[ARC5] The Search for a Retubed, Stable BC-375 Oscillator

[AKLDGUY .]

> Here is a diagram of the BC-375 oscillator in
> its native condition:
> http://home.netcom.com/~arc5/375osc.jpg
>
> This is my first idea- to open the feedback to the
> OSC grid and feed to Aux Osc. in there.
> The original OSC tube will act as a "buffer/amp."
> http://home.netcom.com/~arc5/375AuxOsc.jpg
>
> However-it's a triode.  Neutralization might be
> an issue.  If so, how would you accomplish it?


The osc tube is currently running in Class C. The grid is driven hard from
the lower end of coil 801 via capacitor 803 and resistor 1103.

On half-cycles when it's driven positive, grid current flows and the
resulting
DC bias voltage is developed across resistor 1103 which sets grid current.

This current has a DC return back to the filament (electron source) via what
appears to be a choke (804) and resistor 1109 to the 'ground on key-down'
line. When the key is up, -170V is fed to the grid via the same path,
stopping
oscillation.

Thus, the tube's oscillation provides its own bias at key down.

First question: are you sure you're going to get enough drive from your
solid
state oscillator and amp? The level is critical if you're going to drive
the tube
to Class C. Not enough drive and the tube may destruct.

I strongly recommend you to check the level of drive like this. Disconnect
the
lower end of resistor 1109 and connect it to negative side of a milliamp
meter,
with positive of meter to the keying line. There is no RF on the meter leads
so don't worry about that. Meter scale 0-10 mA will probably be appropriate.
Key the BC-375 and note the meter reading. This is the grid current.

Now turn off, do your mod. Run the filaments only if you can, or disconnect
the 1,000V plate supply to this tube. DO NOT key the BC-375 or transmit
with it any way. Start your oscillator and check the meter reading. If
you get
the same level of current or more, you have enough grid drive.

If not enough drive, Class A operation may be better as it's more sensitive.

The easiest way to implement Class A would be to detach the lower end of
resistor 1109 and connect enough 9V dry cell batteries in series (to the
keying
wire) to bias the tube to Class A. Negative towards grid of course. It might
take 5 to 10 batteries to accomplish that, but they can be worn out
ones since
no current flows. ((What, you don't save those worn out batteries for use as
bias supplies?)) Start with a high number of batteries and reduce to get to
the
desired Class A condition. It won't hurt to start high (the tube will
simply be
biased fully off).

Class A operating conditions are beyond what I have space to describe here.
Neutralization might not be necessary in Class A. The tube's plate voltage
is
1,000 V and could be reduced if you have enough drive and neutralization is
a problem.

Warning: If you plan to key the original key-down wire with the solid state
unit
attached, be aware that -170 volt grid blocking voltage will get
transferred via
the coupling capacitor. The unit might be susceptible to damage.

73 de Neil ZL1ANM

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