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Hi, when I finally get around to a pair of Dynakit PAM's for rebuilding, I've heard get rid of the Selenium rectifier. Some even said it's dangerous. Can any of you explain that or if it's degrading to the sound....thanks Mark K.
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For those preamps, a working selenium will have minimal sonic value, either positive or negative.
For the many phono and even full-function vintage preamps which used seleniums for high voltag, their sound, when working, is not bad. Compared to a silicon replacement, adjusted for proper voltage with a resistor, the selenium actually does yield a slightly warmer sound !
Considering that a few "world reference" vintage preamps came with selenium rectifiers for their high voltage supply, if the voltages are proper and hum is not heard, I am hesitant to replace the selenium, at first. I will then increase power supply capacitance and listen critically after coupling caps and voltages are fine.
If a phono stage, like the old chrome cuties, or a GE UPX-003_ is found for restoration, since their B+ selenium is working with less than 4 mA at under 150V DC, why replace it ? Because, someone says they are toxic ? If the voltages are fine, stable and the unit sounds fine, I might even leave the selenium alone for another decade....
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?
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.
Selenium rectifiers are ticking toxic time bombs. It is only a question of when, NOT if they will fail. When they fail, both toxic gas and toxic dust are produced. The STENCH of a failed selenium rectifier is horrible.
Eli D.
While this is all true, a silicon rectifier is not a direct drop in replacement. First of all there is considerable forward resistance as well as reverse leakage in a selenium.
With silicon you are going to have a higher rectified voltage, and a higher peak current as the diode turns on. This may make noise in the supply.
Most effective would be a resistor in series with the silicon. I did this and it worked, but that was an ultrasonic cleaner. For audio I would also put a capacitor across the silicon, not sure of what value, that would depend on the application as does the resistance and wattage of the resistor.
One thing to really watch is the voltage. This equipment (in the US) was designed to operate on 107-112 volts, now nominal line voltage is 125. This wreaks havoc on old tube guitar amps no matter what kind of rectifier(s) they use. I have had to bias quite a few of them almost all the way down (in current) to get them to stop redplating at idle. What happens when they turn it all the way up is beyond my control. Ideally they would buy a bucking transformer but they are cheap. At least where I used to work on them. When the ticket says "call if over $20" you know they are not going to spring for a transformer. In fact that is one of the reasons I no longer work there. That and the fact that most of them are clueless, they have no conception of mic/guitar level, line level and speaker level. They have used guitar cords for speaker connects on 200 watt amps, and then speaker ¼" lines for a guitar and wonder why it hums.
Anyway, a choke in series with the silicon rectifier might work well, but there will have to be some capacitance across it to keep the inductive voltage in check.
Changed resistance to get the voltage right is important. Noise is easily solved. Use Schottky diodes, with ZERO reverse recovery spike. :> D
Eli D.
Thanks you guys. I got some great ole advise. Mark K.
I agree with Uncle Mike,
They are nasty. If you smelled a failing one you would not want to again. If you want to keep the look, just gut the old selenium and embed a silicon diode inside the remains of the old nasty thing.
In the '50s when I was a kid I worked in a TV shop where we got paid 1/2 the labor charge on what we worked on. One of the techs would refuse the replace the seleniums which failed in TVs for fear of the toxicity. I guess I thought he was paranoid and my old Ford needed to be fed so I did them. Today I might also pass them by depending how hungry my ride and I were.
Phil
Hi Phil,
We are still around, having been affected by some nasty seleniums' farting. While their smell may be toxic, so would a blown power supply cap, or oil cap, or solder fumes !
If the voltages are fine, stable, plus if the current draw is minimal, why replace the selenium ? There are some very fine vintage preamps, some still considered world reference, which came with seleniums for their High Voltage Rectifier. Moreover, they were often 1/2 wave rectified ! I agree they may be old and tired, and could smell bad soon, but, they do sound "warmer" than a silicon rectifier adjusted for the same voltages...
Hi Interstage Tranny,
Please don't shoot me. I have one of those fine vintage preamps (at least I think it is) of which you speak. It is a Harmon Kardon Citation 4. It had a selenium bridge followed by an RC filter feeding the 6 12AX7 heaters in a series/parallel arrangement. The B+ supply was a 1/2 wave selenium rectifier feeding an RC filter network fairly typical for preamps of the day (~1960). When I got it the heater supply rectifier bridge was on the way out as the heater voltage was low so I replaced it with a silicon bridge. The dial lamp was part of heater supply filter. As tubes were heating the lamp would be very bright. I took a notion to use a 24 Volt three terminal regulator for the heaters as there was plenty room with the selenium bridge gone. The lamp was across the resistor between the filter caps and now across the three terminal regulator. With that arrangement the lamp would start dim and get brighter as the tube heated. All was well except that when the fridge came on my subwoofer went nuts. I decided that I needed to regulate the B+ supply. After the second design iteration I ended up with a MOSFET pass device controlled by a TL430 via an opto-isolator. I replaced the selenium rectifier with a silicon bridge and set the regulator to the input of the OEM RC B+ filter to the value on the schematic. The TL430 got its power from the now regulated heater supply. All was well and my subwoofer and fridge could co-exist. A few years later the twist lock high voltage electrolytic caps started to go South. During that re-fresh I used modern electrolytics mounted on copper clad board and replaced the incandescent dial lamp with a green LED. OK, not OEM but I liked the look. The heater supply electrolytics were still good so I left them.
That was all good for few years until the preamp developed an intermittent hum. A move was on the horizon so I had no time to deal with it so I got a Hafler DH100 off eBay. All sand but it seems to be a nice preamp.
I have recently dragged out the Citation and started troubleshooting it. I found one of the heater supply twist lock electrolytics to come in and out as I stressed its leads. Since the heater supply now also powers the TL430 that could account for my intermittent hum. Also I discovered that the three terminal regulator was coming out of regulation at ~105 Volts. I can't have that so I found a state of the art low dropout regulator from Linear Technology. Now both my B+ and heaters stay in regulation down to about 95 Volts AC input.
The front panel was getting rusty so I spent silly money to get a graphics file generated, the panel sanded out, chrome plated,and silk screened.
I'm closing in on it now, there are some OEM electrolytic caps as cathode bypass caps. I want to replace them, run some signals through it, and check the RIAA accuracy on phono, then I can listen to it.
Phil
Hi Phil, You did exactly what should be done. When there is trouble, we fix the trouble. For heaters, seleniums are not recommended to stay in. With "high" current passing through, seleniums are not stable. The indicator of trouble was the low voltage. Any low voltage off any selenium means get rid of the selenium, if the electro and/or dropping resistor is not bad. Of course....
Twist-Lok originals are now usual suspects for trouble also. They have outlasted their expected life-span long ago. Yet, many still work fine. Cathode bypass caps look so innocent, sitting across resistors and often measuring fine. Yet, replacing them with some modern equivalents will really open our ears. I tried the Silmic II and Muse BP with fine results. Their temperature ratings need heeding though, when used inside the hot-runners.
Vintage gear is like vintage sports cars. If you leave the fifty year old, structural or moving parts in place, they probably will eventually fail. Old electronic parts, especially capacitors will, most likely eventually fail. Just like vintage cars, staying aware and vigilant, replacing parts before they strand you, keeps us on our journeys...
If it works, it's time to go. Besides the high forward voltage drop they aren't very good with reverse voltage. When they fail a toxic gas is emitted that smells really, really bad. Unless you want to maintain your gear as vintage there really is no good reason to keep them. You can do a search on them and read the same story over and over.
And, Half-Wave Rectified to boot ? Yes, I am talking genuine, stateside made stereo preamps. The kind anyone of us would be proud to own. I am thinking of two units which begin with sequential letters...
Wasn't that a good hint ? Yes, I will spare you the brain power...
Lafayette KT600/KT600A and Marantz Seven(7C) originally come with Half-Wave Selenium Rectified Power Supplies. That means only one, small stack disc for the High Voltage supply.
If you replaced these with silicon diodes and adjust voltages with a resistor, you might have preferred the sound with the selenium rectifier.
I am not condoning either method. I am simply posting this here, so all can consider the facts. Surely, conversion to silicon or modern bridge diodes might be beneficial electronically. Soundwise, what if you actually prefer the half-wave selenium with more capacitance added to the power supply cap banks ?
If you were restoring a Marantz 7C, you might prefer to "bypass" the selenium, but leave the selenium in place for that vintage look the buying collector wants to see...You all have seen the prices of the 7C rising rather sharply through the years, right ? There must be something about the sound of that unit which keeps demand going, right ? BTW, the Lafayette KT-600 is no slouch in the sound department either....
Hi Interstage,
I'm going with your idea on the matter. I'm not rejecting the other way to go but by going by my manual it's easier for me to finally get the project done.
You guys have given me tips that really can't be found in books. Any output from you all is greatly appreciated and noted....thanks Mark Korda.
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