[ARC5] Power Supplies for the ARC-5 Rx...and others.

[Kenneth G. Gordon]

On 29 Jan 2011 at 10:10, J. Forster wrote:

> I can see a physics argument that would support your conclusion.
> 
> Part (not all for sure) of the noise in tubes can come from electrical
> discharge in residual gas. This is why Thyratrons are so noisey.
> However, the getter scavanges things like O2 and N2 which are
> chemically reactive, leaving the Noble gases. The major Noble Gas in
> the atmosphere is Argon, which ionizes in the 160-180 V range. Once
> ionization starts, there is an avalanche effect, which would increase
> the tube noise.
> 
> If the HV is dropped below where that happens, the receiver noise
> would go down.
> 
> Following this thread, there seems little point in going below about
> 70 V where Neon breaks down.

Agreed. 

Although when I was experiementing with lower B+ on ARC-5s, I certainly 
made no thorough tests, the "empirical" conclusion at which I arrived was as 
I have stated: that the receivers appeared to work "best" at some voltage 
around 180 VDC.

 As I remember it (this occurred some years ago) I tried regulated 150 VDC 
with a view to increasing the stability of the receiver as much as possible, but 
that did not work as well as raising the B+ above that point by several volts, 
to around 180 VDC.

I was using a very well-hacked BC-453 which I intended to use as a monitor 
receiver in our on-going tests for our 600 Meter Research Group. (A number 
of us are licensed to operate in that band now.)

I suppose, if I have time, I really should do a thorough instrumented test to 
see what really happens with the lower voltage.

>From the discussion here, it sounds as though by adjusting the voltages on 
the oscillator and mixer, 150VDC would have given better results, but I did 
not want to do any modification to the original circuit.

Thank you, John, for your comments. They are always very reasoned and 
thoughtful.

As I have often said, as far as I am concerned, the ARC-5 receivers are the 
finest single-band receivers ever built. 

There are, of course, a couple of places they could be improved by more 
modern technological methods; the mixer stage (too darned noisy), the 
detector, and possibly the AGC, but as it is, they are more than adequate 
today, and served the original designers' purposes perfectly.

Speaking of the mixer stage, I have posted an almost "plug-in" mod to the 
mixer on my website. This mod uses a small dual-triode, and reportedly 
reduces mixer noise by a substantial amount. I did not design this.

Here is the link:

http://www.w7ekb.com/glowbugs/Military/ARC5MixerMod.pdf

Some here may also find this of interest:

http://www.w7ekb.com/glowbugs/Military/arc5pages.htm

Also, this may be of interest:

http://www.w7ekb.com/glowbugs/rx/receivers.htm

and click on 11) The Novice Q-5er down at the bottom of the page.

Ken Gordon W7EKB