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Questions about tubes and gear that glows. FAQ

Suggestions

May I suggest that you call Leonard at ARC customer service and order an owner's manual with schematics. Also ask for his suggestions, he is very helpful.

I have a few suggestions:
With schema on hand, measure B+, Bias supply, screen supply, DC balance and bias points.

-There are four coupling caps connected to the 6550 grids (one per tube).
Measure DC Volts from ground to 6550 grid. Your meter should be stable at zero volts. Any DC voltage or fluctuating reading means a bad coupling cap> replace. Since those caps drive grid resistors, the output tubes do not need to be in place to measure DC leakage.

-Most likely defect: the bias supply or the screen supply is defective (since all tubes turn cherry-red). Measure both supplies and compare to specs on the schematics.

Places to look:
SCREEN SUPPLY:
-weak screen supply 6550 regulator tube;
-the screen supply 6550 must be similar to output tubes. If it takes longer to warm up than the output tubes, the OPT plates will glow red. This tube MUST be of the same manufacture and vintage as the OPT tubes.
-likewise if the screen supply 12ax7 conducts immediately (typical of european 9 pin tubes), the TL071 opamp will conduct too soon and will "latch-up" the bias supply. You need an USA-made controlled warmup time 12ax7. On this particular application, avoid european tubes that flash upon turn-on.
BIAS SUPPLY:
-measure the bias supply before the bias adjust trimpots. If I recall correctly it should be around MINUS 70 volts (double-check the schematics).
-bad cap in the bias power supply, easy to find out: paralell a good cap with the suspect cap.
-last but not least: bad zener diode in the bias PS. I believe it is a 12V zener, check the parts list to make sure. If the zener diode measures above spec, it is open.

You may remove the output tubes to measure the screen and bias supplies.
I suggest that you use rubber gloves at all times, your amp has very high voltages and large PS caps, take care.

Never turn on a tube amp without an output load, this may burn the output transformer(s). A 10W 8 ohm resistor from Radio Shack across the output is OK to adjust bias, if you do not want to risk your speakers.

Also, use a pair of shorting plugs (cheap RCA plugs shorted center-to-ground) when measuring bias. Bias must be measured with zero input signal.
I hope this helps
Good luck
Carlos


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