My Sherwood integrated amp had a meltdown and I have got it up and running but I had to replace two diodes in the power supply. I replace diodes labeled SM334 with IN4007's. Now the voltages for the B+ are high by about 50 vdc. I replaced all the filter caps and resistors out of speck. I couldn't find any reference on the SM334 so I was wondering if the IN4007"s were not a direct replacement. I have a service manual but it gives no info on these diodes. Any help would be appreciated, Tom??
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If these are then-standard metal "top hat" diodes (1N1763 diodes were used in S5000), 1N4007s should be fine.
Higher B+ is most likely the result of something else - power amp not working or disconnected, part of PT primary short-circuited (check heater voltages) or something.
Higher B+ is most likely the result of something else - power amp not working or disconnected, part of PT primary short-circuited (check heater voltages) or something.
I find many old power trannies tend to run over-voltage with the current AC voltages in North America. I usually start with adding a low ohmage wire wound resistor on the primary side until the filament voltages are at or slightly below nominal.
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I'm not an expert on diodes but, in general, it's not unusual for modern types to drop less voltage than vintage types. The solution is to use an appropriately sized resistor to drop voltage to the appropriate level. I would also suggest adding a thermistor to the primary of the PT in order to soften the startup current surge. This will also drop the voltage just a bit.
Modern wall voltage is also higher today. Schematics usually specify that the voltages listed are based on a wall voltage of 117v. So when you're evaluating the voltages in a vintage amp you should have it on a variac set to 117v.
Once your voltages are in spec, which is usually considered to be +/- 15%, you can either continue to operate the amp on a variac or build a voltage bucker using a 6.3v filament transformer to keep the voltage closer to 117v. Or, I suppose, you could modify the PS using higher value resistors to get the voltages in line using modern wall voltage.
Depending on the specifics, some amps might not run the tubes too hard even if the voltages are a bit high. You'll need to calculate the dissipation to see how hard they're being run. Generally, if you get the heater voltages in spec, the other voltages aren't so far out of line that you need to worry.
Although it won't affect the voltage you might consider using diodes that are quieter than the 1N4007, which is considered relatively noisy today. The UF4007 is cheap and a better choice. Also look into various Schottky types which are supposedly even better. Again, my knowledge is pretty limited so you should do a little research to find the best solution.