is an inverse relationship between plate current and plate voltage. I think your tubes may be running cold. Try warming up the bias voltage to -25-ish. You're plate voltage will drop even more, but see how it sounds. Before you get all stretched out over what you hear, bias the tubes closer to 44mA each. You might be surprised what happens or maybe nothing will happen. I figure, this adjustment is a freebie and needs doing anyway.

I didn't look at your filament string very carefully when the pictures were open, but I thought I saw an artificial CT at the pilot light. I'd lift this and ground the filament center tap (you taped it off, that is what it is, isn't it?). If you have hum, this might just do something for it.

Just my 2 cents. Good luck with this project.

[martin manning]

HH, your voltages look good but low compared to the schematic. Like Phil says, is the PT up to the spec? What is the AC voltage on the secondary with no load (standby switch open)?

Re the ground scheme, you could try splitting- off the screen filter cap ground and take it over to where the power tube cathodes are grounded.

In your picture of the cap board, I can't see the other 220k balance resistor for the pair of 100uF 350V caps... is it there somewhere?

[ToneMerc]

I'm late coming into this thread and not as expert as many of the others. What makes you think the 90v drop is a problem? Yes, I understand you want more plate voltage, but there is no way to get into the 400's when you are starting at 390. The AA864 schematic I'm looking at says Va=420.

What is the PT secondary rated for? Or is it unknown?

I agree and so the question is, what were the voltages before he rebuilt it?

TM

[C Moore]

I will measure the unloaded secondary of the PT.
Never saw this amp running. It had a bad tube socket and several other issues associated with an amp of this age.....plus it had been F'd with a few times. But I still could have gotten some of those numbers. A mistake I will not make again.
That other 220k resistor is there. That first filter cap is leaning over the top of it. Again, just out of a quest for knowledge, what might be the outcome if it were not there.?
Will get back with the unloaded voltage ASAP.
Thank You

[martin manning]

If the other balance resistor were missing, the the voltages across each cap could be somewhat different, that's all. I just couldn't see it, and wondered where it was.

Can you get the DC voltage at the standby with the standby open too? That should be the highest DC voltage the transformer can put out, and it should be ~1.41x the AC since the only load is the balance resistors and the cap ESR, plus a tiny bit of bias supply current.

[rdjones]

If there's an oscillation (caused by lead dress or positive feedback) at an ultrasonic frequency, you wouldn't hear it (obviously).
But it would cause additional current draw, and therefore, a voltage drop.
The reason I don't suspect this is the low cathode current.

Looks like a little over 20% voltage drop, or put another way, worse than 20% regulation.
That's a fairly mediocre regulation number but not unheard of.

rd

[C Moore]

OK, All tubes are pulled.

SB switch open.
PT secondary 680 VAC
Junction of SB switch, rectifier, and first 2 filter caps 312 VDC.
Other side of SB switch is 0 Volts.

SB switch closed.
PT secondary 680 VAC.
Both sides of SB switch are 465 VDC.

[martin manning]

It looks like something is wrong with the wiring between the PT secondary and the standby switch. With the standby open you should be seeing the peak of the secondary AC voltage at the top of the reservoir/input to the standby switch. Since you have 340VAC RMS on the each half of the PT secondary, you should be seing ~480 VDC there. When you close the standby (with no tubes in, so no additional load), you are almost getting there, I think because the other filters are connected.

[Firestorm]

Discharge the caps and then check the wiring of your "totem pole" filter. In particular, make sure you have continuity between the cathode of the "top" cap and the anode of the "bottom" cap (not 220K between them). And make sure the main ground is good. The fact that your voltages go up when you connect the other caps (which are grounded separately) makes me think the problem is with the main filter.

[C Moore]

There is ONE of my problems fixed....
You guys are right. It was with the series caps. Whenever I get lazy, it comes back to haunt me. I tore out everything from that filter cap board EXCEPT for the jumper for the "totem pole" connection. The solid core wore was broken under the board. So there was no series connection between those two caps.
The hum problem is better, but still there. I will have to try a few different things to try to improve that.
The bass channel that I "modded" has horrible sounding, low, erratic, output. I will have to come back in another post for that.
Thank You So Much For The Help