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I found this info on the web.? It's from the document "HT-40 Preferred
Upgrades.pdf".? You can search for it.? I think it's for the plain HT40
and not the Mark 1.? (I've still been trying to figure out the function
switch without any luck.? Clean it and make sure it's switching properly.)

>>>>>The Buffer-Multiplier-Driver Stage

The Buffer-Multiplier-Driver stage V1B uses the same 6CX8/6HF8 as
described above except the pentode section is used as the Buffer-
Multiplier-Driver. It buffers the oscillator from the Final, multiplies
the frequency when necessary, and also drives the grid of the 6DQ5
Final. See Page 2 of the manual for the frequency multiplication process.

Without the buffering function of this stage, impedance changes at the
output of the transmitter would be reflected back to the oscillator,
changing the crystal's oscillation frequency and V1A's stage gain.

Some Turkey-Bird owner had replaced R4 with a 1 Megaohm resistor.
Undoubtedly this contributed to the demise of R5 since this stage was
operating in saturation rather than in class C. Furthermore, with the 1
Megaohm resistor value, I doubt any frequency multiplication could
occur. You know, if 100k is good, then 1 Megaohm has to be 10X better!

After the necessary component changes and power-on, there was no RF
output even though the filament was lit. I checked this original tube
once again in the tube checker and wolla, it was failing on all counts,
most likely due to the R4 component change and subsequent suffering.

So a NOS tube went in and power output was restored. Always measure the
cathode resistor R5 and the screen resistor R6. I found R5 was
715 ohms so it was replaced with a metal film resistor of the stated
value of 470 ohms. R6 was right on and miraculously, I have found this
12K resistor to be stable in most HT-40's. In this case, and due to the
failure of the original V1B, I replaced both R6 and L2.
Another Turkey-Bird moment was found when the expected stage gain wasn't
as high as it should be. If this was originally a Kit, then
Turkey-Bird forgot to solder in C11 from the cathode Pin 6 and to
ground. With C11 now in circuit the stage gain was higher, with less RF on
the Keybias rail.

This stage feeds the 6DQ5 Final's grid via a pi-net input circuit. RF
feedback from the Final is fed to the junction of C14 and C15 to curb
any parasitic oscillations and is a Neutralizing circuit. I have never
experienced any neutralization problems in an HT-40 with or without a
stub at the 2 lug terminal strip next to the Final.

The Cathode Bias Rail:
This transmitter uses cathode bias to shut off the
buffer-multiplier-driver and Final tubes on Keyup. In Keyup a high
voltage is on the V1B
and V2 plates but very little current is flowing through the tubes, but
it is enough current (0.7 mA) to pull the cathodes up to a voltage of
32V across R2. This voltage reverse biases the tubes and keeps them from
amplifying RF.? On Keydown, the “Keybias” voltage goes to zero allowing
the V1B and V2 tubes to conduct.

--
Joe Hutchens
AJ8MH - Radio
Marquette, MI