[Collins] Power level ratings for ART-13 modulation transformer

[Dr. Gerald N. Johnson]

On Sun, 2006-06-25 at 09:36 -0700, C E wrote:
> Hello to All:
> I just wanted to know if using an ART-13 transformer
> would be feasible in a modulation circuit using a pair
> of 811A's modulating a pair of 812A's. 
> 
> The approximate Z of the ART-13 final plate was around
> 7300 ohms while the pair of 812A's is about 6500. 
> 
> I'm more worried about the power levels associated
> with the 812A circuit. The amp is rated at 500 watts
> input while the ART-13 mod xfmr usually ran around
> 100-150 watts on the 813. Also, my modulators will be
> using 811A's vice 811's. Any significant differences
> to worry about here? Thanks in advance. 
> 
> Cal, n6KYR/8
> 
Based on the physical size of the ART-13 modulation transformer, I'd say
it was rated at no more than 75 watts of audio, not the 250 it will take
to modulate 500 watts input power of the 812As. Even then at 75 watts
they probably had to take care to raise the audio LF passband with small
coupling capacitors partly to protect the transformer and partly to
discriminate against aircraft engine rumble.

The other thing is core saturation from the unbalanced PA plate current
in the modulation transformer secondary. Surely the 812As are going to
draw 3 or 4 times what the ART-13 PA did. You could stretch the power
rating (and reduce the saturation and the DC dissipation in the
modulation transformer) by using a separate choke in the PA plate lead
to carry the PA plate current, with a suitable coupling capacitor to
couple only audio from the modulation transformer. This was standard
practice in the BC industry. Probably 5 to 10 Hy in the choke, 10 to 20
mfd in the coupling capacitor.

Then you need to watch the secondary winding DC voltage to ground. There
was only about 1Kv in the ART-13, you will want to be running more to
get the 812As to draw enough plate current for 500 watts input.

I think you need to run lower power or find a bigger modulation
transformer. Except for the unbalanced DC in the PA winding, sometimes a
power transformer has the right turns ratio and enough core to work.


-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer