Re: Valve frequency multipliers

john larkin <JL@gct.com> wrote:

On Fri, 31 Jan 2025 13:51:25 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

[...]

The whole thing must be done with the minimum number of valves and no
semiconductors.

 
Why?

Because that is the challenge I have set myself.

 The Colpitts-derived xtal oscillator is an EF91 and the

multiplier stage(s) can be either another EF91 or an ECC91. 

 
Why not use a 150 MHz xtal?

a) I don't have a 150 Mc/s crystal, but I do have one for 15 Mc/s

b)  The same crystal is used on transmit and receive, but there has to
be +/- 2.5 Kc/s deviation for F.M.  when transmitting and a 100 Kc/s
offset on receive.   I wasn't at all confident that a 150 Mc/s crystal
could be pulled that far whereas these variations become 10 Kc/s and 250
c/s respectively if the crystal is followed by a x10 multiplier and I
knew in advance that it should be feasible with a 15 Mc/s crystal.

[Block diagram at:
http://www.poppyrecords.co.uk/other/Transceiver/Blockdiag5c.gif]

c)  Trying to develop a reactance valve for 15 Mc/s was a real
challenge, as most of the conventional circuits won't work above about 2
Mc/s. *  I would have had to use some other method such as saturating a
ferrite core with audio or physically vibrating a capacitor.  Another
constraint is the H.T. dynamotor, which gives 60 mA at about 250v, so
there isn't a lot of H.T. current to spare for the modulator.

~~~~~~~~~~~~~
* Long explanation:

A reactance valve works by feeding a variable amount of 90-degree
phase-shifted current into the oscillatory circuit.  This is uaually
achieved by having a resistor-capacitor phase shift network with the
resistor from anode to grid (with an appropriate blocking capacitor) and
a capacitor between grid and earth.   The audio is also superimposed on
Grid 1 to vary the R.F. gain of the valve.

At a Mc/s or two this is satisfactory, but at 15 Mc/s the divider action
of the network attenuates the signal reaching the grid.  Even with no
external capacitor, the internal grid-earth capacitance is large enough
that virtually no signal voltage appears on the grid.  If the resistor
is lowered in value, it imposes such a load on the anode that the output
from the valve is negligible and the resistor loads the oscillator
heavily (and may stop it altogether).

I was saved from this dilemma by the "Electronic Designers Handbook" by
Landee, Davis & Albrecht.  It noted that the 90-degree phase shift could
equally well be achieved with an R-C network or an R-L network.  As
there was a low impedance source of oscillator signal at the cathode of
the oscillator, I used an R-L network to inject a phase-shifted signal
into the cathode of the reactance valve.  This had the twin advantages
that the reactance resistor also acted as the cathode resistor for both
valves and it freed-up Grid 1 for injection of the audio signal from a
high impedance source.

[Circuit diagram of prototype VXO and multiplier at:
http://www.poppyrecords.co.uk/other/Transceiver/XtalOsc3d.gif]

The multiplier is one of several I've tried that haven't worked.

~~~~~~~~~~~~~

[...]

>
Does anyone know how to determine the optimum conditions for generating
the 5th and 2nd harmionics in valves?

 
One dual triode could make two injection-locked oscillators.

I had considered that, but if they became unlocked for any reason
(failure of the oscillator) I risk transmitting out of the allocated
band or drifting all over the place.  Not a good thing to do.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk