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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Sat, 01 Feb 2025 13:30:25 +0000
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On Fri, 31 Jan 2025 16:28:38 -0800, john larkin <jl@glen--canyon.com>
wrote:

>On Sat, 01 Feb 2025 00:21:10 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>On Fri, 31 Jan 2025 16:12:35 -0800, john larkin <jl@glen--canyon.com>
>>wrote:
>>
>>>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.
>>
>>Well, there's AI and there's AI. The above was Grok. I did try to get
>>the 'new kid on the block' (DeepSeek) to regale us with its wisdom on
>>the subject, but it didn't seem to have any. Chat GPT might be worth
>>asking....
>
>Won't it "learn" from amateur posts to chat sites? Collect opinions?

Well, Grok at any rate cites its sources so you can make a qualitative
judgement for yourself (assuming that's what you're asking).