C4S LED and voltage issue

[PassionForSound]

No problems, Paul. Only using the LEDs as an indicator. Happy to focus on voltages now you've told me what to measure. I'll test as soon as I can and report back

 

[PassionForSound]

Original orientation
OA: 3.32V
OB: 2.28V

Centre board turned 180 degrees
OB: 2.22V
OA: 3.2V

Other measurements with centre board rotated 180 degrees:
+275: 286.6
+6.3: 6.29
IA: 289 (A) and 289 (C)
Breg: 220.3 (B side, now closest to A socket) and 157 (A side, now closest to C socket)
-reg: 0.4mV both sides
Kreg: 4.45 (B side, close to A socket) and 1.56 (A side, close to socket C)

 

[Paul Birkeland]

Breg:  157 (A side, now closest to C socket)

Kreg: 1.56 (A side, close to socket C)

These have changed.  Can you pull the 12AU7 and recheck?

 

[PassionForSound]

You might be onto something...

Kreg B 1.16 and A 1.04
Breg B 286 and A 286

 

[Paul Birkeland]

With no 12AU7, are you still seeing that super low voltage at OB on the offending channel? 

 

[PassionForSound]

With no 12AU7, are you still seeing that super low voltage at OB on the offending channel?

189V at OB on socket C
For reference, it's 262V for socket A

 

[Paul Birkeland]

OB on the two outboard C4S boards shouldn't be that high.  You need to run the amp with no 12AU7 but with the 6C45PI tubes. 

OB on the outboard C4S boards is of interest under these conditions.

 

[PassionForSound]

That was with 6C45Pi installed on both sides.

Confirming measurements WITH 6C45s and WITHOUT 12AU7. All references to sides are based on the A and C sockets on the amp to avoid confusion from the rotated regulator board. Apologies for any redundant info. Just trying to provide a complete picture

+275 = 288
+6.3 = 6.3
IA = 287 (A) 288 (C)
Breg = 285 (A side)  285 (C side)
-reg = 0.3mV (both)
Kreg = 1.16 (A side) 1.02 (B side)

OB: 259 (A) 187 (C)
IB: 285 (both)
IA: 286 (A) 287 (C)
OA: 284 (both)

 

[PassionForSound]

That was with 6C45Pi installed on both sides.

Confirming measurements WITH 6C45s and WITHOUT 12AU7. All references to sides are based on the A and C sockets on the amp to avoid confusion from the rotated regulator board. Apologies for any redundant info. Just trying to provide a complete picture

+275 = 288
+6.3 = 6.3
IA = 287 (A) 288 (C)
Breg = 285 (A side)  285 (C side)
-reg = 0.3mV (both)
Kreg = 1.16 (A side) 1.02 (B side)

OB: 259 (A) 187 (C)
IB: 285 (both)
IA: 286 (A) 287 (C)
OA: 284 (both)

Just rechecked all my wiring because of your concerns about OB voltages. OB on C4S boards remained as per quoted post here.

OB on regulator board reads 2.24V (connected to C socket) and OA is 3.37V (connected to A socket)

 

[Paul Birkeland]

OK, so the OB voltage that really wanted to spend the rest of its life at 6V has popped back up without the regulator in the way.

I believe your issue will be resolved by replacing the 431 regulator on the offending channel, with the slightest possibility that you'll also need a 12AU7. 

-PB

 

[PassionForSound]

OK, so the OB voltage that really wanted to spend the rest of its life at 6V has popped back up without the regulator in the way.

I believe your issue will be resolved by replacing the 431 regulator on the offending channel, with the slightest possibility that you'll also need a 12AU7. 

-PB

Thanks Paul - I appreciate your help and patience. I've ordered the required parts and will advise once installed

 

[PassionForSound]

I've replaced the TL431 and swapped in a fresh 12AU7. All voltages are correct EXCEPT Kreg (B side) is still reading only 6V.

All tubes are new compared to when original issue arose.

 

[Paul Birkeland]

Are you able to adjust the bias per the instructions in the manual?

 

[PassionForSound]

Yes! I think the A-side trim pot was out of range if there's such a thing. A few extra turns and I'm back in range.

Thanks so much, Paul. Can I ask one more question?

Should I be wary of trying any of the previous tubes back in the amp? If the tubes have failed could they cause problems if I reinsert them?

 

[diynewbie]

Good question, looking forward to Paul's reply.  I would suggest that you download data sheets and use your meter to check for shorts.

 

[Paul Birkeland]

The amp should tolerated shorted 6C45PI tubes pretty well.  Things will heat up if you leave them in there running for a long time though.

 

[PassionForSound]

The amp should tolerated shorted 6C45PI tubes pretty well.  Things will heat up if you leave them in there running for a long time though.

Thanks, Paul. I'll fire it up quickly, check for sound (with sacrificial test headphones) and respond accordingly