BA-6A meter zero problem/question

[Martan]

HI all,

I've finally got my BA-6A functioning but with one little hiccup. The meter is reading a 10 on the GR scale at rest and I cannot get it to zero. Its a Simpson Model 27 0-200uADC. I've read that the original BA-6A used a 0-500uADC meter, but should that affect the zeroing? I've also read that making careless mistakes with these (expensive) meters can be regrettable. My meter circuit matches the attached schematic. Increasing the zero pot value increases the problem, and jumper-ing the 82 ohm resistor has no effect. The screw on the front of the meter does very little. I've read a bit about shunting these meters when they are mis-matched to the circuit, but I don't think I understand properly how to do it. The unit has little to no noise/hum in the audio path and seems to compress properly.

Any help zeroing the meter would be appreciated. Would increasing the value of the 82R resistor be effective/safe?

All the best

 

[Martan]

Lowering the value of R12 gets the meter about half way to zero but also lowers my V1 cathode voltage to 3.1 (should be over 5V)

 

[emrr]

Bias is really dictated by the control voltage in operation, versus non-operation resting point. Cathode bias with any random set of tubes in 2023 is a crap shoot. I see lots of old limiters that won't hit zero with all other components in spec, draw is not as high as expected. Once there's compression it's swimming around anyway. if meter zero is all the way to one side, shorting R60 makes no difference, and lowering R12 also doesn't get you there, it suggests there's another problem (guessing you've eliminated B+ variations and other component values as culprits), then the tubes you're using just aren't 'in spec' enough. Short R12 and R60 with the zero set to 100R and see what you get. If that scares you, put another 500-1K pot in line and reduce it's value with the meter zero set to 50 and see if you can make it line up, if so, then listen to results with it there and with it raised so the meter isn't hitting zero, see if you detect compression differences.

 

[Martan]

Thank you for the reply! I will try some other 6SK7’s and then pull the resistors if needed. In the end, am I just chasing a cosmetic issue? I’ve done all the balancing/voltage testing internally and don’t plan on using the meter for precise readings.

 

[Martan]

I tried shorting R12 and R60 by replacing each with a jumper. The needle went the wrong way. Tried a second set of 6sk7's from the same vendor. No change.

 

[emrr]

Dropped instead of increasing?

 

[Martan]

Went from 10 to pegged at 25-ish (top of scale)

 

[emrr]

Confused, 25 is lower on the scale, and not marked. 0 is top of scale.

 

[Martan]

Ah, sorry. My thinking is backwards here. Removing those two resistors moved the needle lower on the scale. I estimated the 25.

 

[Martan]

Still though, my understanding is that with no compression the needle should be at zero, since the meter should be reading DB gain reduction. That is correct, isn't it?

 

[emrr]

Yes