[AMRadio] Mod xmfr?

John Coleman, ARS WA5BXO wa5bxo at pctechref.com
Sun Oct 10 12:18:54 EDT 2004

The load that is represented by the final is Ep/Ip = load impedance in

The tube manuals gives the proper plate to plate load impedance for a
given voltage and class of operation for your modulators.

The impedance ratio for your XFMR would be the 
(PTP impedance) : (final EP/Ip) 

Turns ratio would be sqrt of impedance ratio.

Example  - only examples not necessarily real values for 813s

Push pull modulators need plate to plate load of 10 K ohms

Final amplifier is 1500 volts and .300 ams = 5000 ohms

Z ratio = 2 : 1

Turns ratio = sqrt of 2 = 1.414 : 1

Where the modulator side has 1.414 times as many turns as the final

This can be tested by putting 10 volts AC in final the side and
measuring 14.14 volts on modulator side.

Some times it is easy to compensate for ratio differences by raising or
lowering the voltage on the final while maintaining the same power

For example suppose you could not find an exact transformer of Z = 2 : 1
But you found one that was 1.5 : 1

This means the final would need to have a higher voltage and less

10 K ohms / 1.5 = 6667 ohms

Consider raising the plate to 1700 and lowering the current to .255 amps

This would get the impedance up to where it is a match for the
XFMR/modulator yet still run about the same power. 

The turns or voltage ratio would be sqrt of 1.5 = 1.22

Good questions Steve


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of
StephenTetorka at cs.com
Sent: Sunday, October 10, 2004 7:12 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] Mod xmfr?

Hi all:

I need to better understand the workings of a plate modulator for AM.

Considering the RF amp with 2 x 813's and the modulator with 2 x 813's -
are the equations and math to work out the required wattage, impedance,
ratio etc.

And, just how much can one depart from the ideal arrangement and with
effects? ( should I not be able to find the right xmfr for this amp I'm
to build ).

PS: I am seeking that modulator xmfr at this time.

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