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Zeroing in on the reasons for 5AR4/GZ34 arcing in the ST-70 "upgrades."

Anyone have the DCR numbers from their new PA060 Dynakit or Triode trannies, each side?

Thanks.

Have arching on start up? What is the size of your first cap in the power supply? What 5Ar4 rectifier are you using/brand?

I have put together a number of these and haven't had a problem with the correct size of the first cap and a reliable 5Ar4. The Chinese 5Ar4's will show fireworks easily and should be avoided at all costs.

I have put together a number of these and haven't had a problem with the correct size of the first cap and a reliable 5Ar4.

6BG,

Thanks for the help.

The rectifier arcing that has been talked about here extensively is something I'm trying to get a grip on before starting a build. The input capacitor size limit and the "reliable" GZ34 are 2 important factors. The problem with trying to identify a reliable brand of rectifier is the "not knowing" under what conditions other folks had their problems. So I'm puting a list of all the factors that seem to be a factor in rectifier failures.

Do you have any DCR numbers from the PT's you've had good or bad luck with?

Thanks much

No doubt it is imperative to identify the exact conditions under which an arc occurs, before any progress can be made as to why. Arcs that occur literally at turn on, after warm up, under light loading, heavy loading, etc. imply different concerns. I went through much the same process years ago when I set out to determine why output tubes arced all too frequently. Identifying the exact conditions present at the time of an event is important to finding the cause.

With new tubes today, you can't assume proper manufacturing conditions, or proper manufacture to a given tube's standards either, so manufacturing source becomes part of the equation.

On the other hand, if a tube is well manufactured, then the issue will either come down to one of excessive current flow or PIV breakdown relative to safe area operation for the tube and choice of filter used.

Regarding the size of the first filter cap, the concern for hot switching is what places the biggest limitations on the size of that cap. When hot switching occurs, that is when the greatest peak currents occur -- hence, the need to limit the cap's size to limit peak current under that condition. It is also why the use of current in-rush limiters are so crucial when the size of these caps are increased.

Dave

Puzzle piece just in...

Triode PA060 DCR nearly identical to original Dynaco specs, reply from Chad at Triode.

@1R primary and @32R per 1/2 secondary.

Thanks Chad

When hot switching occurs, that is when the greatest peak currents occur -- hence, the need to limit the cap's size to limit peak current under that condition. It is also why the use of current in-rush limiters are so crucial when the size of these caps are increased.

And beware thunderstorm season with momentary power flickers from lightening. But this brings up a question of how quickly can the hot switch be done? If the input cap is still charged, does that nullify the need for a current limiter? What is the typical discharge rate?

It takes multiple seconds for the volume to drop off after turn-off. Is that from cap discharge primarily or from heater cooling? Need to monitor the B+, I guess, to answer that one. A quick off then on may be safe as opposed to a 5 second off then on. Then once the cathode has cooled enough to block conduction it would be safe again, like perhaps 20 seconds off.