Hello,


I am putting together a push-pull power amplifier based on the 813 power
tube. It will have about 1.5 to 1.7kV on the plates and put out at least
260 watts into a 9k load.

I am not new to high voltage tube circuits, but this is the first time I
will be working with potentials this high (above 1kV). I am aware of all of
the usual safety precautions, but I was hoping to get some specific tips for
putting together a design like this from some people who work with these
sort of voltages, hence the posting here.

For the plate supply, I have a Hammond power transformer laying around here
rated at 1000-0-1000 volts @ 200mA. From the ap notes on the 813, I am know
that the tubes will idle at about 50 mA and peak at about 305 mA. Would
this Hammond be ok for this type of use? I dont *think* I would be drawing
300 mils anywhere near continuously, but you never know. This amp will be
part of a bass guitar amplifier for live performance use.

I want to use solid state rectifiers and in studying some older ARRL
handbooks, they tell you to use equalizing resistors and small caps across
series-strung diodes to get up to the peak inverse voltage you need.
However I have seen a newer part in Mouser, made by Rectron, rated at 800mA
and 8kV PIV. Would something like this be all I would need (four in a
bridge configuration)? I would think it would be easier than making all of
those strings up from individual diodes. Or is there a better way?

I want to use a choke-input supply and I happen to have a large old filter
reactor made by Chicago Transformer rated at 20H @ 250mA, with a test
voltage of 6kV.

As for filter caps, I guess the cheapest way to go is to string together
series electrolytics with balancing resistors.

Also I have seen a lot of supplies use soft-start relays in the primary -
would you think this would be necessary in this application? I am looking
to get about 1.6kV out of this supply.

So basically you have a 2000vac secondary feeding a bridge of high voltage
rectifier diodes, then a 20H choke, and then a string of 'lytics. How much
capacitance would be sufficient to elliminate the hum in the plates? I know
that with push-pull and beam power tubes you can probably stang higher
ripple in the plates but is there a good ballpark figure?

Also since the 813 is directly-heated, would it be ok to have one 10v, 10A
filament transformer to light them both or would it be best to have two
separate ones to allow for hum balance -pots or for some other reason?

I also will have to make a screen supply for 750 volts at 45mA max. Would
this have to be a regulated supply?

I had planned on ordering some Belden 18awg high voltage test lead wire
rated to 10kV for all of the plate supply wiring. Would this be ok, or is
there a better wire for this application?

By the way I am having Heyboer custom wind an output transformer for this
thing. It was fairly expensive (around $200) so I want to protect it as
best as possible - fuses, etc - any ideas there?

Basically I'm looking for any ideas that might not be at first obvious, in
terms of providing safety to me, the operator, or the most exensive
components -

Any and all ideas/tips/etc. most welcome

Dave