Voltage test problem + 2 LEDs dark [solved]

[George Napalm]

Hello everyone,

I finished building my Mainline a couple of days ago. Power supply test and resistance check were fine. But the voltage test brought the following problems:

+275vDC on the Power Supply Board: 278
+6.3vDC on the Power Supply Board: 6.2
IA on the A side C4S Board: 276
IA on the A side C4S Board: 274
Breg Regulator Board A side: 216
Breg Regulator Board B side: 142
-reg Regulator Board both sides: 0
Kreg Regulator Board A side: 11
Kreg Regulator Board B side: 58

In addition to that, 2 LEDs on B side of the right-side C4S board are dark. I've seen similar problems with other users, but their voltage readings were quite different. I've rechecked all connections on the Regulator Board and the right-side C4S board, as well as B and C sockets. I also resoldered some suspicious-looking connections on the rights-side C4S Board and Breg Regulator Board B side, but the problem persists.
I also need to mention, that I seem to have different 431 regulators: A side one has "WS TL 431A", B side has "FKB31 LM 431" written on them.

Does anyone has ideas what is the problem here? Thanks in advance.

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[Paul Birkeland]

Pull the 12AU7, then measure the voltages at IA, OA, IB, and OB on the B side C4S board.

[George Napalm]

IA: 285
OA: 283
IB: 283
OB: 172

[George Napalm]

And I got similar results on the left-side C4S board, but OB on the left (A) side was 180

[Paul Birkeland]

Can you use the bias trim pots to trim out your OA voltages to more like 150V?

I'm just whittling down possibilities here.

-PB

[George Napalm]

A-ha! I reinserted  the 12AU7, then set the bias to 145 on both sides - and all my LEDs are now on! And those offending voltages are normal now: Kreg on both sides are 11V and Breg are both 217V.

I guess I'm all set! Thank you so much! I'll do listening tests tomorrow.

[George Napalm]

I did the listening test yesterday and had a bit of a scare. There was a big channel imbalance (on all volumes). After verifying that attenuators are built correctly I decided to double-check the tube bias. And indeed it drifted to 155 and 135 volt on left and right sides. I must have done the initial biasing before the amp warmed up properly. I waited for 10 minutes and set the bias voltage back to 145v on both channels. It totally fixed the problem.

The amp is up and running and produces beautiful sounds :-) Thanks Bottlehead for terrific product and excellent support!