[Boatanchors] Diode replacements for 866 tubes, ratings needed ?

[D C _Mac_ Macdonald]

Putting the choke in the C.T. lead is the cheapskate's way to use a choke with far lower insulation ratings!
 
Mac, K2GKK/5 in OKC

Sent from my iPhone

On Nov 10, 2014, at 19:38, kb1fqg--- via Boatanchors <boatanchors at mailman.qth.net> wrote:

Yay Rick

You bought that cheap amp on craigslist and saved it from coming home with me!!!

John KB1FQG

Sent from my Verizon Wireless Droid

-----Original message-----
From: Rick Poole WA1RKT <wa1rkt at comcast.net>
To: boatanchors at mailman.qth.net
Sent: Tue, Nov 11, 2014 01:18:12 GMT+00:00
Subject: [Boatanchors] Diode replacements for 866 tubes, ratings needed?


I'm rebuilding an old Heathkit HA-10 Warrior amp (4x811A, 1700 VDC on the plates).  I plan to replace the 866 rectifier tubes with solid state rectifiers.  I would prefer to replace them with plug-in solid state replacements but am having trouble finding them at a reasonable price and as I am coming up on three years of unemployment I really need to do this on the cheap.  So I figured on building in two strings of 1N5408 diodes, one string in each leg of the center-tapped plate transformer secondary.

I have seriously limited (though not quite "none") experience in high-voltage power supply design, and have some questions...

1.  The Warrior power supply uses a swinging choke in the center tap lead of the power transformer (I must admit, I haven't seen that kind of "choke-input" filter before).  I'm told that choke-input filters tend to generate impressive back-EMF voltage spikes that the rectifiers need to be able to handle.  So, how many 1N5408 diodes (1 KV PIV, 3A) do I need for each leg for a full-wave rectifier?

2.  Related to (1), does the choke in the center tap of the transformer, rather than at the input to the filter, provide any relief to that "impressive back-EMF voltage spikes" issue noted above?

3.  Older Handbooks show a 0.01-uf cap and a several-hundred-Kohm resistor across each diode in the string.  I understand why the caps but not sure about why the resistors.  I've read lately that the resistors are not needed.  True or false?

4.  One of those "older" Handbooks (1991) states that choke-input filters are not normally used with silicon rectifiers.  True or false, and if true, why?

Thanks...

Rick WA1RKT
Londonderry, NH
Catch me on 14.300 MHz Intercon/MMSN most days
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