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In Reply to: RE: 6X4 Rectifier Questions posted by FlaCharlie on April 17, 2017 at 10:36:00
Thanks for the clarification guys!My query stems from a thread on another site: http://www.diyaudio.com/forums/instruments-amps/5246-el84-substitute-7189a-2.html
The poster is trying to rebuild a Japanese Teisco Express 5 guitar amp. After viewing a schematic he posted of a Teisco 10 (which seems to be the same amp) and other pics of his amp, I looked at some schematics of similar amps (Kalamazoo Models 1 and 2 and a Silvertone 1421) and noticed a similar use of the 6X4. While I don't own any of these amps, I hadn't ever seen the 6X4 - or any other rectifier tube for that matter - used like this.
If you read the thread you'll see that my main concerns were the use of a 40uF first cap in the PS (the data sheet calls for 10uF) and the owner's seeming reluctance to replace the infamous grey oil caps that the Japanese were so fond of using.
Edits: 04/17/17Follow Ups:
Abuse of vacuum rectifiers was, and remains, a problem. Look at some of Cary's PSUs. :> ((
The cheapskates would not use proper CLC filters that don't abuse the vacuum rectifiers.
Eli D.
40mfd is going to kill the 6X4. But, prolly the 6X4 was less than a couple bucks, when those Japanese amps were being made. So, the manufacturers didn't care.Still, the mil spec NOS Tungsol 6X4W --- my favorite --- is not that $$$.
8^)
Edits: 04/17/17
The stock configuration on the Teisco was 40-10-10. Fortunately, I was able to convince him to switch the first cap with the last one so now he has it 10-10-40, all 450v parts.
The others I mentioned used 20uF as the first cap, not quite as extreme but still too high.
That's the value in the old Akai decks that I mentioned. They followed the 20uF with a small choke, then another 20uF. The 6X4s in those never fail.
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Buy Chinese. Bury freedom.
Does the ability to, apparently safely, exceed the data sheet specs by 100% have anything to do with current draw or other operating conditions? Or is this just an especially tough rectifier tube or ??? Do you think rectifier data sheets are deliberately ultra conservative in this regard?
Another rectifier, the 5AR4, has a max first cap rating of 60uF. Are there designs that safely use 120uF with this? The highest I've seen was with a Pilot 500 console amp that I restored (kind of mini version of the 232) which came with 80uF stock. I used a smaller cap when I rebuilt it.
I've always figured that going, maybe, 25% higher than a data sheet recommends would be about as far as any given rectifier should be pushed. That's just based on a vague philosophical belief not on analysis of technical data, though.
6X4 data sheets show 10uF as the typical value for a cap-input filter. To determine the actual maximum for a given application, it's necessary to use additional published data. The 6X4 is rated for a maximum transient peak plate current of 1.1A per plate. Charts published by the manufacturer(s) use this peak rating to show the required plate supply resistance in ohms VS the AC RMS voltage per plate, relative to a 10uF cap. Additional capacitance can be used as long as the peak plate current rating isn't exceeded. Simulation software is ideal for determining this.
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Buy Chinese. Bury freedom.
Boy, the cost of these things... I would not design an amp with more than 40mfd in the first stage. 120mfd is crazy! At that point, I'd seriously considered solid-state rectification.
Edits: 04/18/17
When I encounter the 6X4 in an amp, I re-punch the socket opening, install a 9 pin socket and change to a EZ80 tube. I do not trust the 6X4 for over 150 volts max.
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