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

[J. Forster]

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.

FWIW,

-John

===============


> On 29 Jan 2011 at 6:45, Military1944 at aol.com wrote:
>
>> In a message dated 29/01/2011 05:06:28 GMT Standard Time,
>> vk2bcu at operamail.com writes:
>>
>> I'm  dithering between a B+ supply value of 60 and 120 volts.  I don't
>> think  90 to 120 volts stresses the set much, and it may work a little
>> better at the  higher voltage.
>
> In my experimenting on this matter, I determined that the receivers worked
> best at a voltage of between 160 and 180 VDC.
>
> IMHO, 250 VDC is really too much, and not necessary even when the
> receivers were new.
>
> Now.... I have mentioned this next in the past, but will reiterate it here
> for
> your benefit.
>
> There was an article in one of the ham magazines some time ago, possibly
> as much as 30 years ago, in which the author, using a Drake 2B lowered the
> plate voltage in steps, while carefully documenting the results of careful
> tests
> during the procedure.
>
> He found that the Drake 2B continued to operate just fine down to as low
> as
> 12 VDC B+, although the audio output power was severely reduced at that
> point.
>
> The most important of his findings was that internally generated noise was
> drastically reduced as he reduced the HV, and a small reduction in HV
> resulted in a much greater reduction in noise. In other words, the effect
> was
> not linear.
>
> I distinctly remember some of his words: he said that at one point in his
> reduction series, he turned the receiver on, let it warm up and thought it
> was
> dead, as he could hear nothing at all on the 20 meter band....until he
> tuned
> in a signal, when that suddenly "popped up" to full clarity. The receiver
> noise
> was simply at such a low level he THOUGHT it was dead. It most certainly
> was not.
>
> He finally raised the B+ back up to about 50 VDC and that is where he left
> it.
>
> His contention (and others' I have read) was that receiver B+ was simply
> too
> high, having been chosen to be 250 VDC mainly because "that is the way we
> always did it." and it had become an industry standard for no really good
> technical reason, except perhaps for audio amp stage power output, and
> that
> with modern tubes and components, much lower voltages would be better.
>
> Both National and Collins used voltages around 180 VDC in some of their
> later receivers.
>
> Ken Gordon W7EKB
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