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

Tue Nov 11 15:11:59 EST 2014

I used a Johnson 500 supply/Modulator to power my HB 572B amplifier ....

I loved the 866 mercury vapor rectifier tube sit was REAL HAM RADIO....they 
were just very cool and could follow my CW ......

When I ran out of 866's I tried 3B28's and they were not as much fun so I 
replaced them with solid state rectifiers.......don't over think this.....

Jim

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
______________________________________________________________
Boatanchors mailing list
Home: http://mailman.qth.net/mailman/listinfo/boatanchors
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Boatanchors at mailman.qth.net
Message delivered to jbrannig at verizon.net

List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com **

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html 