[ARC5] Modified BC-454

[Neil Barnett]

Hi Brad

----- Original Message ----- 
From: "Brad Hernlem" <alihernlem at hotmail.com>

> I hope that it is OK to enquire about mods on this list. I have a beater
> BC-454 that I am cleaning up and surveying for functionality. It has been
> through a lot of turmoil in its tortured life. Amongst other things (many)
> there appear to be changes to the bias of the 1st tube and the audio
output
> tube. Specifically:
>
> a) the cathode resistor of the first 12SK7 has been shunted with a 390 ohm
> resistor (original spec is a 680 ohm resistor). The shunting resistor
looks
> of the same type as the rest of similar size on the rectangular terminal
> pads.

Actually, the original spec for the cathode resistor (R-1)
was 620 ohms, 5 % tolerance, 0.5 watt, composition.

I do not know why this mod has been carried out.
I suspect that the reduction in value was not primarily
related to a need for a change in the tube bias conditions,
but was rather a reduction in the RC time constant of the
receive-transmit switching, to make this first stage cut off
more promptly at the changeover from receive to transmit.



> b) the cathode resistor of the 12A6 audio tube is changed to a power wire
> wound 400 ohm unit. Original small (1.5K?) resistor clipped from terminal
> pad.

The original spec for this resistor (R-21) was 1500 ohms,
10 % tolerance, 0.5 watt, composition.

The value of 1500 ohms is NOT an appropriate value when
the 12A6 is required to drive a loudspeaker load consuming
several hundred milliwatts or more.

In airplane service, the 12A6 was driving headphones, which
consume minimal power, and therefore the 12A6 did not
need to deliver much power. The high value of 1500 ohms was
chosen so that the tube drew much less current than it normally
would draw with correct bias under normal circumstances.

The 400 ohm resistor is the correct value when a loudspeaker
is being driven, based on the following calculation:

According to the 1963 ARRL Handbook, the 12A6 delivers
up to 3.4 watts of audio when it is biased at a grid-1 voltage
of 12.5 volts.
At this bias voltage, the tube draws a combined plate and
screen current of 33 mA or so. The Ohms Law calculation is
12.5 divided by 33/1000, which gives a cathode resistor of
378 ohms. The value of 400 ohms appears to be the nearest
old-style value to that value. In modern times, 390 ohms would
be entirely suitable.
The resistor is dissipating about 0.4 watts in these conditions:
12.5 volts  x  33/1000  =  0.4125 W, so it makes sense to use
a wirewound resistor rather than a half-watt carbon version.

You can see that 33 mA is a considerable imposition on the
aircraft power system, so it made sense to reduce the 12A6
current consumption when only minimal power was required
for headphone service. This had the added advantage that the
12A6 ran cooler, was less stressed, and lasted longer.

To sum up, if your 12A6 output transformer has been changed
to a loudspeaker type and you're using a 'speaker, 400 ohms
is the appropriate value. Retaining 1500 ohms in conjunction
with a 'speaker transformer and loudspeaker usually results in
weak audio output.


> c) the grid 1 resistor of the 12A6 is changed to 500 K from 2 Meg original
> spec.

The value of 2 Meg appears to have been chosen so that the
diode detector/final IF transformer was not unduly loaded,
which would have led to a widening of the IFT bandwidth.
However, 2 Meg is not an appropriate value when the 12A6
cathode resistor has been changed in the manner described
above.

With any tube, a high value of grid-1 resistor can lead to the
effect known as "contact-potential" bias. In basic terms, a tiny
current always flows from cathode to grid, even though the grid
is negatively charged. Even a current as low as one micro-amp
will develop a change in the bias of two volts if the grid resistor
is 2 Meg.

This effect becomes much more likely when a power output tube
like the 12A6 is being operated at high plate current in order to
achieve maximum power efficiency. For this reason, power tube
output stages seldom use grid resistors greater than 500 K ohm.
The change of the 12A6's cathode resistor to 400 ohms greatly
increased the possibility of this effect, and the reduction in value
to 500 K is therefore a wise choice.

This tiny current flow is, of course, a source of noise. The higher
the value of grid resistor, and the greater the tube emission
(power output tube), the more noisy the stage will be.


> Sounds like these were attempts to "improve" the output level of the
> receiver. Do these mods sound familiar? Mod a) almost looks "stock", as if
> it were a revision change by the manufacturer. The others are plainly done
> by someone else. Can anyone shed some light on these changes?
>
> Brad
> KG6IOE


73 de Neil, ZL1ANM
in Auckland.





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