[AMRadio] Re: 4-65's

Larry Szendrei ne1s at neandertech.com
Fri Apr 6 17:00:24 EDT 2007

Brett gazdzinski wrote:
> 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.
Yes, but they're such a cute little toob. Actually I'm still trying to 
decide whether it will be the pair of 4-65s or a single 813.

> 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.

In my (planned) dual 4-65 rig*, I will run a 2500V common supply for 
both the 811 modulators and the RF PA. I've run 811s at 2500V before, 
and as long as you don't exceed plate dissipation ratings, they seem 
perfectly happy about it.

*Actually, I'm still trying to decide whether it will be the pair of 
4-65s or a single 813.

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

> 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.
Or, if you do it that way, flip around the primary and secondary 
(assuming the secondary has a center-tap). Or is that what you mean by 
"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.
The possibilities are virtually endless...

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

> Brett

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