[Hammarlund] Ballast tube replacement....

[Kenneth G. Gordon]

On 6 Jul 2013 at 11:26, Charles Ochs wrote:

> Of course all these old radios will suffer from changes in AC input
> voltage to some extent.The R-390 (dreaded 3TF7)and the HRO-60 (dreaded
> 4H4-C) included current regulator "ballast" tubes in an attempt to
> minimize this problem.

Some time ago, while refurbishing a couple of SRR-13s, I ran across an 
RCA-designed replacement for its ballast tube. I had never seen one before, 
and was really intrigued by it. 

It consists of two back-to-back zener diodes, arranged/biased so that the 
output is a modified-sine-wave. It outputs 6.3 VAC RMS using an 
approximately 14 VAC RMS input. Input voltage can vary a lot more for this 
circuit with no change in output, than the old tube-type ballast tube could 
handle. But there is less heat produced, and it is far more reliable.

What it essentially does is to keep the peak of the sine-wave constant no 
matter what the input does.

Receiver input-voltage fluctuations and their effect on the HFO or the BFO 
tube's filaments are thereby pretty much eliminated.

The circuit is very simple, requires very few parts, and works extremely well.

I intend to use this circuit in those of my receivers which need it when I finish 
restoring them.

I have seen other attempts to replace the ballast-tubes with solid-state 
circuits for other receivers. Most of those are, to my mind, much too 
complicated.

I will be happy to send a PDF of the schematic to anyone who asks for it.

Ken W7EKB