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

[Al Parker]

Hi,
	I bought some diodes (again) from K2AW "Silicon Alley" 2 yrs ago, his 
daughter was still running the business then.  Good stuff, reasonable 
priced.  "K2AW “Silicon Alley” HVPR10-MX12 / 8541 10KV, 750 mA HV Diodes"
	They don't need any "balancing" resistors or capacitors.  "Modern" 
diodes in a string don't need the resistors any more.
	Putting the choke in the CT lead reduces the voltage rating needed for 
the choke.  Don't know if it changes the spike issue at all.
73,

Al, W8UT
www.boatanchors.org
www.hammarlund.info

"There is nothing -- absolutely nothing -- half so much
worth doing as simply messing about in boats"
Ratty, to Mole

On 11/10/2014 8:16 PM, Rick Poole WA1RKT wrote:
>
> 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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