Last voltage check (getting there) [Solved]

[Suger]

Hello, me again. We're now at the last stage. All the resistance checks went smoothly, but the voltage checks are (consistently) too low: At least, there is symmetry between both sides, so there is some consistency. Here are the measurements:

IA 94V
IB 91V
OA 60V
OB 60V
OC 20V
OD 20V
OKA 20 V
OKB 20 V
OKC -15mv
OKD -5 mv

One thing to note: in a previous step, as some will know, I plugged a pair of 220K/1W instead of 220ohms. I removed the 220K, but as they were not in a good shape, I decided to replace them. I couldn't find 220K/1W, so I put 220K/2W, but, if my understanding is correct, this is only a question of heat dissipation (and size), so it should not affect the voltage.

Any hint as to where to look would be appreciated.

[Paul Birkeland]

Remove the blue wires from IA and IB on the front C4S board and recheck the regulator board above the D socket.

[Suger]

I'm sorry, I feel a bit daft, but what do you mean by "recheck the regulator board"? Check that I get the correct voltage from the first test?

[Paul Birkeland]

The regulator board above the D socket is the regulator board above the D socket where the 12AU7 lives. You should get about 220V on the OA/OB and Breg terminals, and something more than 2.5V and less than 20V on each Kreg terminal.

With the rest of your amp being fed from that regulated power supply, that regulated voltage is being pulled down, so the first step is to double check that the high voltage regulated power supply is still functional.

[Suger]

Oh right, makes perfect sense. Well, I did that, and I get a weird behaviour, usually a spike when I touch with the probe, but then it goes quickly down to 0. I'm going to try again, because it feels like I'm doing something wrong.

Quick question: I've only put the 12AU7 in, should I put all the lamps?

[Paul Birkeland]

You can leave all the tubes in.   Also be sure the meter is set to DC volts and not AC volts.

[Suger]

Call me stupid, but I'd somehow not done a correct connection to the ground tab. Anyhow, here are the measurements:
OA 316
OB 216
BREG A 316
BREG B 216
KREG B 2.60V
KREG A 1.36V

So if I understand correctly, the B side is fine, but the A side isn't.

[Paul Birkeland]

You're back to where you were in your first post with a regulator board that isn't working properly.  I have seen this exact situation when the Kreg wires going down to the socket were hooked up backwards (or the red wires leaving the board and going to the D socket could also be switched), so you'll need to double check everything there.  I would not hook the blue wires back up that feed the front C4S board until the board above the D socket is fully debugged and working.

[Suger]

Somebody had left a huge (but really huge) blob of solder between 29L and 29U when adding the blue wire. Without these, the readings are

OA 217V
OB 217V
BREG A 217V
BREG B 217V
KREG B 2.72 V
KREG A 3 V

So I'll reconnect the blue wires now.

[Suger]

And all measurements are now in range. Testing it with Miles' Porgy and Bess. Thanks a million for the help (and for the great design)

[Paul Birkeland]

I sincerely appreciate your determination to chase things down and especially that you came back to let us know that all is well.