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In Reply to: RE: Upping downstream capacitance in PS.. posted by gkargreen on July 02, 2015 at 14:28:40
Is the PSU filter CLC? An inductance of substantial size "isolates" the rectifier from the remainder of the PSU filter. Increasing reservoir capacitance (within reason) is a good thing under those circumstances.OTOH, quite a few units use CRC filtration. A resistor is not nearly as good as a choke in "isolating" the rectifier. Make the reservoir capacitance large enough and an excessive turn surge will occur. I don't know if my calculations were 100% correct, but no failures have been reported of the B- supply in "El Cheapo" (schematic uploaded) and that rail is CRC filtered. The reactance of the 4.7 μF. 1st filter cap., at the 120 Hz. ripple freq. is considerable smaller than 1 Kohm and the cap. passes the vast bulk of the turn on surge, without incident. Resistance values in power amp CRC filters are smaller and care MUST be taken in increasing the value of the reservoir capacitor.
Eli D.
Edits: 07/02/15Follow Ups:
thanks, Eli, yes, these are CLC supplies inside my vintage Fisher and Scott units, the cap values after the 1st cap are around 20 ufd, some 30 and 10 ufd, thanks!
If you keep the first cap after the rectifier to whatever spec the particular rectifier has for the max value, then upping the other values downstream after the dropping resistors will not be a problem in the old vintage gear. All the old 10 and 20 uF cans can easily become dual 32 or dual 50 F & T caps. It will generally sound better and I have never had any problems with tube rectifiers failing. Really, the first cap is the critical one as long as you are not going crazy upping the values. Bypass the first one with a good small film cap too....... Change any old diodes out for modern quiet ones like the GI856 that McShane sells, but you probably already did that.....
cheers,
Don
I can also attest to no rectifier failures with sensible increases of capacitance at the first cap filtering point. Moreover, the grade of power supply caps and diodes available has steadily increased over the years; especially within the last two decades.
From a vintage collector perspective, more capacitance downstream always benefits bass control and impact, increasing dynamic range; even lending a smoother sound. Splitting the HV power supply storage capacitance rails, with separate storage "banks" for each channel, also has many benefits, approaching the sonic potential of dual mono power supplies...
Yes, I often split the B+ rail with diodes and then separate filter caps for each channel if there is room. Also beefing up the bias supply filtering and using much better diodes will clean up the old amps. Often you can add an extra pot in the bias supply to make it more adjustable in many designs where that was lacking. Then you really start to hear the quality of the iron in many of these vintage amps....
thanks, Don, I will go with that plan!
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