[R-390] SS replacment for 3TF7 - back-to-back Zeners.

[Kenneth G. Gordon]

David Wise wrote:

> There's regulation and then there's regulation.

Absolutely!

>  The shunt clipper
> you describe below is good to, oh I don't know, maybe a few percent,
> which is IIRC slightly inferior to a 3TF7 at the top of its game.

Could be, all right. I haven't compared them too closely. BTW, the 
ballast tube in the SRR-11 isn't a 3TF7, and I can't exactly remember 
which one it is either.

> Why?  Let's say the supply voltage goes up.  The zeners continue to
> lop off the top of the sine wave, but the part they don't, lasts
> longer per cycle and therefore delivers more power to the heaters.

Right. However, in this case, the SUPPLY voltage is 17 VAC.

> An exact answer requires integral calculus.  When ballast tubes
> went out of the mainstream of new instrument design, RCA judged
> the clipper "good enough" for the SRR.

Yup, and it isn't all that stable, either, especially on the upper band 
which includes 32 Mhz.

> 
> There are heavy-duty approaches that can beat this by several orders
> of magnitude.  Will you notice?  Depends, probably not.

As you say, it depends. With the selectivity set at its narrowest, a little 
drift IS noticeable.

In the case of the SRR-11 Zener regulator, I think it regulates well down 
on the more "linear" slope of the sine wave, not at the sharply curved 
top. None-the-less, it is pretty crude, IMHO.

As I said above, the supply voltage is 17 VAC for a 6.3 VAC filament.

In any case, I have not yet tested the regulation, but will when I can.

Ken Gordon W7EKB