Re: Valve frequency multipliers

On Sun, 9 Feb 2025 10:23:28 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

legg <legg@nospam.magma.ca> wrote:
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[...]

 
I'd be more selective of the crystal frequency. Two triplers might get
you there with a lot less grief.

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You were right: with a crystal frequency of 16.656 Mc/s and two triplers
the results are much better.
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New block diagram:
<http://www.poppyrecords.co.uk/other/Transceiver/BlockDiag6c.gif>
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New oscillator circuit:
<http://www.poppyrecords.co.uk/other/Transceiver/XtalOsc4c.gif>
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The stages are: EF91 Reactance, EF91 Oscillator, ECC91 Tripler+ tripler.
>
The new crystal necessitated changes to the component values in the
'pulling' circuits, but the basic concept remain unchanged.   Over large
frequency swings the reactance valve characteristic appears curved but
with 0dBm audio input at 400 c/s  there is no audible distortion and the
frequency swing is around 15 Kc/s pk/pk, which is three times larger
than the permitted deviation in the amateur bands.
>
I tried putting a parallel-tuned circuit in the anode circuit of the
oscillator, to resonate at the third harmonic, but there was very little
signal, so I decided to use one triode of the ECC91 as the first
tripler.  I then resonated the anode tuned circuit of the oscillator at
crystal frequency but discovered that this made it unreliable at
start-up.  Eventually I found that just an aperiodic [untuned] anode
choke gave plenty of drive to the grid of the first tripler and allowed
the oscillator to start reliably.
>
The parallel-tuned circuit of the first tripler and series-tuned circuit
between the second tripler and the output cable both have comfortably
gentle tuning, which is an advantage because they should hold their
settings without adjustment for a long time.  The output voltage is only
around 300mV rms into 75 ohms, but this can be stepped up when it gets
to the mixer grids by a further resonant circuit.

You seem to have got all your ducks in a row.

Tripling within the oscillator seems to have been pretty popular
back in the day. I'm not sure what kind of stress they were putting
on the crystal to do this, but they wanted >5V from a substituted
VCO to get similar downstream responses.

Unable to find RAH'69 on the web, I've copied two related articles:
http://ve3ute.ca/query/RAH_'69_VHF_rx-tx.zip

 You'll see an anaode choke or two. Preference was pentode for xtal
and triode for next buffer/tripler stage.Combined with a dual
pentode PP-tripler, this gave one comedian a 'two-tube' PA box.

If ever you do fool with LTspice, the oscillating HV PSU file should
run 'out of the box'. Tube models were typed into the schematic to
get around software revisions and modeler's individual file handling
pecadillos. Schematic legibility obviously suffers as a result:
http://ve3ute.ca/query/RM504_power_supply_sim.zip

RL