[ARC5] PS Hum in BC-453-B

[Ian Wilson]

Reposting my approximate calculations. I have added a note ([1] below).





*Robert is correct about the input trimmer: this is in parallel with the
tuned circuit,and the antenna connects to that through a small fixed
capacitor. My bad. Theimpedance of the input at resonance is not determined
by the grid leak resistorof the first tube, but by the parallel effective
parallel resistance of the coil, namely2pi * L * Q *[1]

*. Guessing that the tuning capacitor is around 300pF at 500kHz, thenthe
coil inductance will be on the order of 350uH. With a Q of 50 the
equivalentparallel resistance will be around 55k ohm*.

So the effective input circuit will look like 8.5pF in series with about
55k ohm
(to ground).

[1] at the frequency of operation of the BC-453, we can ignore the real
component of the complex input admittance of the RF amplifier pentode. At
higher frequencies, various losses and effects such as transit time and lead
reduce the effective input resistance of tubes radically.


73, ian K3IMW

On Fri, Jan 30, 2015 at 9:16 AM, Kenneth G. Gordon <kgordon2006 at frontier.com
> wrote:

> On 30 Jan 2015 at 21:54, Brian wrote:
>
> > The antenna input circuit of a Command Rx is very high impedance;
>
> I thought at one time we had either calculated that, or found it in a
> reference,
> and that the value was 4K ohms. Am I in error?
>
> > mismatch. If you must use coaxial cable to bring in your antenna signal,
> then
> > use a small unun to step up the impedance to something near that of the
> Command
> > set's antenna input.
>
> Have we yet come to the design of such an un-un? I think not.
>
> I would love to see a practical example of a 4K to 50 ohm un-un.
>
> The way I have operated is that if the front panel "Align Input" makes a
> substantial and obvious difference in signal strength (as noted by ear),
> then
> the match is "adequate". Am I wrong here?
>
> > The likelihood of mains hum transfer from the overhead mains catenary to
> any
> > length of horizontal antenna is small and calculable. However, the
> likelihood of
> > antenna-borne mains hum getting through the RF amplifier, the inter-stage
> > transformer and the IFTs, is negligible.
>
> Yes, therefore, the hum "path" must be something in the AF system.
>
> > Please remember that the originator of this stream told us that the hum
> started
> > immediately he turned on the power supply, which was well before any of
> the
> > tubes had started conducting. Therefore, the hum was coming from the B+
> line
>
> Not necessarily: in my experience such hum can come from such an unlikely
> source as the connections at the AF gain pot, if such exists. In his case,
> his
> hum may very well be coming from some source on the output side of his
> audio transformer.
>
> > All this talk of radiation and capacitive pickup is very interesting,
> > but quite irrelevant to the problem as originally posed.
>
> Well, I agree with this statement...
>
> Has Les yet determined the source of his hum?
>
> None of my receivers exhibit noticeable hum output, even at extremely low
> audio output levels.
>
> But, of course, I am using "Low-Fi" military headphones, either ANBH-1 or
> R-14 based 'phones.
>
> The only time I attempted to use modern hi-fi headphones with any of my old
> receivers (in this case, a Heathkit HW-16) 60 Hz AC hum was VERY
> noticeable. When I switched back to my ANBH-1-based 'phones, there was
> none to speak of.
>
> Ken W7EKB
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