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24.27.41.140
In Reply to: RE: Power Supply Rebuild Question 222D posted by mr9iron on March 14, 2025 at 13:08:37
You can determine the current flow through each pair of output tubes by measuring the voltage across the 10Ω cathode resistors. I can't find a spec for this voltage in Scott literature, but I would expect to see about .16V or .17V at that point in each channel. This will tell you if the output tubes are drawing sufficient current. Ideally, each cathode should have its own current sense resistor, but Scott didn't design it that way.
Follow Ups:
I think a math error crept into your calculation TK. With a 10 Ohm cathode resistor .17 volts across it would mean a total current of 17 ma or 8.5 ma per tube (assuming both tubes were passing equal current). That's way low!Assuming the plate voltage was at "spec" (420 volts) and assuming you wanted each tube's total dissipation to be 10 watts (a reasonable level for a 7189 or EL84M Sovtek) then you'd want each tube to be biased at about 24 ma.; that would make the total current through the 10 Ohm resistor 48 ma.
48 ma. across a 10 Ohm resistor will mean a voltage across it of about .48 volts.
If the plate voltage was 480 volts instead of 420 and you decided to bias them at 24 ma per tube then the tubes would have to dissipate about 11.5 watts. While I wouldn't set the bias there for regular use it is okay for a short period of testing and adjusting/repair. But 480 volts is pretty high, well above the plate voltage spec.
So using the variac to drop the line voltage to about 117 would be what I'd do. Then I'd bias the output tubes to that 24 ma. setting and recheck your voltages.See where you stand at that point.
Jeff, if you do that and need help or want to know how to get to the next step (what you are doing is more complex than it seems at first glance) drop me a note and I'll be happy to help.
Edits: 03/19/25
Jim,as always, thanks for the info, I missed this reply- so sorry, but you answered my main concern about what I was dealing with - thank you. I have since been able to get passed this issue and I am getting closer to having this thing running very well and in spec. I am back on the forum though because I am about to post another question about this amp re-build -lol.
I sincerely hope you are doing well Jim and that everything is headed in the right direction!
Edits: 03/28/25
Math error indeed, and not a small one. No idea how I arrived at that number. Hope you're doing well, and thanks for the correction!
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