Setting bias on Sherwood S-7100A

[CCook]

I have a decent level of knowledge on restoring old tube equipment. But SS brain cells are a bit dated, hence the question here.

The Service Manual has an elaborate setup with a distortion analyzer, which I do not have. The second method utilizing a watt meter on the AC line, I have on with a 0.1 resolution. No idea how accurate that in setting bias, but sounds a bit wonky to me. How much of a shift in AC power will indicate you have hit the correct setting? Is it a cliff/vertical rise that you will see?

I do have a good scope, Voltmeter, and signal source.

A question that was posed earlier but not answered. Can you use voltage across the emitter resistors on the output transistors as a guide? If so, what voltage would indicate a correct bias?

Or, is there a generic way to set bias on thise types of output configuration?

Thanks in advance.

 

[chazix]

Is it a cliff/vertical rise that you will see?

No, more like a hockey stick - assuming a starting point with the bias trimmer set for minimum, you'll generally be able to turn it up a ways with no increase in AC line current, and then at some point the AC current will start to increase. When it increases, it will usually be fairly gradual, but you want to go slow. Does the manual give you a target for the level of increase in AC current?

Can you use voltage across the emitter resistors on the output transistors as a guide?

Sure. Just apply Ohm's Law. I think the main reason they didn't write service manuals to use this method in the era of your Sherwood is that technicians couldn't be expected to have millivolt-resolving meters.

If so, what voltage would indicate a correct bias?

Well, there's the rub, and the reason that it's hard to use the emitter resistor voltage method - you need a target value. If it were on my bench, I'd probably go for a voltage that corresponds to 10mA of current. This is probably on the low side, but it's conservative in the sense that it's unlikely to make anything run too hot.

 

[CCook]

Thank your for your help here.

The manual has no mention of target AC line current. Not sure why that is.

How did you come up with 10ma as a proper emitter current?

Forgive my lack of knowledge of solid state here....

 

[chazix]

The manual has no mention of target AC line current. Not sure why that is.

My take would be that they are just calling for the bias to be increased (from minimum) so that the AC current just barely starts to increase. That will mean that there is at least SOME bias current. That is, the bias current will be more than zero. In the absence of a more precise method, "more than zero" is a target that is conservative, but will still greatly decrease distortion.

How did you come up with 10ma as a proper emitter current?

Experience-based empiricism, I guess I'd say, to try to make it sound psuedo-scientific . Again, it's a conservative (low) target, but I'm confident that it is enough to greatly reduce distortion (compared to zero emitter current).

 

[CCook]

Thanks! Some troubleshooting still in progress. Likely a bad cap cutting input to PA section.I understand that old electrolytics are all suspect. Just got a cheap cap tester, but think I will take the time and just replace all in the audio path.

 

[rickl]

My notes say:
Set Bias: R (R602b): 12.2mV, L (620a): 12.4mV

not sure where I found this (maybe try searching AK) but this was on a s7100a I did a year ago.

 

[CCook]

My notes say:
Set Bias: R (R602b): 12.2mV, L (620a): 12.4mV

not sure where I found this (maybe try searching AK) but this was on a s7100a I did a year ago.

Thank you! Fighting a few bad caps, some off by 80%. Great learning experience. I remember being a very poor college student, with a Zenith tube FM radio. My roommate had a Sherwood receiver and a pair of small JBL speakers... 'some day' I use to tell myself.