[ARC5] [Milsurplus] BC-348 PSU questions

[Ian Wilson]

For the record, the DC resistance of the audio choke is about 300 ohms. The
developed negative bias is used only for the audio o/p tube grid, as far as
I
can see on the schematic.

73, ian K3IMW


On Tue, Sep 29, 2015 at 6:24 PM, Roy Morgan <k1lky68 at gmail.com> wrote:

>
> On Sep 29, 2015, at 8:50 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:
>
> >>
> >> NOTE WELL: For an AC supply in this receiver, the negative side of the
> >> supply must be left floating.  It goes to ground through a bias filter
> >> network which is part of the audio output transformer (I hope yours
> still
> >> has the original transformer...?).
>
> The B + filter choke (actually the B MINUS choke) is inside the audio
> output transformer case - it is not electrically part of the output
> transformer.  The resistance of the choke carrying the normal receiver
> plate supply negative lead current creates the bias applied to the audio
> output tube (the bias may well be used in the AGC circuit or else where - I
> can’t remember).
>
> IF the B Minus lead of the supply (home brewed or dynamotor) is grounded,
> there will be no choke in the circuit and no bias on the audio output
> tube.  This lead to many ruined output tubes due to excessive plate
> current, and very bad audio in BC-348’s.
>
> If there is no choke in the circuit, or it is shorted, I would expect hum
> in the audio, especially if the B+ supply is not otherwise filtered. Two
> factors would contribute to the hum: excessive plate supply load, and lack
> of adequate filtering in the supply voltage.
>
> Testing with an external supply that has a floating minus connection (as
> do many Heath variable, regulated supplies) is a very good idea.  Just make
> sure the “floating” negative connection goes through the choke., or provide
> the needed negative bias some other way.
>
> Knowing the nominal DC resistance of the choke and the approximate plate
> supply current would lead you to the expected value of bias voltage.  Of
> course the manual may well have that information.
>
>
> Hooray for another BC-348 being in the hands of a caring owner!
>
> Roy
>
> Roy Morgan
> k1lky68 at gmail.com
> K1LKY Since 1958
>
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