Vacuum Tube To Digital Oscillator Adapter

[N1BCG]

Sounds sacrilegious, doesn't it?

I present to the Brain Trust the challenge of modifying a Collins 300G broadcast transmitter oscillator to low-level amplifier using a DDS VFO with a high impedance 80mW output as the new frequency source.

Why? Every try locating FT-243 crystals for 160M? Nuff said...

The circuit uses a 6F6 in electron-coupled configuration with approximately -60 VDC on G1 and 165 VDC on G2 (effectively the oscillator plate).

The current oscillator schematic is attached below.

Thoughts?

[Opcom]

It can be done. Dennis Brady W5FRS uses a DDS VFO (simple board with LCD and litle pushbuttons) in a plate modulated transmit setup ending with a 250TH. There are a couple stages of amplification before driving the 250TH. I think one is a 6Y6 because it has more sensitivity on RF than a 6AG7, but I don't have any more info. Probably one decent tuned (or not) stage will get you the drive to the sensitive 6F6.

Tube circuits of wideband amps like video output drivers for old black and white picture tube TV sets have response to 4 MHz.

On 160M, you could probably copy part of a vertical deflection amplifier of a simple scope like this one to drive the 6F6 grid.

Only a single 6CL6 is used. (or 6AQ5 /little power pentode as used in some small scopes).

[W2JBL]

Clark- you need to change the oscillator to a buffer. Remove everything from the grid circuit except the 47K grid leak. Short out the cathode RF choke. Up the cathode resistor to about 300 ohms. Short out the 1K resistor in the plate circuit. Remove the 10K resistor from the screen to ground. Drive the grid through a .001 cap and you have a class A buffer amp.

[WBear2GCR]

Might be easier than that...

If one adds a switch to open the connection of  C64  to the grid, you end
up with a gain amplifier. At least I think it ought to work as-is. You probably
want to unmesh that variable cap (C65) at the input. There's already a DC blocking
cap in series with the xtal... so the only thing left to do is to pad down
the level of the DDS output with something like a "T network" of resistors to
hit the requisite output level.

The switch needs to be a low capacitance type - not a simple toggle switch.
So that leaves a rotary wafer switch.

[WBear2GCR]

Plan B.

When the cathode resistor = plate resistor the circuit (minus the cap doing the
feedback to make it oscillate) becomes a paraphase inverter circuit, aka unity gain.

So you could modify it by A) making the cathode resistor set up with a switch to
make a series ~900 ohm resistor bypass (for normal operation with a xtal)
or
B) add in a 5 watt or so pot of ~1kohm in series with the 100 ohm resistor...

one still needs to switch in and out the upper feedback cap off the grid.


That was Plan B.