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In Reply to: RE: True Mullard 5-20 design. posted by Michael Samra on February 27, 2015 at 13:04:59
I was told that the voltages are both higher - one around 480 and the other closer to 500V. I certainly have some more questions to ask about what is going on in the amps.
Question: could the EL34s themselves, cause instability if they are out of spec, somehow? Or is this unlikely?
Big J
"... only a very few individuals understand as yet that personal salvation is a contradiction in terms."
Follow Ups:
only the 500vdc one is a problem. When these amps were made,the typical AC line input voltage was about 5% to 10% lower many cases than it is today.For instance,your AC main voltage is 230vac correct? At the time the amps were made the line AC voltage was probably around 217vac to 220vac so the additional 10vac to 13vac would explain the higher DC voltage of 480vdc.Many amps have a line voltage setting on them for this very reason but from the schematic I looked at your amps do not.
This is not much of a problem with the amp reading 480vdc because it is well within tolerance of less than 5%.
You asked if the EL34s were bad or in a weakened state,would this raise the voltage? Absolutely because if the tubes don't pull enough current,the voltage will read much higher.
Swap the tubes around in the amps to see if the voltage problem follows to the other amp.If it stays the same,you then need to find out why and that is usually because cathode resistors that have gone up in value.
I have also seen different brand GZ34 rectifier tubes vary the voltage out by 15vdc to 20vdc but it's rare.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 02/28/15
Got that. In the case of the RFTs, the problem follows the two tubes. Substituting 2 Svetlanas cured the issue so this would seem to lend support to the idea that the tubes are not behaving normally .
Yes, 230V here in Auckland, though it can vary.
Big J
"... only a very few individuals understand as yet that personal salvation is a contradiction in terms."
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