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207.151.14.213
In Reply to: RE: There is an easier way to do this. posted by thermionic emitter on July 13, 2016 at 22:10:36
Could have weak Reflektor tubes.
Follow Ups:
Look at the 299B schematic. The only reasonable O/P tube option is the Russian 6p14p-ev, AKA EL84M. Nothing but a genuine 7189 equivalent belongs in that amp. Scott beat the living guano out of the "finals".
Eli D.
EL84 or 7189, just to see if you can get into proper bias range. It really sounds like he has a weak set of new tubes.
These reflektor tubes are new and test at 95 to 105% of specified plate current on eico 667 tester set for 7189. The thing came with JJ el84 which is not rated for the voltages in this amp. It came to me with a blown JJ rectifier. The JJs are still OK having not been in there very long but I am not leaving them in there for very long. The current and bias voltage relationships I have described for the reflektors apply to the JJs as well.
As to another suggestion that separate resistors be in place for all four tubes I completely agree and might end up doing that. Thank you all for your interest in this project.
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.
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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