why would diodes be attached to a rectifier tube socket?

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why would diodes be attached to a rectifier tube socket?

Postby Unit_1 on Thu Mar 05, 2015 7:23 am

Oh what a long strange trip it's been. Started with a layout and schematic from a guy that obviously made several mistakes and finally found a better, mistake free layout from ceriatone.

And on that layout it has diodes and a cap attached to the bottom of the rectifier socket. Why is this? It seems odd to rectify the juice going to a rectifier....

and why in the world would you need a 3000 volt capacitor for 290 volts???

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On a personal note I think I'm finally over this wretched rhinovirus from HELL as my brain actually started working again tonight for the first time in nine days. man I thought this one was going to kill me, no $#!+....
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Re: why would diodes be attached to a rectifier tube socket?

Postby pdf64 on Thu Mar 05, 2015 12:43 pm

They are protection diodes, in series with the tube plates.
A common failure mode for tube rectifiers is a plate-cathode short, which would cause fault current in the B+ winding, and hopefully pop a fuse after a while.
The Vac on the reservoir cap / B+ string / tubes may result in collateral damage.
Series silicon diode maintain rectifier function if there's a short, keeping the show on the road and protecting other components.
The downside is that the user may not realise there's a been a problem and that the VB+ has likely increased, increasing dissipation.
But on balance, the benefits may be seen to outweigh that drawback, eg RG Keen has it as a recommended mod http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#ampmods

The cap is probably intended to act as a snubber for high frequency switching noise from the silicon diode/s, in the case of one or both taking over from the tube, in the case of it shorting.
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Re: why would diodes be attached to a rectifier tube socket?

Postby Unit_1 on Thu Mar 05, 2015 7:58 pm

pdf64 wrote:http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#ampmods


WOW! what an awesome link!!!!! :not_worthy:

Thanks for the answer pdf64, makes perfect sense. Snubber caps ordered and I already have some diodes

I'm going to put this mod on all my tube rectified amps, especially my 68 sfdr - I don't think I'm even going to play it again till I do, murphy's law and all that...
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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Re: why would diodes be attached to a rectifier tube socket?

Postby pdf64 on Thu Mar 05, 2015 10:19 pm

Glad the link was helpful; lots of great stuff at geofex!

I think modern Fenders have these protection diodes as standard, so they've probably done the analysis and found a cost benefit advantage in fitting them.

With higher power amps, unless they're in series arrays, 1000V diodes are insufficient (hence RG advises 1500V types).
The voltage would only appear across them if the tube failed, but it would be sad for the mitigation to fail too!
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Re: why would diodes be attached to a rectifier tube socket?

Postby Unit_1 on Fri Mar 06, 2015 3:53 am

akkk

I only have 1000v 1a and 1000v 3a

1500v 1a ordered!
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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