[ARC5] [Milsurplus] BC-348 PSU questions

[Roy Morgan]

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