813 modulators
 

My Ranger driven 813 modulator is working very well but now I want to
move on to a self-contained mod.

I have ordered a 15 W solid state amp but need some help picking a
transformer for between it's 4 ohm output and the 813's running as triodes.

I am sure this has been done many times so perhaps someone can save me
the pain of trying to determine the impedance needed for the output of
the trans.

Thanks,

JACK K9ACT

--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

813 modulators
 

On Mar 9, 8:02 am, Jack Schmidling wrote:
My Ranger driven 813 modulator is working very well but now I want to
move on to a self-contained mod.

I have ordered a 15 W solid state amp but need some help picking a
transformer for between it's 4 ohm output and the 813's running as triodes.

I am sure this has been done many times so perhaps someone can save me
the pain of trying to determine the impedance needed for the output of
the trans.

Thanks,

JACK K9ACT

Wild guess: in pentode mode, a PP pair of 813's in AB1 needs approx
160 v peak-to-peak to develop 250 watts of power.

15 watts at 4 ohms is about 20 volts peak-to-peak.

So you need a voltage step-up of 8 or more, round up to 16 for a
little headroom. Don't round up too much or you'll lose some current
ability. If you're looking for impedance ratios they are voltage
ratios squared, so 1:64 (250 ohms, sounds low) to 1:256 (1K ohms,
about right). Note that 600 ohms is a commonish value, you're in luck
if it has a center tap!

As a sanity check, typical 811A (high mu triode) class B pairs take
about 10 watts at 1000 ohm impedance for driving too.

Generally the voltage ratio is more relevant than any impedances
printed on the transformer. If you find a hi-fi or 70.7V PA
transformer that's great, but lots of people have pressed filament
transformers into use too. A center-tap on the output side (even if
it's really the input side!) is really nice, you'll note that
"international" filament transformers have two 120V windings that can
be put in series for 240V or in your case to make a center tap.

My question for you: what kind of modulation transformer do you use to
drive the plate for the 8000? I'm jealous that you apparently had one
just sitting around :-).

Tim.

813 modulators
 

On Mar 9, 8:02?am, Jack Schmidling wrote:
My Ranger driven 813 modulator is working very well but now I want to
move on to a self-contained mod.

I have ordered a 15 W solid state amp but need some help picking a
transformer for between it's 4 ohm output and the 813's running as triodes.

I am sure this has been done many times so perhaps someone can save me
the pain of trying to determine the impedance needed for the output of
the trans.

Jack,

I agree with Tim's comments, and will add just a few words.

One concern I have is that the 15 watt amp will see a load of varying
impedance from the 813 grids. If you are running them Class AB, their
input Z is near infinite until
grid current flows. That reflects back to the 15W amp, which may or
may not affect it.

I suggest putting a resistor across the secondary of the driver
transformer, so the amp always sees a load. The resistor then sets the
impedance of the transformer.

Some back-of-the napkin numbers:

As Tim says, 15 W at 4 ohms is about 20 volts peak-to-peak. To give
some headroom, let's use 10 volts.

Again using Tim's numbers, figure about 200 volts peak-to-peak at the
813 grids. That's a 1:20 voltage step up, or a 1 to 400 impedance step
up.

The output Z is therefore 1600 ohms or so. The resistor should be 1600
to 2000 ohms, 20 W. Could be a bunch of smaller resistors in series-
parallel. The 20 W rating is very conservative, too - 10 W may be
enough.

Besides the transformers Tim mentions, keep an eye out for Stancor
"rectifier transformers". These have been around forever, and have the
numbers ranging from RT-201 to RT-208 or so. They are all the same
windings, the difference is in the power rating. An RT-201 is more
than enough VA for this application.

The RT series are well-documented, common and not too expensive, and
have the desired dual primaries (which you use in series to drive the
813 grids). They also have lots of secondary taps so you can try all
sorts of ratios.

One request for your website: MORE PICTURES!

73 es GL de Jim, N2EY

....that 8000 is just beautiful, btw...

813 modulators
 

Tim Shoppa wrote:

If you find a hi-fi or 70.7V PA transformer that's great....

What does 70.7v mean?


My question for you: what kind of modulation transformer do you use to
drive the plate for the 8000? I'm jealous that you apparently had one
just sitting around :-).

No such luck but I did find someone else by trolling who did. It's a
UTC SS22 and about right for the job but I would feel more comfortable
with something a little bigger.

I only need about 200W of audio but I would like to run it at 2000 volts
to get legal limit out of the final but I am a little reluctant to push
the mod trans. I am currently running at 1600V and everything is happy.

JACK K9ACT


--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

813 modulators
 

wrote:

One concern I have is that the 15 watt amp will see a load of varying
impedance from the 813 grids. If you are running them Class AB, their
input Z is near infinite until
grid current flows. That reflects back to the 15W amp, which may or
may not affect it.


There is some idling grid current flowing.. about 30 ma if that helps
any. This is a zero bias set up with screens and grids tied together.

I suggest putting a resistor across the secondary of the driver
transformer, so the amp always sees a load. The resistor then sets the
impedance of the transformer.

Someone else suggested a resistor in series but I forgot why. Are you
addressing the same problem?


One request for your website: MORE PICTURES!

Thanks but I have been doing a Photo of the Week this for 12 years and
that's about 624 to date. If you just want to fall asleep watching
them, try the slide show.

...that 8000 is just beautiful, btw...

Very satisfying to turn a bookend into the heart of a real transmitter.

JACK K9ACT



--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

813 modulators
 

Jack Schmidling wrote:

Tim Shoppa wrote:

If you find a hi-fi or 70.7V PA transformer that's great....


What does 70.7v mean?


It is a standard method of distributing multiple PA or music speakers
from an audio amplifier. If you start out with a 100 watt amp with a 100
watt/70 V audio output transformer you can run small gauge wire to the
remote speakers. At each remote speaker a 70 V transformer steps down to
the speaker impedance by way of a tap corresponding to the power you
want delivered to the speaker. If you have 10 speakers, use the 10 watt
taps etc.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

813 modulators
 

On Sat, 10 Mar 2007 00:46:14 -0500, **THE-RFI-EMI-GUY**
wrote:

Jack Schmidling wrote:

Tim Shoppa wrote:

If you find a hi-fi or 70.7V PA transformer that's great....


What does 70.7v mean?


It is a standard method of distributing multiple PA or music speakers
from an audio amplifier. If you start out with a 100 watt amp with a 100
watt/70 V audio output transformer you can run small gauge wire to the
remote speakers. At each remote speaker a 70 V transformer steps down to
the speaker impedance by way of a tap corresponding to the power you
want delivered to the speaker. If you have 10 speakers, use the 10 watt
taps etc.

That must be some kind if americanism, since I have only heard about
100 V lines, for distributing audio power at a reasonable level around
a large house,

Paul OH3LWR

813 modulators
 

On Mar 9, 11:52�pm, Jack Schmidling wrote:
wrote:
One concern I have is that the 15 watt amp will see a load of varying
impedance from the 813 grids. If you are running them Class AB, their
input Z is near infinite until
grid current flows. That reflects back to the 15W amp, which may or
may not affect it.

There is some idling grid current flowing.. about 30 ma if that helps
any. *This is a zero bias set up with screens and grids tied together.

Then you don't need the resistor I mentioned.

I suggest putting a resistor across the secondary of the driver
transformer, so the amp always sees a load. The resistor then sets the
impedance of the transformer.

Someone else suggested a resistor in series but I forgot why. *Are you
addressing the same problem?

No. I was suggesting a resistor across the transformer secondary -
from grid to grid. But since your setup is zero-bias Class B, my
concerns don't apply. The 813 grid/screen connection you are using
provides a load
for the driver.

I think the series-resistor idea meant a small (~100 ohm)
noninductive resistor in series with each grid lead, installed right
at each 813 socket, in case of instability. You obviously don't have
that problem.

One request for your website: MORE PICTURES!

Thanks but I have been doing a Photo of the Week this for 12 years and
that's about 624 to date. *If you just want to fall asleep watching
them, try the slide show.

They're GREAT!

...that 8000 is just beautiful, btw...

Very satisfying to turn a bookend into the heart of a real transmitter.

Yep, I know the feeling.

73 de Jim, N2EY