[AMRadio] Re: 4-65's

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Thu Apr 5 20:29:20 EDT 2007

These days, you typically build what you have or can get
parts to complete.
4-65's seem a poor choice, high voltage and low power.

If you were to build an RF deck that ran 3000 volts, its best to
build a modulator that runs 3000 volts, otherwise the mod transformer
has a huge voltage difference between primary and secondary.

You would also likely have a very large impedance difference
between primary and secondary.

Say the 811's run 1500 volts, that's 12.4 K ohms, run them at
1200 volts and its around 10K.
The 4-65 at 2500 volts and 110ma is at 22.73K ohms.

Most setups have it the other way around, with the modulator being higher
than the RF side.
You would need a multimatch mod transformer so you could turn it around.

I have built a pair of 813's modulated by 4cx250b's, works good,
2000 volts on both.

I built a push pull 812 rig modulated by 811's, worked very well.
1500 volts on both...

I also built the 3x 4D32 rig modulated by 811's, the 4D32's run
1250 volts, same as the modulator. Works good.

What the 4-65 wants is a pair of 75TH's, very rare tubes!


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