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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: RE: Sherwood S-5000 Potential posted by dls123 on November 13, 2014 at 20:27:22:
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.
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Follow Ups
- RE: Sherwood S-5000 Potential - Sherwood Forest 10:07:06 11/20/14 (1)
- RE: Sherwood S-5000 Potential - DaveV 12:07:09 11/20/14 (0)
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