fisher 500-b selenium rectifier
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century tek
Addicted Member
EDIT: Ok, some confusion here: A selenium (two legged) rectifier is not the same as a bridge (3 or four legged) rectifier. To replace the selenium rectifier, you can use a diode such as a 1N4007.
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Utterly and completely incorrect. A selenium rectifier can be a bridge rectifier, which is just four diodes in one package. Selenium is just the material the diodes are made of, these days almost all are silicon like other semiconductors. If a selenium rectifier has four legs, it's a bridge rectifier.
If you don't believe me, the main rectifier in the 800-B service manual, SR1, is called out as "Selenium Rectifier, Bridge Type."
century tek
Addicted Member
I was thinking of the all-too-common transformerless radio two legged type. I know a bridge has four diodes in one package as I have built 100's of units using replacement 1N4007 diodes.
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century tek
Addicted Member
Thanks cademan, that is exactly right. I was thinking of those multi-fin square blocked grey thingies that look like a giant resistor. Hence the two legged type.
The post I responded to said that a "selenium rectifier is not the same as a bridge rectifier." That info is wrong, and could greatly confuse novice technicians. The comment wasn't "I don't think that uses a selenium bridge," it reads as "selenium was never used to make a bridge rectifier."
Selenium was used for all sorts of power rectifying devices before silicon gained prominence - I've seen diodes, 1/2 wave rectifiers and bridge rectifiers made of selenium. Fisher could be really odd - in at least some of the "B" series receivers, they used a silicon diode as a 1/2 wave rectifier in one part of the power supply and a selenium bridge elsewhere in the same unit. I think all of the "C" series receivers (including the 400) used silicon rectifiers everywhere, but I'm not positive.
century tek
Addicted Member
No, the info is not wrong, just mis read! A selenium rectifier is almost always a two legged diode, not four as in bridged. Now a selenium bridged rectifier has 3 or four legs, not two. Sam, you read into it wrong and jumped the gun. A selenium rectifier is not the same as a bridged selenium rectifier. When I see "selenium rectifier", I take it to be just a two pronged like a diode. I have seen and replaced thousands of the old style selenium rectifiers with single 1N4007 diodes. If it was a selenium bridge rectifier, then I replace it with four 1N4007 diodes.
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cademan
Lunatic Member
I didn't read it that way! I knew exactly what he was talking about. I took it as a single type diode no matter what kind of material it is/was made of.
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cademan
Lunatic Member
I knew what you meant!
OK, we're just reading things differently. I think of any rectifier using selenium instead of silicon as a "selenium rectifier," not differentiating the types - as they all need to be replaced. We're on the same page here. I can get a bit defensive about making things clear for those of us who haven't been at this as long as others - I'd hate for someone to think "oh, that bridge rectifier is fine, I'll leave it alone" and subsequently have a part failure that damages an irreplaceable component.
A purpose-made silicon bridge rectifier is another good option for replacing selenium bridges, and can be easier to mount than four diodes. :thmbsp:
Well, in most cases. In the case of the Fisher 400 and 500, the additional voltage is WELCOME, as it increases the bias negative voltage to the output tubes. Given that they run really "hot" in stock trim (partially due to increased line voltage from the wall now, compared to the time they were built), this can help dial back the idle current in the output tubes.
Regards,
Gordon.
Spot on. Best to check what kind of current your OP tubes are drawing through 10ohm resistors on the cathodes before adjusting resistance to dial in the negative voltage for the OP tubes. The one exception is the heater voltage to the phono 12ax7's is almost certainly above spec due to the increased efficiency of the silicon rectifier. My 400 has a 15ohm wire wound resistor coming off it stock and a slight increase will put the voltage right back into spec....
If my memory serves me correct tonight after replacing the bridge rectifier with silicon it approaches 27V instead of the neighborhood of 24v you're looking for, so a slight increase in resistance is called for.
Kory
Bluelobster
Well-Known Member
If you are talking about the "top hat" rectifiers that Fisher used they were Germanium, not silicone.
Fisher used some silicon diode rectifiers. They're called out in the 800-B service manual, for one.
