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12.10.199.11
In Reply to: RE: Have you tried a know, good set of EL84? posted by thermionic emitter on July 14, 2016 at 09:12:34
All of JJ's Octal production is, at best, inconsistent. The failed 5AR4 is not at all surprising.
I don't know what you put in, but I suggest you scan the archives for the, several times posted, series SS diode tweak. The tweak allows the use of the reasonably priced Sovtek 5AR4, in complete confidence. The weak spot in the Sovtek variant is PIV handling, which is shored up by the "sand". In all other aspects, the Saratov, Russia, product is highly satisfactory.
Eli D.
Follow Ups:
Or you can simply install a new production Tung-Sol or Genalex - problem solved!BTW, if it was my amp I'd remove the 18 Ohm resistor and install a 10 Ohm 1 watt resistor at EACH EL84 tube cathode so the cathode current goes through the resistor to ground. Then you can measure the voltage at EACH tube's cathode and use Ohm's Law to calculate the voltage:
Cathode current = voltage at pin 3 divided by 10.
So if you wanted 30 ma per tube you simply use the bias and DC balance pot to adjust so you see 0.35 volts at pin 3 of each tube. You'll have to go back and forth with the bias and DC balance pots to get both tubes right since they interact. Set the DC balance pot at midpoint then adjust the bias pot until you see close to .30 volts at the cathodes. Then adjust the DC balance so the tubes show equal voltages at the cathode. Finally a last adjustment of the bias pot should get the tubes spot on - if not repeat the sequence once more and you are set.
If you can't get enough bias current then either the tubes are tired (put some EL84M Sovteks in there, they are VERY rugged and can survive plus they sound excellent), the plate or screen voltages are too low, or the heater voltage is low.
PLEASE read my FAQ post on this topic, the link is below. Item #4 is particularly important.
Take a look and let me know what questions you have. Do this resistor change once and life becomes MUCH easier both now and in the future!
Edits: 07/14/16
Jim, that link isn't working for me.
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Buy Chinese. Bury freedom.
nada aqui
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Buy Chinese. Bury freedom.
It's fixed - it's the Tube Asylum FAQ.
Try the 1 provided below.
Eli D.
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