[AMRadio] Pw Supply

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Wed Dec 6 20:57:19 EST 2006

> > Why the big hurry, wait to find a good transformer,
> > they are around.
> I have wanted one of these since 1955.  I have waited long enough 
> besides, at my age, time is at a premium.

Looking for a transformer since 1955, or a power supply?

> > What ratings are you looking for?
> Good question.  Actually, I was going to post a question so I could 
> answer that one.
> I can't make any sense out of the 813 specs, either in the 
> Handbook or 
> the data sheet I got on line.
> As near as I can tell, the "typical" voltage for AM is about 
> 1300 but I 
> see the Maul talking about 2000.  I see plate current at 
> about 150 but I 
> see elsewhere 250.  I am sort of confused but think that 
> 2000v ct at 1/2 
> amp would do the job.

In plate modulated AM service, the 813's will run nice
at 2000 volts on the plates at 400ma for 2 tubes.
About 600 to 700 watts of carrier output, 3000 watts pep
is easy if you have the audio power to drive them.

You can drop the voltage or current and just get less
power out. You can match them to the mod iron that way
if you don't want maximum power out.
The 813 is very rugged, I have run them at 2500 volts
and 500 ma for short times, they run 2000 volts
and 400 ma all day long without problems, and they don't
seem to go bad or wear out, I have had the same pair 
in for the last 15 years...

For the modulator I use a pair of 4x150a/4cx250b tubes,
600 watts of audio out at 2000 volts in AB1, no driving power
needed, very clean audio..

> > Is it supposed to fit on the deck over that big hole?
> Roger.
> > Fair radio used to sell a nice oil filled Collins
> > transformer, 2250 volt ct at 650ma, 110 or 220 primary.
> > With a variac that would give 0 to 1500 volts.
> I don't see anything like that on their site but I will  
> check further.

Lots of stuff is no longer in the fair radio catalog.
But lots of people bought the stuff they used to have,
so its floating around.
The Collins iron would be on the low side voltage wise,
and light on the current, but would work at lower powers.
1200 volts at 600ma, you could run 1200 volts at 300ma
on the RF deck, and modulate it with 811A's in zero bias.
But you would want a separate voltage output from
the power supply, through a swinging choke for the modulator.

> > Its always much better to go with choke input, much better
> > voltage regulation, much lower startup surge current.
> > With a cap input, the voltage will increase quite a bit,
> > but will sag big time under a load.
> Good info.  I read somewhere about needing some sort of step 
> start and 
> mine has a time delay relay but it doesn't seem to have 
> anything to do 
> with charging caps.  It connects to something in the RF deck which I 
> haven't run down yet.  It has a high/low switch which applies 
> either 110 
> or 220 to the primary of the HV trans.  Would this be 
> adequate step start?

You can do that I suppose, a time delay relay that
switches between 220/110, I just use a resistor, like
5 to 10 ohms, the relay shorts the resistor out.

> > What are you going to run off the supply?
> > 8UF is not very good for a modulator, I always go
> > around 40UF for energy storage.
> Unfortunately, I want to run the modulator and rf deck from 
> this supply. 
>   That would be 813's/811's.
> This package I bought was a linear so that could explain the 
> too low cap?

I don't think the supply would be big enough.
ssb amps are light duty, even the real big ones are light
duty. The peak current could be over 700 ma, constant current
will be 400 ma for the RF deck, and the modulator resting current
of about 40 ma, and the 811A pulls about 350ma peak.
Plus the bleeder resistor, say 100ma, that's 850ma 
on peaks, 540 ma under no modulation.
The transformer, choke, rectifiers have to be able to take
that current and duty cycle.

Its lots better to have separate supplies for rf and modulator.
You can run different voltages, use a swinging choke 
on the modulator power supply, and parts are easier to get
when they are not huge.

With the parts you have, I suspect it would run
a single 813 with 811A modulators at about 200 watts out.

Amplifiers also tend to have low voltage plate tuning caps,
but in AM service, the voltage doubles under modulation, and
really, on peaks can get to 3 times the plate voltage.
You can use a 2000 volt cap on an amp running 2000 volts, 
but need a 6000 volt one under plate modulation to be safe.

Most amplifiers can run 1/4 the power of their ssb
rating on AM, and I would guess that is so even if
you plate modulate it, its just not good for the power.


> js
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