[AMRadio] Building

Geoff/W5OMR ars.w5omr at gmail.com
Sat Dec 1 20:46:02 EST 2007

jeremy-ca wrote:
> Old Handbooks, especially the "Radio" version have extensive charts of 
> the PP load of most tubes used a modulators and you already know that 
> the VT-127A's are basically 100TH's so you can easily determine their 
> load impedance. If you loaf them at 500W input using say 1700V @ 300ma 
> for instance (rounded off) the mod xfmr secondary must be close to 
> 5666 Ohms.

A fine and fitting job for the RCA 1:1 modulation transformer, with 
5,500 ohms on primary and secondary.

> Then knowing the PP load of the modulators and the turns ratio of the 
> xfmr you can determine what works.

If you want to get 'close enough', then, at 1500v, run 4 100TH's in 
push-pull parallel in the modulator for effectively 4400 ohms, 
plate-to-plate impedance, according to the Eimac tube data sheet.  
According to it, a pair of 100TH's in AB(1) has a plate-to-plate load 
impedance of 8800 ohms.  That's kinda high to work into a 5500 ohm 
transformer.  4400 ohms is -much- closer.

Driving your AM Rig without a scope, 
is like driving your car at night, without headlights. (K4KYV)

73 = Best Regards,

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