Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Fri, 31 Jan 2025 16:28:38 -0800
Organization: A noiseless patient Spider
Lines: 144
Message-ID: <qiqqpjtrdk0b75l2h82a59sg0ej5mor9oj@4ax.com>
References: <1r71194.rtliy6v9cf4N%liz@poppyrecords.invalid.invalid> <4doqpjd9e3h5u9lt9tklkjs0k9unutbkct@4ax.com> <aapqpjppf0elhu02dp7274ltu2ra77tp1b@4ax.com> <s3qqpjhb8olq1pcuj0fen5i722ck7n1oje@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Injection-Date: Sat, 01 Feb 2025 01:24:44 +0100 (CET)
Injection-Info: dont-email.me; posting-host="e9d28b5c702138d4ce28d5e4825b254d";
	logging-data="3950682"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19+orhbU55MqDW8TRn7/li6"
User-Agent: ForteAgent/8.00.32.1272
Cancel-Lock: sha1:IrjqaoFNGHsGcwZDzSFSUZjwVUo=
Bytes: 7671

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?