Stereomour channel imbalance

[bqc]

I have been enjoying the Stereomour for about 3-4 years now. Everything works great until recently when I noticed the left channel is softer causing the stereo image to shift to the right. I have swapped the tubes L <-> R and the left is still softer. I have put in another set of 2A3 tubes and still have the same issue. I have bypassed the input selector and volume control pot and issue is still there. I have tried different source and still same issue. I put in a different amp and the issue went away so it sounds like it is an issue with the Stereomour amp.  Some time the imbalance is not noticeable when first power up, but after a few hours, it tends to be more noticeable. Any help is appreciated.

[Paul Joppa]

Voltage measurements can provide clues to what is going on. My initial suspicion is that it's an issue with the driver (12AT7) or nearby. The fading after some on-time makes me suspect a depleted cathode.

[Doc B.]

Yup, I concur. Take voltages measurements after the amp has run long enough that it sounds imbalanced.

[bqc]

BTW, I have also replaced the input tube 12AT7 and it made no differences.  Here are the voltage measurements

Term   Measured in V DC
Hv+        384
1            356
2            203
3             0
4             0
5            58
6             0
7            0
8            0
9            0
10          0
11          57
12           0
13           0
14          215.7
15          358
16          58
17          386
18          0
19          386
20          57

A1          59
A2           357
A3           8 mV
A4          59
C1          57.6
C2          358
C3          9 mV
C4          57.5

IA          386
IB          386
OA          202.5
OB          215.8

Kreg (both)    2.49
-reg  (both)    0

Thank you for reviewing these numbers and let me know what could be wrong.

[bqc]

So it looks line terminal 2 and terminal 14 dont have the similar voltage. Terminal 2 is 203 and Terminal 14 is 215.7. Are they they connected to OA and OB respectively because OA and OB seem to have the same voltage as Term 2 and Term 14. Is that what causing the imbalance, if so what component in the active load PC board that could have failed to cause this imbalance?

Thanks