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In Reply to: RE: In the PAM & PAS, selenium only used for filament voltage... posted by Interstage Tranny on January 21, 2017 at 15:37:13
Purely for safety sake. I've done quite a few 1940-50s era amps with UF4007 or UF5408. No problems. Change to response and tone seem positive. Never negative. IME.
Esp true when working on client's equipment. No?
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Of course, for amplifiers ! In the interest of reliability, amps require stability. There is no argument that a selenium rectifier in a fixed bias supply deserves to be replaced. Their failure can be catastrophic to amp parts. In fact, any old "top hat" or early silicon diode deserves the same replacement, for the same concern of reliability.
Low current, vintage preamps might deserve some consideration. A 12AX7 preamp draws about 1 mA per tube section. A 6SC7 tube draws about 2 mA per section, at the most. For those phono only preamps which use one or two of these tubes, if the voltages measure fine and the unit shows stability, I may be hesitant to replace the selenium.
Fender Lover, your priorities are concerned with a full amplifier, comprising preamp stages, tone control/ambience stages and power output on one chassis. That means "high current" draw, even with a Fender Champ amp. For the performing stage atmosphere, reliability is imperative. There are few moments as depressing as when an amp or speaker blows at a performance and the audience starts Booing. Amps deserve quality and reliability, as well as great sounding parts.
So, while your experiences with musical instrument amps are significant, the phono preamplifier concerns are usually important within the home hifi world. Sometimes, replacing that selenium diode does not sound better, or increase desirability.
The next buyer for a Marantz 7C, if there are even many units available stateside, will probably want that original selenium and all original coupling caps still in place. I have no 7C, nor do I intend on getting them. If I did find one, I would not hesitate to replace the HV selenium disc and any offending leaky coupling caps within. I would save the bad parts for collector interest, and probably leave the selenium in situ (but bypassed). When I check them for friends, I recommend replacing the selenium and any leaking couplers, as well as beefing up the power supply cap banks. Usually, 7C collectors prefer to keep as much originality as possible inside, in spite of any future catastrophe.
Let us think about this, for a moment. Do we automatically replace a tube rectifier with ss diodes ? Electronically and theoretically, ss diodes have come a long way and have been preferred by most "licensed" engineers. Opinions aside, subjectivity with sound equipment is quite personal. While many might always choose the ss diodes, many might always prefer the tube diodes. The analogy is a good one, don't you think ?
Although I never found a failure, they are grossly inefficient and must go. Used in amplifier bias circuits, bias is the most important parameter. New solid-state diodes will last forever.
Perhaps the failure rate for bias rectifiers is lower. HOWEVER, if you get poisoned, probability factors mean very little.
Replace all selenium rectifiers as a matter of routine! Obviously, whatever alterations are needed to keep things in order should be made too.
Eli D.
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