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[email protected] themend@gmail.com is offline
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Default Restoring a Sherwood S-5000

Hello all,

Made some progress on the bias supply yesterday, but didn't get around to posting it. First, the selenium rectifier he
https://www.dropbox.com/s/27opjx5auy..._Rectifier.jpg

has been replaced with a pair diodes and a pair of caps mounted to a terminal strip as seen he
https://www.dropbox.com/s/bfynay796k...eplacement.jpg

There is nothing special about these diodes, they are rated for 1000v (total overkill) and 1.5 amps. Since silicone diodes tend to generate a bit of switching noise, two .01@1000v mica caps have been added to shunt that noise to ground. While the center post of this terminal strip is mounted to the chassis, I find that using that mechanical connection as a ground can be hit and miss, so instead, I run a ground wire inside the chassis and solder it to the bottom of a cap can. Here is the diode bridge installed in the amp where the selenium rectifier once was:

https://www.dropbox.com/s/nzy9mw08cq..._installed.jpg

"But wait!", you say "Aren't you going to replace the capacitor cans?" Well, yes we are. But we are only going to replace the two HV+ capacitors, and while we will also replace the caps in the bias supply, we will not remove the old capacitor can. Why? Because it is a very rare capacitor can with COMMON POSITIVE. All other can caps are common negative, and you simply will not find a replacement for this bias supply can. So, we will have to rebuild the bias supply using axial and/or radial caps stuffed underneath the amp. Don't worry, there's room as we'll see later. For now solder the ground to an unused common terminal on the bias cap can as I did with the green wire he

https://www.dropbox.com/s/1knvs3qlc6...for_diodes.jpg

In order to mount the new diode bridge, I first had to cut 7/8" off the old mounting bolt for the selenium rectifier in order to use it. I just used the bolt cutter on my strippers. Check it out:

https://www.dropbox.com/s/eqwe6e7vgf..._off_screw.jpg

Ok, so I got all that done, now it's time to fire up the amp and take measurements. Oh crap! One of the output tubes is arcing over now! Better get the tester out!

After testing my output tubes, it turns out that it was a complete coincidence. One of the tubes had already had a part come loose inside, and all I had to do was operate the amp upside down to make the short happen. After testing, 3 tubes flunked the test. One arcs over, the other tests with shorts, and the last one is weak. Dang, and they're all Amperex Bugle Boys..... Anyway, after digging through my stash, I found a replacement for these three guys he

https://www.dropbox.com/s/y1suozb1di..._bad_tubes.jpg

Ok, now the amp fires up without any drama. As I expected before, my bias voltage has now risen up and is now -24.85v, about where I expect it to be with silicon diodes and no dropping resistor. Yes, you can get a couple more watts out of this thing with a lower bias, but the wear and tear on the output tubes and the heat generated makes it scarcely worth it, IMHO.

The DC filament voltages are now 26.1 and 12.7 volts at V2 and V1 respectively, up from 11.06 and 22.74. These might be just a touch high, but they're within the realm of sane. I may in fact try to put a slightly higher value dropping resistor in front of these.

I have also made some interesting observations regarding the voltages listed on the schematics, versus real life and I will get to that in my next post.