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Sherwood S-5500ii Blowing Power Tube (7868)

Sterling245

Sterling245

AK Subscriber
Subscriber
Hi all,

Today I was listening to my Sherwood S-5500ii when all the sudden I heard a hum from the right speaker. Before I could get over to it to shut it down, the main fuse blew. I went ahead and tested the output tubes, and found one of them had shorted. I opened up the amp to check and see if anything obvious had failed internally, but everything seemed fine. I'll be completely honest here - I was running this amp completely original. All the caps and other components had been tested though, and were good. I went ahead and tested caps again - coupling caps showed no leakage at working voltage and electrolytics all tested good. I went ahead and changed out several though while I was in there, figuring it was easy enough and these parts are very old.

After not finding anything wrong, I put in a new 7868 and fired it up, watching the tubes and current draw of the amp. Amp seemed to be drawing a bit too much current - around 1.5a, but all output tubes looked happy - no red plating. Then all the sudden I heard the hum and shut it off, but not before the tube I had just put in shorted.

I am at a loss as to what could be wrong here. I have tested just about every component, and it seems like such a simple little amp. Does anyone have any idea what the issue could be?
 
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Just to be clear, have the coupling caps to the grids of the output tubes been replaced? Have you cleaned the output tube sockets?

Jack
 
TriodeLuvr said:
Just to be clear, have the coupling caps to the grids of the output tubes been replaced? Have you cleaned the output tube sockets?

Jack
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No, they are original, but I did a leakage test on them at working voltage and they are still within original spec.
 
Sterling245 said:
No, they are original, but I did a leakage test on them at working voltage and they are still within original spec.
Click to expand...

you may want to consider replacing the coupling caps. You can also take one of those dead tubes and cut off a single pin. Use that pin to test the tightness of each receptacle on the octal socket in question. If any are loose, you'll need to replace that socket. If the bias pin is making poor contact it could cause some of the issues. If you replace the socket try to find a NOS US made octal socket. They were made much better than the current Chinese made sockets.
 
Good call. I think I have some on hand so I will go ahead and replace them this evening and report back. Will also check the tube sockets. Thanks for the help!
 
With the output tubes removed, check the bias supply to all the output tube grids!
Check the resistors in series with the grids.
Replace the coupling caps.
What brand are the output tubes? I've had problems with JJ tubes drawing lots of current.
 
dr*audio said:
With the output tubes removed, check the bias supply to all the output tube grids!
Check the resistors in series with the grids.
Replace the coupling caps.
What brand are the output tubes? I've had problems with JJ tubes drawing lots of current.
Click to expand...
Will do! I checked the resistors last night and they are dead on value. Unfortunately, the tubes were Sylvania....
 
I've had good luck with these type of coupling caps but they are pretty damn old at this point and its far from impossible one is ailing.

The sockets are also a common source of problems on these. Sherwoods get really hot and the sockets just bake. The ones in mine are in OK shape, but the insulators don't actually stay in the metal brackets anymore so I have to hold them down with a screwdriver when trying to remove a tube. I don't think you're finding good new production sockets from anywhere though.
mcgjohn said:
each receptacle on the octal socket in question
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These are Novar sockets, think oversized 12ax7 9 pin, and there isn't much demand for the things. New 7868 use a Magnoval socket which has fatter pins on the same spacing. Probably stuck looking for old production. I would avoid the Chinese ones.

If you poke around on ebay for "novar socket" you'll get someone selling red ones that are identical to the originals and a bunch of Chinese stuff. I'd go with the originals if you need to replace your sockets.
 
dr*audio said:
With the output tubes removed, check the bias supply to all the output tube grids!
Check the resistors in series with the grids.
Replace the coupling caps.
What brand are the output tubes? I've had problems with JJ tubes drawing lots of current.
Click to expand...
It's about what I would have said. Sylvania makes good tubes and at the time yours were manufactured, the tube companies were at their best. As mentioned, bias is where I would certainly check as well as suspicious coupling capacitors. But the vintage of your amplifier, using 7868 tubes, Sherwood was probably using Mylar capacitors. But how many decades was that?
 
also you might want to consider breaking the original cathode to ground connection and putting in a small value resistor to give you current sampling and so it might act as a fuse in the event of a tube short. 1/4 or 1/8 watt flame-proof 10 ohm will do it. As-found, the only thing that was in not proper working condition in my 5500-II were the bias supply filter caps, which caused low bias voltage and excessive current.
 
Update:
I replaced the coupling caps with some Panasonic films I had. Checked bias - -18.6v on the grid of each tube. Sockets all look good too.

Now the amp is only drawing 0.5a and tubes are not getting hot enough. Sounds very distorted. Also, there is ~300v AC and ~250v DC from the chassis of the amp to ground! What is going on here??
 
Are all 4 tubes actually woring? Are they flowing reasonable current? Too low and it distorts badly.
 
gadget73 said:
Are all 4 tubes actually woring? Are they flowing reasonable current? Too low and it distorts badly.
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All 4 are definitely working, but not flowing enough current and I am not sure why. I think it may have something to do with the high voltage on the chassis but not sure what is causing it. I am completely at a loss here.
 
Do you have a scope? You should look at the signal in the previous stages to see where the signal gets distorted.
Having said that, the chassis is floating. If something in the power supply is shorted to the chassis, you may damage your scope by connecting the ground to the chassis, so you should check that out first. What I would do is measure the current between the chassis and ground.
 
Sterling245 said:
Now the amp is only drawing 0.5a and tubes are not getting hot enough. Sounds very distorted.
Click to expand...
The amp wasn't distorted until you replaced the coupling caps? Did you use the same value as the originals?
Sterling245 said:
Sockets all look good too.
Click to expand...
It's not enough to visually inspect them They need to be cleaned or replaced.
Sterling245 said:
Also, there is ~300v AC and ~250v DC from the chassis of the amp to ground
Click to expand...
The schematic doesn't show a separate electrical connection to chassis, only ground symbols. If the chassis isn't electrically connected to ground, it should be earthed by installing a three-wire line cord. This can be checked with an ohmmeter between the chassis and the cathode pins of the output tubes. The 7868 filament pins should also show continuity to ground. Do this with the amp disconnected from the line, not just turned off.

Jack
 
dr*audio said:
Do you have a scope? You should look at the signal in the previous stages to see where the signal gets distorted.
Having said that, the chassis is floating. If something in the power supply is shorted to the chassis, you may damage your scope by connecting the ground to the chassis, so you should check that out first. What I would do is measure the current between the chassis and ground.
Click to expand...
Yes, I do. I think the distortion has to do with insufficient current to the output tubes though. They are running incredibly cool, just getting warm, not hot as usual. The current is definitely more than it should be from chassis to ground. Not sure what is shorted though - everything is testing okay!

TriodeLuvr said:
The amp wasn't distorted until you replaced the coupling caps? Did you use the same value as the originals?
Click to expand...
Yes. Exactly the same value.

TriodeLuvr said:
It's not enough to visually inspect them They need to be cleaned or replaced.
Click to expand...
I cut a pin off one of the dead tubes and used that to check the socket connections, all of which seemed tight. I will clean them tonight.

TriodeLuvr said:
The schematic doesn't show a separate electrical connection to chassis, only ground symbols. If the chassis isn't electrically connected to ground, it should be earthed by installing a three-wire line cord. This can be checked with an ohmmeter between the chassis and the cathode pins of the output tubes. The 7868 filament pins should also show continuity to ground. Do this with the amp disconnected from the line, not just turned off.
Click to expand...
Will check this tonight and report back. Thanks!
 
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