Re: Valve frequency multipliers

On Fri, 31 Jan 2025 23:50:56 +0000, Cursitor Doom <cd@notformail.com>
wrote:

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

I am trying to use a 15 Mc/s crystal oscillator to generate a 150 Mc/s
signal, the obvious multiplication ratios are x5 and x2.   The 150 Mc/s
has to be distributed to two other units by a 120-ohm screened cable.
>
The whole thing must be done with the minimum number of valves and no
semiconductors.  The Colpitts-derived xtal oscillator is an EF91 and the
multiplier stage(s) can be either another EF91 or an ECC91. 
>
I have tried picking the x5 signal (75 Mc/s) off the anode of the
oscillator with a tuned circuit but can only get a couple of volts
pk/pk.  This isn't enough to drive the ECC91, which I  had hoped could
be used as a 'push-push' doubler, it also won't drive an EF91 over
enough of the curved portion of its characteristic to give sufficient
frequency-doubled signal.
>
Alternatively, I have tried using a parallel-tuned circuit at 15 Mc/s in
the anode of the xtal oscillator to drive one of the triodes of the
ECC91 which can then act as the multiplier.  There is a whopping great
15 Mc/s signal going into the grid of the triode (about 25v pk/pk) and,
with the cathode earthed, this develops enough grid-leak bias that the
valve is conducting anode pulses of over 20 mA about 10% of the time.
>
I would have thought that under those conditions the triode would have
given a large signal at 75 Mc/s in an anode circuit tuned to that
frequency - but it doesn't appear to.  I can't use the triodes as
straight earthed-cathode amplifiers at those frequencies because of the
Miller capacitance effect, but they should be perfectly satisfactory as
multipliers where the grid and anode circuits are  tuned to different
frequencies.
>
Does anyone know how to determine the optimum conditions for generating
the 5th and 2nd harmionics in valves?

>
Nowadays, just ask AI!
>
Generating 2nd and 5th harmonics from a thermionic (vacuum) tube
involves specific operational conditions and circuit configurations.
Here's how you can achieve this:
>
Conditions for 2nd Harmonic Generation:
>
   Non-linear Operation:
       Grid Bias: The tube should be biased to operate in a
non-linear part of its characteristic curve. This typically means
biasing the grid slightly negative so that the tube's operation swings
into the non-linear regions of the plate current-grid voltage
characteristic.
       Signal Amplitude: The input signal should be of sufficient
amplitude to drive the tube beyond its linear operating region,
causing even harmonics (like the 2nd) to be generated due to the
non-linear response of the tube.
   Circuit Configuration:
       Single-ended Amplifier: A simple single-ended triode or
pentode amplifier can naturally produce even harmonics if driven into
non-linearity.
       Cathode Bias: Using cathode bias can help in achieving the
desired non-linear operation by introducing a dynamic bias that
changes with signal level.

Crazy.

>
>
Conditions for 5th Harmonic Generation:
>
   Non-linearity and Symmetry:
       Higher Order Distortion: The 5th harmonic is an odd harmonic,
which means the tube's operation needs to be asymmetrical around zero
to produce it effectively. However, tubes naturally produce less odd
harmonics than even ones unless manipulated.
       Signal Drive: A very strong input signal might push the tube
into regions where higher-order harmonics become significant, but this
approach might also introduce unwanted distortion.

Dumb AI. We WANT distortion. Specifically about a 10% plate conduction
duty cycle.

   Circuit Techniques:
       Feedback: Using feedback in the right way can amplify certain
harmonics. Negative feedback can reduce even harmonics while
potentially emphasizing odd ones if designed for such.
       Multiple Stages: Cascading multiple tube stages can increase
the harmonic content by compounding the distortion at each stage. Each
stage could be designed to push further into non-linearity.
       Harmonic Enhancement Circuits: Specific circuits like those
with transformer coupling where the primary and secondary windings'
interaction can be tuned for harmonic enhancement. Also, using
resonant circuits or filters tuned to the 5th harmonic can help.
   Tube Selection:
       Tube Characteristics: Some tubes are naturally more harmonic
rich than others. Tubes like the 12AX7 might be used for their gain
and non-linearity characteristics, but for more harmonic richness,
sometimes pentodes or specific triodes known for distortion
characteristics are employed.
   Operating Point:
       Grid Voltage: Adjusting the grid voltage to operate closer to
cutoff can increase harmonic distortion since the tube's response
becomes more non-linear near cutoff.

Operate it deep in cutoff, off most of the time. A high amplitude
drive and grid-leak bias would be good.

   Power Supply Design:
       Power Supply Stiffness: A less stiff (or intentionally loose)
power supply can allow tube operation to vary more with the signal,
potentially increasing harmonic distortion.
>
>
In practice, generating a specific harmonic like the 5th is
challenging because tubes are inherently more suited to producing even
harmonics when driven hard.

Why is that?

To get a pronounced 5th harmonic, you'd
often need to tailor the circuit design, including feedback, load
characteristics, and perhaps even use of additional harmonic
generators or filters post-amplification.

AI platitudes, harvesting conventional wisdom.