PT frying SS diodes?

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joshdfrazier
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PT frying SS diodes?

Post by joshdfrazier »

I've replaced a 6ca4 with two uf4007 on a PT with a 660v CT. the amp will work totally fine for a couple of days, but eventually the diodes short (always both simultaneously), thus blowing a fuse and stressing this already old PT. Admittedly, I ordered ALOT of these on ebay for very cheap. Could it be cheap diodes?
teemuk
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Re: PT frying SS diodes?

Post by teemuk »

Too low current handling?

The rectifier tube is indirectly heated and has "slow startup" that limits the initial inrush current. Solid-state replacement just draws a HIGH current spike when you power on.

Try inserting a suitable NTC resistor to the mains line.

Could be too low reverse voltage rating too. For 4007 it's above 660V alright, but the initial inrush spike has a habit of also introducing a higher voltage surge when you power on. Usually you see several 4007 diodes connected back-to-back in high voltage supplies, which at least is a slight attempt in increasing maximum reverse voltage.
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Reeltarded
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Re: PT frying SS diodes?

Post by Reeltarded »

1 amp rating. triple up!
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pdf64
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Re: PT frying SS diodes?

Post by pdf64 »

Yes, could just be dodgy diodes, consider dumping them.

Are you are that the caps and power tubes are coping with the increased voltage / dissipation?
Could be too low reverse voltage rating
That can catch people out.
The peak voltage for 660V is 933 but is it really 660, especially before the tubes warm up, at times of high line etc?
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matt h
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Re: PT frying SS diodes?

Post by matt h »

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surfsup
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Re: PT frying SS diodes?

Post by surfsup »

Could be too low reverse voltage rating too. For 4007 it's above 660V alright, but the initial inrush spike has a habit of also introducing a higher voltage surge when you power on. Usually you see several 4007 diodes connected back-to-back in high voltage supplies, which at least is a slight attempt in increasing maximum reverse voltage.

my rule of thumb is triple. 660 x 3 = 1.8A

The diodes are probably shorting due to this reason. Did you use one on each leg? If so this is probably your problem.
tubeswell
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Re: PT frying SS diodes?

Post by tubeswell »

What surfsup said. Not high-enough PIV rating
He who dies with the most tubes... wins
joshdfrazier
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Re: PT frying SS diodes?

Post by joshdfrazier »

This is the same univox amp that I've posted about a few times. 6973 tubes that I switched to el84. Added octal sockets to accommodate 6v6 also. Some other mods to manage the higher b+.

Surfsup, I did use only two. Are you talking about running them in series? I tried that and the b+ dropped considerably, which is not what I was expecting..
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Reeltarded
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Re: PT frying SS diodes?

Post by Reeltarded »

three take the place of each.. same direction
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Leo_Gnardo
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Re: PT frying SS diodes?

Post by Leo_Gnardo »

Two or three in series 1n4007. Two of these to simulate each "half" of the tube rectifier. Done it a million oh xxx don't exaggerate, well, 50 or so times. Each rectifier drops the B+ by 0.6 volts. Two thoughts - 1) if you have a pile of 1N4007, no big deal to use up a couple. (Hey I buy 'em by the hundred or thousand from Mouser for pennies apiece.) Thought B: If your source was dodgy and the little black cylinders you have aren't the authentic thing... well who knows what will happen. Heck some of em might even be zeners then they will really goof up a power supply. Instead of buying from ebay, get authentic parts from Mouser, Digikey, or any of a number of other reliable sources. There was a terrific thread on here about dodgy capacitors, reads like a horror stories. See what you get when you buy cheap from unreliable ebay sources:

http://ampgarage.com/forum/viewtopic.ph ... c&start=15

http://www.audioasylum.com/forums/tubed ... 16991.html
down technical blind alleys . . .
surfsup
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Re: PT frying SS diodes?

Post by surfsup »

Surfsup, I did use only two. Are you talking about running them in series? I tried that and the b+ dropped considerably, which is not what I was expecting..

:shock:

As Leo says the voltage drop is negligible. Run in series all the same orientation (correct orientation)...do you have a picture you can post?
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M Fowler
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Re: PT frying SS diodes?

Post by M Fowler »

Is this transformer meant to be run half wave?
surfsup
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Re: PT frying SS diodes?

Post by surfsup »

check the op. he said he replaced a 6ca4 with diodes so it should be full wave. maybe he has a pic

http://frank.pocnet.net/sheets/093/6/6CA4.pdf
tubeswell
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Re: PT frying SS diodes?

Post by tubeswell »

4007 will be "1000V-rated", but the PIV could be borderline if a 2-phase FW 330-0-330 supply was used (933V) and the rating of the diodes was dodgy.

Put diodes in series with 1kV .01uF balancing caps in parallel with each diode

[img:258:177]http://www.valvewizard.co.uk/Fullwave2.jpg[/img]
He who dies with the most tubes... wins
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