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Sherwood S-1000II troubles...

clydeselsor

clydeselsor

Lunatic Member
I picked up a Sherwood S-1000II mono tube amp for my mono system that I putting together. When I first received it, it was apparent that new caps would be in order. So I purchased some new orange drops from AES and they arrived today. I replaced all of the coupling caps. When I fired it up again, it sounded great for a few seconds and then began to distort. Two of the EL84s were glowing like red light bulbs, so I shut it down right away. I suppose it is possible that my new caps may be defective, but I doubt it. How can I test this? Any suggestions on what to try next?
 
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Could be that the original output tubes were weak, and all of a sudden they were getting good clean DC current which they were not used to! You can always try the old caps with the old tubes, or get new tubes to try with the new orange drops. Also, I have come across defective new capacitors at times.
 
Does the 1000 have a selenium bias rectifier? And, has it been replaced?

They are generally not to be trusted, and will happily overwork your tubes by providing insufficient negative bias voltage.
 
jon_s said:
Does the 1000 have a selenium bias rectifier? And, has it been replaced?

They are generally not to be trusted, and will happily overwork your tubes by providing insufficient negative bias voltage.
Click to expand...

Yes, I believe it does. Where can I find a replacement?
 
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
 
Agree 100% with Tom here.

Also, you may need to check the resistors (usually like 220K to 470K) that connect the bias supply to the grid terminals of the output tubes... there should be one resistor per tube. If any of these are bad, then you'll get the symptoms you described. Definitely make sure those are in spec (by the schematic)...

Regards,
Gordon.
 
GordonW said:
Agree 100% with Tom here.

Also, you may need to check the resistors (usually like 220K to 470K) that connect the bias supply to the grid terminals of the output tubes... there should be one resistor per tube. If any of these are bad, then you'll get the symptoms you described. Definitely make sure those are in spec (by the schematic)...

Regards,
Gordon.
Click to expand...

Looking at Tom's schematic (Thanks Tom!) are we talking about the 1k resistors R62. R63, R64 and R65?
 
Okay, I tested the resistors and they are in tolerance. I have just noticed that my digital multi-meter has a diode symbol on the dial. Correct me if I'm wrong but if I set the meter to that symbol, and place the probes on the diode leads, shouldn't I get a reading in one direction and O.L. in the other? If this is the case, then the rectifier is bad as I got readings in both directions. Please let me know if this is correct.

Thanks guys!
 
I'd use EL84M - Russian 6P14EB - which are rated for this high voltage. Jim McShane has them.
Click to expand...

This is a likely problem (or part of it) and a good solution. The EL84Ms sound good, too.

Yes a diode should show a 'back to front ratio', infinite resistance in one direction, low in the other. Do remember that a full-wave bridge isn't just one diode. Replace the selenium bridge if so equipped (it is an accident waiting to happen) and 'pad' (so to speak) the voltage drop resistor to get the right bias voltage. You may have zero volts bias; which might make your output tubes unhappy. Been there, done that (once with a Marantz 8B!)
 
Where do I want to put this resistor and what size should it be? Sorry for all of the questions, but beyond recapping, I'm lost!
 
Okay, I removed the selenium rectifier and replaced it with the 1N4007 diodes that I happened to have. The problem didn't go away. Only two of the tubes are glowing like red light bulbs. On a whim, I shut down the amp and swapped the tubes around and discovered that the problem followed the tube. I guess a good lesson here is to never assume brand new tubes are always good! I will check into those EL84M tubes, as I think I'm getting close here.
 
clydeselsor said:
Okay, I tested the resistors and they are in tolerance. I have just noticed that my digital multi-meter has a diode symbol on the dial. Correct me if I'm wrong but if I set the meter to that symbol, and place the probes on the diode leads, shouldn't I get a reading in one direction and O.L. in the other? If this is the case, then the rectifier is bad as I got readings in both directions. Please let me know if this is correct.

Thanks guys!
Click to expand...

Regardless of how it measures, I would replace it. In every amp I've owned with a selenium bias supply, it has dropped too much voltage in practice, and caused the output tubes to overheat. With the Sherwood running fairly high plate voltage, it really isn't worth taking a chance.

clydeselsor said:
Okay, I removed the selenium rectifier and replaced it with the 1N4007 diodes that I happened to have. The problem didn't go away. Only two of the tubes are glowing like red light bulbs. On a whim, I shut down the amp and swapped the tubes around and discovered that the problem followed the tube. I guess a good lesson here is to never assume brand new tubes are always good! I will check into those EL84M tubes, as I think I'm getting close here.
Click to expand...

AH interesting! Let us hope this fixes it.
 
mhardy6647 said:
This is a likely problem (or part of it) and a good solution. The EL84Ms sound good, too.

Yes a diode should show a 'back to front ratio', infinite resistance in one direction, low in the other. Do remember that a full-wave bridge isn't just one diode. Replace the selenium bridge if so equipped (it is an accident waiting to happen) and 'pad' (so to speak) the voltage drop resistor to get the right bias voltage. You may have zero volts bias; which might make your output tubes unhappy. Been there, done that (once with a Marantz 8B!)
Click to expand...

The schematic only shows a half wave rectifier so I only installed two diodes. Is that correct (See Tom's link to the schamatic to see if I'm correct)?
 
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.
 
He could put it right after the (negative) output of the rectifier, couldn't he?
I am ready, fire, aiming again...
 
Sure... but that will lower the bias voltage too. I think this amp pushes the output tubes pretty hard, and higher bias voltage may be a GOOD thing.
 
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