[ARC5] Receiver Tube Substitution

[Kenneth G. Gordon]

On 17 Dec 2016 at 18:19, Richard Knoppow wrote:

>     Your experiments are interesting. Hallicrafters, of course, modified 
> the RF stage for both bias and for correct plate and screen voltage, 
> which are slightly different. Its likely the input and output impedances 
> are different so the transformers would need some change. The older 
> tubes were pushed up beyond 30Mhz in many receivers but I think it was 
> wishful thinking plus some advertising hype to claim something like an 
> S-20R would go up to 40Mhz. Another problem is mixer noise. The old 
> mixers like the 6SA7 and 6A8 are very noisy tubes and need a lot of gain 
> ahead of them to prevent them from dominating the noise figure of the 
> receiver.  I tried substituting a 6SB7Y for the 6SA7 in an RCA AR-88. It 
> made no difference whatever as a straight across change but, of course, 
> it might if bias and other parameters were changed. In the case of the 
> AR-88 the two stages of RF with 6SG7s and low loss coils on the higher 
> bands results in quite low noise.
>     No simple answers for this one.

Boy! I'll say!

As another example and one which I have mentioned here or elsewhere a time or two 
before this,  many years ago, I was given or traded for an essentially new BC-779, with 
power supply. The BC-779 has two RF amp stages ahead of the 6SA7, which as you state 
is a very noisy mixer. (As an aside, the only mixer which is MORE noisy is the 6/12K8 which 
is used in our ARC-5s).

Anyway, my "new" BC-779 had a very serious problem in that the BFO injection was so low 
that even when receiving fairly weak CW signals, the signal would overcome the BFO which 
made it very hard to copy CW.

Not knowing very much at the time, I thought that perhaps a product detector would make a 
difference, so I modified the 6N7 noise-limiter stage into a triode product detector copied 
from a Heathkit SB-100.

The difference was amazing!

That receiver became, very shortly, my favorite receiver for all ham operations. It was the 
quietest receiver I had in the station, and I could copy stations on it I could hardly hear on 
any other receiver I owned. I used it for years for RTTY too.

Yet that noisy mixer tube was left completely stock.

Ken W7EKB

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