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

[Rob Atkinson]

You can also put in 3B28s instead of silicon.

When you have a B+ secondary CT with a choke, the iron is usually
designed with that in mind, i.e. the insulation is intended to be
above ground with ripple voltage at the CT.  Also the filter choke
leads have to be connected in a certain way for the insulation on the
CT end may be different from what's on the end to be grounded.  Now,
understand I am no expert; I am merely passing along what I have been
told.

The thing with soft starts began I think with capacitor only power
supply filters which came into use with RF amps intended for CW and
slopbucket.  That's because they usually had and have 30 to 50 mfd
total capacitance.   At power on they are fully uncharged and there is
a huge current draw.  What's being protected is the plate supply
secondary winding but there is differing opinions about that.  The
important thing for  you is that with a typical choke capacitor
filter, you probably have only around 8 mfd filter capacitance so it
is not an issue.

What may be an issue is regulation with a mode like CW since you are
drawing a lot of current with each dit and dah, then zero signal idle
current.   Most LC filters struggle to keep the plate v. under control
with CW--constant load modes like RTTY and AM are okay.    I am not
familiar with the Warrior power supply filter so perhaps Heath
designed it to regulate with CW.

Rob
K5UJ

On Mon, Nov 10, 2014 at 7:16 PM, Rick Poole WA1RKT <wa1rkt at comcast.net> 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...
>