1) The power supply transformer has a centre tap ,hence is meant to be a
1000 V -200 mA ,hence 200Watts supply device. When you would use it as a
2000V-AC device the primary winding and or the 'iron' will highly
likely be inadequate to produce 400W of power.
So the transformer will NOT be capable of supplying 1600* 0.305 equals
490 Watts. You will need a transformer which can supply at least 2.5
times the power you get from your available transformer .

2)Depending on the transformer voltage available say 1300 V the max
rectified DC voltage (due to the capacitor(s) will be 1.4* 1300 equals
1820 V ,hence the PIV of a bridge diode string would have to be at least
twice as high ,with a safety factor ,say 4000 if not 5000 V
It depends of the type of diode you use but if it is one with a PIV of
1000V you'll need 4 to 5 diodes in a bridge arm string.
Modern avalanche diodes are known not needing parallel capacitors or
resistors . The Recton diodes are highly likely a molded set of lower
PIV diodes . You could find out how many diodes are in series by
applying a low forward bias dc current and measure the voltage across
the module At approx 0.7 V across each diode you then can determine the
number of diodes in the module. You obviously can also use a string of
1N4007 1 Ampere diodes each having a PIV of 1000 V

3)The choke designed for 250 mA might be OK ,although is somewhat
marginal .Its resistance will cause a drop of the DC voltage hence in
order to get the required final DC supply voltage at max load the
transformer voltage has to be adequate . It also means that you have to
determine how much 'voltage sagging' is permissable.

4) If you use a string of electrolytic filter capacitors you need a
sufficient number to suit the rectifier bridge output voltage multiplied
by 1.4 and should apply a decent safety factor. Assuming you use
identical caps ,the overall capacitance is of course the value of 1
divided by the number of caps .
You have to pay much attention to the equalising resistors taking into
account both their voltage and power rating .
A well know RF Power Amplifier designer ,AG6K , recommends MOF =Metal
Oxide Film
resistors , although often one sees high power wire wound resistors on a
ceramic core being used.

5) You also need to decide in which operating Class the valves will have
to work in view of the acceptable distortion level of the amplifier.
Class A would be best but a waste of energy. In the intended push pull
configuration Class B would be OK.

6) The screen voltage of both valves will have to be stabilised

7) You also have to ensure that the valves are being cooled ,by means of
one or more small fans. A good method is to have an airtight
under-chassis pressurised by a small centrifugal fan and the valve
sockets fitted just below the chassis (on stand-offs)such that the air
flows along the valve bases and up. It might also be useful to apply
anode connectors with cooling fins.

8) If amplifier enclosure has a hatch giving access to High Voltage
carrying components it is essential that when the hatch is opened a
microswitch opens ,cutting all external power if left on. Normally of
course power should be cut (power supply plug pulled) and then ,only
after a few minutes, the hatch should be opened, because of retention of
charge by the filter capacitors for some time (depending on the value of
the voltage equalising resistors).

These are just a few of many aspects to consider when building a valve
power amplifier.

Frank GM0CSZ / KN6WH
=================================================
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