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

[Dennis DuVall]

Rick,

Is the rectifier filament transformer in the rig OK.  If so you might consider using a pair of 3B28 rectifier tubes instead of home brewing solid state string substitutes.  The 3B28 works FB as a replacement for the 866.  Not quite as pretty in operation, though.  :^)  Just made a quick check and LOTS of them available on eBay right now at reasonable prices. 

To answer some of your other questions:

1.If the Warrior ran with 1700 vdc on the plates of the 811As then the AC voyage being supplied by the power transformer through a choke input filter would be something around 1900 volts RMS on side of the center tap (3800 volts RMS total). Figure 2000 volts each side side of CT on your plate xformer to be on the safe side.  With a full wave rectifier circuit each rectifier (diode-string or tube) will have to able to handle a peak back voltage across it of 2x1.414x2000 = 5656 volts.  That, in turn means that at least 6, one KV PIV diodes would be required in the string on each side.  Considering the fact that the back-voltage (PIV) rating given for common rectifier diodes is a MAXIMUM value I personally would use at least 10 diodes (iKV rating) IN EACH STRING (and maybe more, diodes are cheap these days as others have pointed out).

2.  As others have said connecting the filter choke in series with the center tap of the HV transformer instead of the center tap of the rectifier filament transformer allows for the use of a unit (choke) with a lower voltage-breakdown specs.  Still a proper “choke input” filter, though, and saves a few bucks.  The filter caps are then connected between the CT on the rectifier filament xformer and ground and B+ (1700V) to the amp plates also comes off the filament CT.  Back EMF spikes have to be addressed  with both arrangements though, see (4) below.

3. Believe this question has been answered by others.

4.  Whether or not one will encounter back-emf (high voltage spike) problems with a choke input filter depends on just how the thing is turned on and off. Sudden changes in the output load will be mostly smoothed out by the filter caps.  The input side is another mater. In operation a series of current pulses are fed to the choke at a 120 cps rate (with a 60 cps primary AC supply) and the energy in these pulses will be  stored, in turn,  in the form of magnetic flux in the core of the choke. Without getting into a prolonged technical discussion here suffice to say that if the input current is arbitrariy interrupted (ON/OFF switch thrown, primary power relay opened) at any instant during which the current input pulse is delivering energy to the choke the flux in the core will collapse, the energy stored therein will emerge as a high voltage “spike" at the input terminal of the choke which, in turn, will have to be dissipated  by some means.  If no special provisions are made for this an arc-over to chassis ground from the choke terminal (or somewhere else in the transmitter wiring) is a typical result and the stored energy is then dissipated (mainly} as heat.  Don’t believe your Warrior is “primary” keyed, though, so you shouldn’t run into trouble unless you do something like throw the main power switch to “OFF” at just the wrong moment while under full load.  As an aside, the T-368 transmitter IS primary keyed on phone and comes with a spark gap across the filter choke (11H, 500ma) to handle just such voltage spikes (Ive seen it fire).

Anyway, Good Luck.

Dennis D.  W7QHO
Glendale, CA

**************

> 
> -----Original message-----
> From: Rick Poole WA1RKT <wa1rkt at comcast.net <mailto:wa1rkt at comcast.net>>
> To: boatanchors at mailman.qth.net <mailto: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 M