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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Fri, 31 Jan 2025 16:12:35 -0800
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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.