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Sherwood S-1000II troubles...

Tom Bavis said:
The voltage at the rectifier should be -30 to -33V. If so it's fine as-is. A resistor in series with the preamp tube heaters will bring them back into range if it's higher. Three 150 mA tubes: 12.6 +12.6 + 6.3 = 31.5V, so if you see let's say -36V, you want to drop about 4.5V at 150 mA or about 30 Ohms, 2W.

Having bias voltage greater than the nominal -30 is not necessarily a bad thing - it will reduce the idle current and the output tubes will run cooler.
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Sorry for the apprentice question, but where exactly would I put the probes to test this voltage? It has been a long time since I studied electronics in high school!!
 
clydeselsor said:
Sorry for the apprentice question, but where exactly would I put the probes to test this voltage? It has been a long time since I studied electronics in high school!!
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Measure at the bias rectifier (common point of the two rectifiers) and look for -30 to -33V, with the other probe on chassis ground.

Two other things - the voltage across the 12 Ohm cathode resistor will tell you the idle current, and I'm guessing they intended this to be about 1V. 1V across 12 Ohms is 83 mA, or about 21 mA per tube, 0.021* 450 is 10 Watts.

And the clever balance test circuit - set the NORM/TEST to TEST and adjust the balance control for minimum hum.
 
Thanks Tom! I ordered a set of EL84M tubes from the Tube Depot last night. I will test the voltage with the old tubes and report back. I don't want to damage the new tubes!
 
From the common leg of the rectifier to ground I have -31.4v. Voltage across the 12 ohm cathode resistor is 1.2v. So it would appear that I am set for now and just have to wait for the new EL84M tubes to arrive. Thank you to everyone for your help!! It is very much appreciated!
 
I am surprised that the schematic does not call out for 7189 tubes instead of the standard 6BQ5/EL84. Were the EL84s in that era a more rugged tube?
 
7189 is the industrial version of the el84 with 400v plate rating there hard to find. If you find any let me know :thmbsp:
 
Tom Bavis said:
A pair of silicon diodes (1N4007 or UF4007 or similar) will replace it - if you get more than 30V for bias, it might be a good idea to add a resistor in the preamp heater string to keep it to 30V. But this amp pushes EL84s pretty hard (452v!). I'd use EL84M - Russian 6P14EB - which are rated for this high voltage. Jim McShane has them.

Schematic is on my page: http://www.audiophool.cjb.net/hifistuff.html
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Which one of those many links shows the diodes?
 
clydeselsor said:
I am surprised that the schematic does not call out for 7189 tubes instead of the standard 6BQ5/EL84. Were the EL84s in that era a more rugged tube?
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In essence... somewhat.

Many old-stock tubes were made with more "exotic" internal materials (that were commonplace then, but scarce/expensive now). As a result, some of them had a greater "reserve capacity" than newer tubes... in some cases.

This, however, is probably a case where they got LUCKY that the older tubes were more "abuse-resistant"... and the amp, in all fairness, SHOULD have been spec'd for 7189s in the first place.

In any case, the Russian military tube specified above should take care of the problem...

Regards,
Gordon.
 
I meant to say earlier that when I came in from the garage, after testing the voltages that Tom specified, I found my wife sitting at the kitchen table, cleaning the face of this amp (since it was removed from the chassis) with some cleaner and a cotton Q-tip swab! She really made it look good and I will post some before and after shots when I re-assemble the amp. She's great!
 
The EL84M tubes came in today! I installed them and this amp sounds great! It's been running for a few hours now and there is no sign of any problems! Thanks to everyone for you very useful help!!
 
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