[Milsurplus] BC-348 Power Supply/Biasing

Sun Jan 2 15:04:20 EST 2022

MikeAl is right, I missed the effect of the inductor's resistance on the bias circuit voltage and then totally missed a second 50 ohm resistor that is in parallel with the first 50 ohm resistor.  This second resistor is located near the 1st detector on the Q model schematic.  But to answer your question, the high voltage current does flow through the total resistance used to develop some 18 volts of negative bias for the grid of the 6K6 audio amplifier.  This contradicts the -8 volts for G1 of the 6K6 as shown in Table C of the BC-348-Q manual but this discrepancy is explained below.
B- is eventually connected to the chassis, what you are calling "ground" but, after a closer look, it is through the resistance of two 50 ohm resistors in parallel (Items 108-1 and 108-2) plus the resistance of the choke that is, as Al has mentioned, part of the audio output transformer.  The two parallel resistors drop approximately -2 volts and the Inductor resistance of about 250 ohms (I measured 235 ohms on a NOS audio transformer) drops another -16 volts.  The parallel 50 ohm resistors, Items 108-1 and 108-2 bias G3 of the 6SA7 pentagrid converter (aka 1st detector) at -2 volts (-1.8 volts according to the voltage chart in Table C of the manual) and this voltage plus the inductor resistance bias G1 of the 6K6 audio power amplifier at around -18 volts.

Going further down the rabbit hole, there are two ways to bias a tube grid.  Self Bias and Fixed Bias.  There are at least two subgroups to these bias techniques.
One method of self bias uses a cathode resistor to produce a negative grid bias with respect to the tube's grid.  Since the tube's grid is referenced to the circuit common by a high value resistor, the grid appears to be negative when the grid voltage is measured with respect to the tube's cathode instead of the chassis or common ground.  Another form of self bias is the grid leak resistor method in which a negative bias is produced by the rectification of the signal voltage applied to the grid by the grid to cathode diode.  This charges the grid coupling capacitor to a negative potential which is also applied to the tube's grid.  Either self bias technique is the preferred bias method since it reduces the chances of high plate current if the tube is somewhat gassy or has another minor defect.
Fixed bias can be produced by either a separate negative power supply or it can be produced by a voltage divider placed in the negative lead of the B+ power supply.  This is how the grid bias is derived in the BC-348 series.  The negative side of the B+ supply current flows through approximately 300 ohms of resistance to produce about -18 volts of grid bias with respect to the chassis common for use in the 6K6 audio amplifier.  This bias voltage is isolated from the grid circuit of the tube by some high value resistors.  This is done to prevent any interaction between the audio amplifier and any other stages in the radio.  This isolation resistance is comprised of three resistors:  Item 96-1, at 50k, Item 87-5 at 1 meg and Item 91 at 250k.  
Measuring this bias voltage is a problem because if an analog VOM is used to measure the voltage it will pull a current from the bias supply and change the tube's bias voltage.  This is evident in that the tube manual suggests a grid bias of -18 volts but the measured voltage in Table C is listed at -8 volts.  A note on the voltage chart says to use the 500 volt range of the VOM to measure the 6K6 grid bias.  It is tough to measure about 8 volts on the 500 volt range of the meter but this ensures that the ohms-per-volt is sufficiently high that the bias voltage is not too greatly affected by the VOM.  If you use a DMM then you should read the true -18 volt bias potential.
Let me know if this has cleared up the confusion or just made matters worse!
Regards,Jim

-----Original Message-----
From: Michael D. Harmon <mharmon at att.net>
To: milsurplus at mailman.qth.net
Sent: Sat, Jan 1, 2022 9:11 pm
Subject: Re: [Milsurplus] BC-348 Power Supply/Biasing

Maybe I'm over-thinking this issue!

All my experiments to date have been with the freshly-built power supply 
just laying on the bench, fed with suicide clips.  I have not installed 
it in the radio.  There are several things I need to fix in the radio 
prior to actually applying power.  According to the documentation, both 
the original dynamotor and the Hallicrafters EP-298 both provide a 
filament winding (with one side grounded) and a high-voltage winding 
with the + and - connections floating.  Since the B- power supply output 
is NOT grounded to the chassis (and everyone says NOT to connect it to 
ground), then how can I use the chassis as a common point to measure the 
various voltages in the radio?  There has to be a "mystery connection" 
there somewhere!

According to the schematic, the B- terminal on the power supply feeds 
terminal 6 on the output transformer (choke winding).  The other end of 
the choke is at terminal 5, and from there, there is a 50 ohm resistor 
to chassis ground.  When I jumper the B- output to terminal 6, and 
jumper terminal 5 through a 50 ohm resistor to the frame of the 
transformer, and turn on the power, I get the B+ voltage between the B+ 
and B- terminals on the power supply, but also all the way down the line 
to the resistor - no change.  I have no idea as to how much current 
needs to be drawn from terminal 5 to read 12~18 volts bias.

Like I said before, I'm missing something here.  Maybe it's just my 
"old-timer's disease" catching up with me, but there's something I'm 
just not seeing here!

BTW, the graphic sent by Al Klase did NOT come through.

Thanks!
>
> Mike Harmon, WB0LDJ
>
> mharmon at att dot net
>
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