[Hammarlund] Submini tubes and the SRR-11/12/13 and FRR-21/22/23

[Kenneth G. Gordon]

On 1 Oct 2014 at 16:02, James A. (Andy) Moorer wrote:

> I might remind folks that the FRR-21, 22, 23 (and SRR-11, 12, 13) used 
> those &$%##! Raytheon subminiature tubes:
> 
> My cursing is not about the tubes, but is about the fact that they 
> soldered them to the PC boards in the modules on those radios.

One of the things I found somewhat....um....really stupid....about the design 
of those radios (I must have 25 of them around here, now) is that the 
designers applied the same voltage to the screens as they applied to the 
plates of all the pentodes and tetrodes in the receiver.

This made the tubes, and the receivers, run hotter than heck! 

Back in the 1970s, I was working with a fellow who was doing VLF 
propagation research. He had bought dozens of SRR-11s (and a few each 
SRR-12s and 13s for parts), and had them set on the Navy VLF station 
frequencies, 24/7/365, recording the output. In that service, they weren't 
particularly reliable.

I tested the tubes and the circuits and found that most of the tubes were 
operating very close to their maximum plate dissipation ratings. I felt this was 
unnecessary.

Just as an experiment, I removed all the modules from one SRR-11, one at a 
time, and installed 56K 1/2 watt bypassed "screen droppers" in every module 
that needed them. This reduced the screen voltages to something more 
reasonable.

The receivers became very noticeably cooler and far more reliable. Other 
characteristics (selectivity, sensitivity, stability) didn't appear to change, 
although they seemed to be quieter.

I can't remember what I did about the triodes, but I also reduced their plate 
dissipation where it was necessary.

I really like those receivers. The dials are very interesting, and the receivers 
are good ones...when they have been made a bit more reliable. They are 
very stable, quite sensitive and selective, and the dial readout is very 
adequate.

As Guido pointed out, the main problem with them are the little cranks that 
operate the band and mode switches. The early ones were made of some 
sort of pot-metal and crack and break. Later ones were much better. 

There is a fairly easy-to-make replacement for those broken ones, though, 
using 1/4" square steel rod. I have drawings of those if anyone needs those. 
Trouble is, you need so many of them for each receiver. Each one has to be 
made by hand, and a drill-press makes the job far easier. I don't have a 
drill-press....yet.

Another problem that has cropped up is failure of the RF, HFO, and Mixer 
coils, despite the fact that they are sealed.

A Morris Coil-Winder makes fixing those less tedious, though.

I have a Morris Coil-Winder and all the cams...

Ken W7EKB