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In Reply to: RE: Sherwood S-5000 Potential posted by Michael Samra on November 13, 2014 at 09:19:02
Another way to skin that cat:)
Bias supply pot added so you can dial the whole thing up or down for a particular quad of tubes. Cathode and screen resistors added. K40 caps are securely mounted so they won't contact chassis. No shrink tape needed on this one. Often needed on others though... Some sytrenes in the signal path. It was a killer little amp. Customer loves it....
Have fun.
Don
Follow Ups:
Very nice job. My only criticism is that I find it very unnecessary to replace the interstage coupling caps on these amps. After like 20 or so of these amps, I haven't seen a leaky one yet. In fact, I have never seen a real reason to replace any of the signal capacitors, even the crappy ceramics, which are notoriously non-linear in their hysteresis curve. How did the PIOs sound compared to the originals, or was it all done at once and no way to tell?
The bias adjust pot is a great mod, I do something similar with power resistors. I don't use a pot because if it ever got dirty, it could cause power tube runaway. However, ability to adjust the bias is very important once you replace the selenium rectifier with silicon diodes; it causes the bias voltage and DC filament to rise, running your phono filaments hot, and your output tubes cold. Doing something to get the bias voltage correct after diode replacement is absolutely critical to making the amp sound right and deliver full power. FYI, the two accessory AC receptacles make great bias test points, just move the dual line cap somewhere else, remove all that extra AC line wire, and once you have your 10ohm resistors on the cathodes, just wire up your test points. There are conveniently 4 terminals, and 4 output tubes.
Here are two pics of my S-5000 bias mods. One is the bias/fil supply, and you see that I've put a 10 ohm resistor to drop the bias and DC fil. voltage down, and shunted it with a 40 ohm resistor. If I want more bias voltage, I can lower the value of the 40 ohm resistor. If I want less, I raise it, or remove it. Basically, this accomplishes the same thing as your pot, but has the downside of being less easily adjusted, and the upside of not being susceptible to pot failure or dirt. I was thinking that I might put a pot in place of the 40R resistor, even if it failed, the 10 ohm resistor prevents total bias failure, and lowers the plate current.
The second pic is how I modify the AC receptacles to be bias test points. What's cool is that they work in tandem with the S-5000 bias balance circuit. You can either measure from each terminal to ground to see how many milliamps you're pulling, or you can measure across two of the terminals on one channel to determine your balance. More accurate than listening for hum, you just tweak the knob until you get 0.00 volts and there's no need to use 'test' mode.
Thanks for your input on this topic. There are now three examples of an S5000 rebuild to evaluate and digest.I can agree with the logic behind all three but I find my newly acquired early S5000 to be quite amazing and altering the sonics too much is not something I want to do so my approach has been very conservative so far.
The voltage correction after replacing the selenium was my next concern and you showed a way to do that. Thank you.There are two things that still puzzle me.
My S5000 has a 17K 1/2 watt resistor in place of the 4.7K resistor as shown on the schematic and in the pics of the S5000's but the other resistor values in the filament/bias supply are the same as the schematic and in the pics.Have you run across this before? I'm wondering what the variation is in my S5000 so that the 17K was chosen. It is an original resistor.
While that value worked, I went down a bit in value because I felt that was a better bias voltage for the 7189A's in there.At about 118 volts in, all of the B+ and low voltage operating voltages are as spec'ed except for the -20 shown right at the 7189's. That voltage would depend on the type and condition of the tubes your using.
Secondly, in your pic with the meter, it looks like one 7189 is running at 20 ma or around 8 watts. Is that how you run your S5000 versus a bit hotter like between 9 and 10 watts per tube?
Edits: 11/20/14 11/20/14 11/20/14 11/20/14
Don, where did you put the added bias pot and what did you use (value and wattage)? It is not too clear on the picture, can you give a little more detail on how you did it? Thanks.
Here is where it is mounted. I can see I used some mounting bracket I had laying around and tied it to the tightening clamp for the big electrolytic cap. I never drill holes in a chassis...
I don't have any notes scribbled for that one, but I can tell that the pot is either a 1K or an 8K from the type. Those are the only two values I have in that type. Looks like I am using half of the pot. I just figure these things out on the fly in an amp when rebuilding. Sorry... It will allow probably a 3-4 mA adjustment in the bias of a quad of EL84. I bet it dials the supply maybe a volt or two? Sorry I cannot be more specific. Usually I keep notes, but my Sherwood directory has only the schematic and the photos..... Wish I could help you more.
Don
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