Weak left channel?

[K-MAN]

I've finally finished installing my brand new pc boards in my SCA35 and used the best caps and resistors I could find plus put in new tube sockets. I got a friend of mine who restored vintage radios to look it over and redo any bad solder connections plus finish wiring up the output transformer wires which were a bit too short. So we fired it up and everything looks good, no hum, smoke or fire Yellow_Light_Colorz_PDT_06 But the volume on the left channel is about 20% lower than the right and we cant figure out why. The wiring has been double checked, the negative feedback is connected properly. The only thing I'm not sure on his I got him to add a cap across the cathode resistor and he moved the lead where it's grounded for space but I dont think that would affect anything. Anyways here's some pics of it all to see if anyone has ideas.

[Shannon Parks]

Have you done all your DC checks from the manual chart?

[TomMcNally]

If all of the voltages check out, and you don't find a
bad component, you might want to temporarily swap
the input wires on your amp boards and see if the
low volume follows the amp or flips. That way you
prove that it's the amp or not.

I would kind of suspect the input stage, not the amps,
and maybe just adjust the balance control until the
sound is balanced, and then re-adjust the knob
with the set screw. I think that's the way Dynaco
did it !

[K-MAN]

I wasn't sure exactly how to check the voltages, do I put one probe to each of the tube pins and check it against ground? Also I'm guessing I do the measurements while the amp is turned on with speakers hooked up right?

[Shannon Parks]

I wasn't sure exactly how to check the voltages, do I put one probe to each of the tube pins and check it against ground? Also I'm guessing I do the measurements while the amp is turned on with speakers hooked up right?

Yes, use speakers or a resistive load. If you don't have a manual chart - Google may find it - just compare pin to pin on each of the sockets.

[K-MAN]

I tested the voltages for all the tubes and the four 6BQ5's had identical voltages and the heater voltages were 6.19vac for all three tubes including 7199 on the left channel and 6.15vac on all the right channel. The only tubes that had different voltage readings were the 7199 tubes but I dont know if it's enough to make a difference. Here's the readings for the two tubes DC voltage.

Left 7199; Right 7199

Pin 1=253 pin 1=253
2=56 2=58.8
3=31.5 3=35.5
6=.71 6=.7
8=66 8=68
9=56 9=60
Also the tubes all were tested last year and worked good before the change plus I've only changed the two output boards nothing else.

[K-MAN]

Ok I found the problem, should have checked this before. So I figured I would measure the voltage to the two 12ax7's since everything else seems to be close. Well the left channel tube was within specs but the right channel 12ax7 had 255vdc on pin#6!! :o
The specs say it should be 106 like the other so something is seriously wrong but I cant figure out what except maybe a shorted filter cap or something?

[Shannon Parks]

Ok I found the problem, should have checked this before. So I figured I would measure the voltage to the two 12ax7's since everything else seems to be close. Well the left channel tube was within specs but the right channel 12ax7 had 255vdc on pin#6!! :o
The specs say it should be 106 like the other so something is seriously wrong but I cant figure out what except maybe a shorted filter cap or something?

If it doesn't have the right voltage on the plate pin, check your plate resistor & your cathode resistor resistances with the unit off. Check that plate pin socket to make sure you are getting a connection, too.

[K-MAN]

Ok I found one of the solder pads for the 12ax7 had lifted so I repaired that and that brought the voltage down to normal on pin 6 and it seemed to help even the channels out a bit but the left is still a bit too quiet. It seems the voltages on pin 1 for both is only around 97vdc when it should be 180. How can the preamp effect the power amp section so much? Cause it is unbalanced using either the phono or tape input.

[K-MAN]

Is there a way I can bypass the preamp section to see if that's where the problem is or not?

[TomMcNally]

Feed an input (something with a volume control)
right into the amp sections - into the spot where
C-17 takes the input from the preamp stage of
the amp.

[K-MAN]

So I ordered a new pc board for the phono section since the traces are too bad to repair. Seems all the movement from taking to tubes in and out of the preamp damaged the traces at the two sockets. I'd really like to know but can't find out if all signals go through the preamp board first because if they do that would help explain why the left channel is affected on all the inputs.

[TomMcNally]

Yep - everything goes through the preamp in that amp.
They attenuate the loud signals to match the lower ones.

The proper way to diagnose the amp would be to feed
an identical MONO tone into both channels, with load
resistors on the outputs, and measure the levels at
every step of the way with a scope and find out where
one side drops off (or the other side gets louder) and
fix it. It's pretty tough to do by ear.

[K-MAN]

Just a update, I got my new phono board and I'm stuffing it right now. I'm reusing the old tube sockets since there still in decent shape after I clean them up but the pins wont fit into the pcb holes, would it be better to drill the holes a tad bigger or grind the pins with a dremel to make them smaller?

[K-MAN]

I guess the better question would be if it's common to drill the holes a tad bigger on pcb boards or not.