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In Reply to: RE: Help needed: 4uF PIO capacitor (Hunts) sought, or PIO replacements? posted by Michael Samra on February 26, 2015 at 22:43:07
Me too. Can they go sufficiently 'out of tune' and stray off value to cause problems?
(PS. Thanks for the advice).
Big J
"... only a very few individuals understand as yet that personal salvation is a contradiction in terms."
Follow Ups:
I have never seen a hermetically sealed cap go way off value unless it was shorted by over voltage.Even if your cap read 3.5uf or 4.5uf,I don't see where that would cause a problem unless the cap has DC leakage,
What kind of problem are you having with this amp and do you have a schematic you can post or a link to one?
"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
I'll post a schematic below.Apparently, 2 of thr RFT EL34s were not behaving, which allowed voltages to runaway internally. This caused cap failures inside.
In the tidy up, the engineer suggested that, in addition, the value of the Hunts caps were not closely matched, which would lead to imbalances between channels. So I don't think these Hunts have failed, per se. Perhaps you could confirm what may have happened as you are likely more familiar with the circuit, given your experience?
IIRC from the visit, the ones which blew are the wrapped capacitors. The mustard ones are all fine. The Hunts cap is on the other side (they sit on top) but the leads are centrally located and can't quite be seen from this picture.
Big J
"... only a very few individuals understand as yet that personal salvation is a contradiction in terms."
Edits: 02/27/15 02/27/15 02/27/15
I kind of thought so..The 4Uf cap you are speaking of is a filter cap.As long as the B+ is ok going into the choke which is 465vdc or a bit higher,that cap is fine.The caps that will cause the output tubes to run away are C7 and C8.Old caps like that tend to leak DC voltage on to the grid of the tubes causing the tubes to draw grid current and therefore start glowing cherry red.If you replace the power supply caps other than the 4uf which I'm pretty sure is ok, then change the coupling caps and change any out of tolerance resistors around the tubes,your tube runaway problem will disappear.
"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: 02/27/15
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."
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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