RadioBanter - View Single Post - 813 power amplifier

Andrew VK3BFA · July 8th 06, 04:59 AM posted to rec.radio.amateur.homebrew

wrote:
On Fri, 07 Jul 2006 18:04:10 GMT, "dave" wrote:

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.

If the XFmer is 1000-0-1000 with rectifiers (Full wave configuration)
you can count on around 1550V at 200ma or ~310W DC power.
You can load that higher if the peak to average is in the 300W range.
If you use a full wave bridge you can get twice the voltage and run
the same 200ma at ~2800-3000V for 600W.

The nasty with a 3000V supply is everything has to stand at least
10KV (allow for transients at least 2x or 3x the supply). Also 813s
may arc over at this high a voltage. (use 4-400s!).

The caveat is for sound amps and music overbuilding is a good thing.
Serious overbuilding is sometime required for commercial (stage)
use. Nothing worse than an amp smoking when the perfromer was.
Most I knew back when used multiple smaller amps rather than one
BIG one all driven from one head.

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?

Either way works. follow the recommeded add xxx percent over rating to
prevent the smoke from escaping.

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.

Ok to 250ma and the power xfmer is 200ma so it's a winner to those
limits. You can go past that but then heating and peak to average
considerations apply (and shorter life is possible).

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

It is the way to go. Lots of them! Don't forget all those resistors
are eating up that limited 200ma, they are needed but it emphasizes
the limited transformer your using.

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.

Yes, saves popping mains fuses from power on surges and also is
easier on the HV components.

FYI: the filliments on the 813s must be up to temp before DC plate or
screen power is applied.

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?

Enough to keep the ripple at full load less than 1%. Start around
60-100uf.

See caveat for ~3000V on plates!

Pushpull will help balance out ripple.

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?

One of adaquate current rating with a 50 ohm pot with the wiper to
ground for balance will do. That 50ohm pot is a high power resistor.
The alternate is a 10V center tapped or two 5V transformers.

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

Absolutely. Likely you will need another transformer, rectifier and
filter for that voltage as you only have a 200ma transformer for the
plates so why waste some of it on the screens. Also the screen
supply must not be applied before the plate voltage.

Not needed if you use 811 or other power triode.

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?

Consider that the environment is not only HV but also HOT. Make sure
the wire (and all your parts) can stand heat too.

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?

Fuses should abound. Minimally, mains input, and DC to plate
transformer.

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 -

ENCLOSE everything with HV on it. Finger proof it. and provide
interlocks. Consider cooling as well.

Any and all ideas/tips/etc. most welcome

Reminder 2 813s with only filliments lit are pumping out some 100W
of heat! (10V at 5A).

The tubes should be biased between class AB1 and B for minimum
distortion. that means at idle you likely standing 20-50ma per tube
(another 50-150W of heat!).

You also need a -V (0-100V) negative to bias the tubes.
Another small transformer as trivial current is needed
mostly for bias pot and bleeders.

Mechanical construction is important as with all that iron
(transformers) it's going to be heavy. I mean wheels on this
beast are a must.

You also need a driver and that will have it's own power needs
though only a few watts need be developed. The low level amps
need to be kept far from the power transformers or it will HUM.

Allison

Dave

Have you got the audio output transformer for this beastie? - if not,
get a price before you start - you might reconsider the project.

It is going to be physicaly big, very heavy, and fragile - 813
filaments dont like being knocked around in the back of a truck (and
you will need a truck or SUV to move it) so if its intended to go on
the road, it might be more trouble than its worth.

But, so saying that, ----- nice lunatic fringe project - worth doing,
just for the hell of it, and ignore practical realities......

Andrew VK3BFA.