Stereomour II - Issue with Shunt Regulator?

[Paul Birkeland]

Are the voltages on the 4 pin socket on that side still solid?  If so let me know and I can run you through how to temporarily install some jumpers to narrow things down better.

[lskiii]

I will check, but one other thing I wanted to ask - is it possible that the issue could be the tube socket itself?  I ask this because:

The left channel has been giving me issues for a while now (I've had the amp for a year).  It started with intermittent hum, which over time became intermittent crackling.  I swapped out many tubes to try and sort out the source of the issue.  Ultimately it became what it is now, which is a weak signal in the left channel that won't go away.

When I was closely looking for any differences between the 2 channels, I noticed that the solder tabs on the left channel socket are much more wobbly than the right channel socket (esp C6 and C8).

I don't know much about how the sockets are internally structured but is it possible that there's a loose internal connection?

Thanks!
Louis

[lskiii]

I believe that these were the readings you were looking for -

A1 57.4
A2 367
A3 14.7
A4 59.9

D1 60.2
D2 367
D3 15.2
D4 57.8

All looks good to me, yes?

Thanks!

[Paul Birkeland]

On A3 and D3, is that 15mV, or 15V?

If you run a wire temporarily between A3 and D3, does sound now come out properly from both sets of outputs?

[lskiii]

Yes, A3 and D3 those were 15mv.

So, I ran a wire between A3 and D3 and played the amp.

I ended up playing some mono tracks to test it.  One channel seemed to have more bass than the other, but they seemed at a similar volume.  The sound was a little bit distorted and there were some crackling/static at points.   When I powered it down there was a big surge in volume and then it faded quickly.

Does that help?

Thanks,
Louis

[Paul Birkeland]

Did both channels exhibit the exact same behavior? 

[lskiii]

Yes, I think.  Should I try it again and listen for something specific?

[Paul Birkeland]

Can you describe the distortion you heard?

Is the "more bass" on the channel that was otherwise working normally or the one that was not?

[lskiii]

OK - spent some time doing much more detailed testing.

The last post I did, when I experienced distortion, I had moved my amp to a different system.  This other system has less efficient speakers (86db vs Klipsch Heresy which is what I normally use with the stereomour.  So, I wanted to remove that as a variable.

So today I tested w/ the less efficient speakers first, here's what I heard:

Source in all cases was a mono Beatles recording.

With just one channel (RCA from DAC) The right channel on it's own sounded normal to me.  Moving the single RCA input to the left channel, I lost all the bass.  Essentially could no longer hear the bass guitar lines. 

I then plugged in both RCA channels from the DAC.  Volume increased dramatically and the sound was badly distorted in both channels.  To me, it sounded like a flubby overdriven distortion that you might get in a guitar amp.    I turned the volume down 9db on the stepped attenuator and the distortion went away but the sound still sounded off (muddy bass, not alot of treble)

To see if there was still a difference in the bass between the 2 channels, I disconnected the left speaker, and then the right speaker to A/B them.  Both speakers sounded the same to me. 

Just for the sake of it, I repeated the above steps with inputs 1, 2 and 3 and in all cases the results were the same.

One other thing - when I turn the amp off, there's a sudden and noticeable volume spike.  I don't recall that ever happening until this most recent issue (of a very weak L channel) came up.


Then I moved the amp back to the system w/ the Heresy's and repeated the above test.

In this case, the loss of bass when running just the input into the L channel vs R was still there.  However, distortion, even with the attenuator completely wide open, went (almost) completely away.

When running both left and right inputs, the overall audio quality still was off.  The bass was over emphasized and "flubby" and the treble was muted.  I have an A/B amp switcher (on/off/on) and I tested an NAD amp against the Stereomour and the differences were extreme.  The NAD sounded correct, the Stereomour was muted/flubby (which is not at all how it normally sounds to me).


I hope that helps?

Thanks

[Paul Birkeland]

With the jumper between pins 3 on each 4 pin socket, repeat the test but run the tube with a 12AT7 in one socket and listen, then shut the amp down and move the 12AT7 to the other socket and retest.

[lskiii]

Here are the results:

Tube in Socket B only - sound from both speakers - weak volume, but sound seems relatively normal

Tube in Socket C only - sound from both speakers - louder volume, but very flubby bass / muddy sounding

[Paul Birkeland]

We will have to address the lower volume with just a tube in socket B only.  Leave the jumper wire in that you have now between A3 and D3, then add another jumper between terminals 10 and 11.  With the two jumpers in, let me know if the volume level coming out of the amp is evened out.

-PB

[lskiii]

I added the jumper between 10 and 11 and retested.  Same results. (Tube in B = weak volume / Tube in C = louder but flubby w/ weak treble)

[Paul Birkeland]

Well, that would have me suspecting that you have a broken wire or a bad solder joint on or around the 9 pin socket on the offending side. 

[lskiii]

So, I re-soldered pin 9 on the C socket (the left channel) and when I tested it, both the L and R channels sound the same (which is good).   Thinking that the problem was solved, I removed the jumper between the 3 pins and the 10/11U and the problem came back (L channel is weak).  So, I re-installed the jumpers and the problem went away again. 

What should I try next?

Also - I hope this helps - I keep suspecting that there's a problem with the C socket.  While music was playing just now, I touched the tube in the C socket and got a ton of crackling noises.  If I do the same thing to the tube in the B socket, nothing happens. I also noticed that if I applied light pressure to the C socket that there was a large volume drop.  Again, doing the same on the B socket produces no change.

To go back to the very beginning of this thread, I had suspected all along that there's something wrong w/ the C socket - it's either a bad solder joint or something with the socket itself.  So, before even opening this case, I re-soldered the whole C socket.  When doing that, I believe I accidentally swapped the wires going to the shunt regulator.  I believe that this is why I started this thread thinking that the issue was with the shunt regulator (as it was measuring wrong).