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124.171.134.238
I got my old Sears SE el84 amp out of storage, turned it on and and the rectifier valve (EZ81) started to go crazy and burn out. I bought another one, brand new and the same thing happened, so the problem must be with the amp.
Would anyone be able to shed any light on this?
Cheers
steve
Follow Ups:
Lots of excellent suggestion, will follow up after Christmas
Nt
"I know just enough to get into trouble. But not enough to get out of it."
As 6AS7_6SN7 said. The caps in your rectification circuit are likely dried out and shorting causing way to much current flow. Replace those old caps and be glad that it toasted your tube, not the transformer. Hope it all works out for you.
roN
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Have Fun and Enjoy the Music
"Still Working the Problem"
Maybe, next time you are starting up an old unit which has been in storage for years, use a Variac and ramp it up. Early voltage signs of odd behavior of the rectifier... could have saved it.
Anyhow, good luck with the recap. May want to use a DVM to measure B+, to be sure you are in the ballpark.
I use a Sencore PR57 with switchable metering.
I monitor the load current as I ramp up the voltage.
First sign of undue current draw and I switch it off before harm.
Then it's time to check the caps.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Also be sure the replacement cap is correctly sized for the rectifier. I think for 6ca4 it is 40 uf.
There is no fixed 'max input capacitor value' for a rectifier tube.There is a fixed 'max peak repetitive current rating' for a rectifier tube.
If the voltage is low and the current is low you can use a "large" input cap and still not exceed the 'max peak repetitive current rating' for the rectifier used.
It's best to look at the issue on a case by case basis with the 'max peak repetitive current rating' for the rectifier tube (in use) in mind.
P.S. The max input cap ratings found in the data sheets are for the situation given and the voltage and current draw in that one scenario are stated in that data sheet.
Not all data sheets use the same scenario and that's why you will find different numbers found for the 'max input capacitor value'.
When you find the same number on 2 different data sheets (for the same rectifier tube) if you look close you will see that they are using the same scenario.
It's not the cap value alone, it's the cap value and the voltage and the load current that causes the 'peak repetitive current' to be what it is and as long as that is not exceeding the 'max peak repetitive current rating' then, it's OK.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 12/09/17 12/09/17 12/09/17
Have a peek at the EZ81 datasheet, here mac input cap is 50uFhttps://frank.pocnet.net/sheets/010/e/EZ81.pdf
The reason this ez81 overloaded was most likley an old electrolytic cap
that has de-formed ( lost the isolating surface). This is reversible
providing the electrolytic still is present.
Reforming the cap(s) via en external DC supply and with a resistive
current limiting ( 10k or so) over a period of some hours might
give the cap back to life. Reforming should be gentle, leakage
will heat the cap and if applying voltage to fast, local heating
might permanently destroy the aluminium foil.
If no external DC is available ; then replace the cap(s) in question.
Edits: 12/09/17
Yes, 900 volts CT with 620 ohms total HT winding, EZ81 rectifier (PRC rated at 500ma.), full wave, 50uf cap and 100ma. current draw.
Peak repetitive current.....370ma. 130ma. under max rating.
Change only winding resistance to 50 ohms total and the PRC goes up to 620ma. 120ma. over max rating.
Change only the current draw to 175ma. and the PRC goes to 920ma. 420ma. over max rating.
On the other hand change the current draw to 50ma and the input cap to 100uf and the PRC is only 370ma. 130ma under rating.
Note; Results according to PSUDII.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
From the OP questions, i did not want to get into the technical aspects of that.
I understand, I just couldn't help myself.
The issue just happens to be one of my pet peeves. :-)
I'm sorry for hijacking.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
by me anyway. I appreciate a broader outlook as it keeps me from making limited choices or from a too narrow point of view.
Check (and smell) for a bad electrolytic.
ecc230
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