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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Tue, 04 Feb 2025 10:11:56 -0800
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On Tue, 04 Feb 2025 09:02:01 -0500, legg <legg@nospam.magma.ca> wrote:

>On Sat, 01 Feb 2025 07:23:57 -0800, john larkin <JL@gct.com> wrote:
>
>>On Sat, 1 Feb 2025 10:50:22 +0000, liz@poppyrecords.invalid.invalid
>>(Liz Tuddenham) wrote:
>>
>>>john larkin <jl@glen--canyon.com> wrote:
>>>
>>>> On Fri, 31 Jan 2025 23:50:56 +0000, Cursitor Doom <cd@notformail.com>
>>>> wrote:
>>>[...]
>>>
>>>> >    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.
>>>
>>>That is exactly what I am doing and it doesn't appear to be working.
>>>With 100v on the anode,  15 Mc/s at 25v pk/pk on the grid and a 22k grid
>>>leak, the peak current for one triode of an ECC91 is around 20 - 30 mA
>>>at the positive peak of the grid swing.  The average anode current is
>>>around 2.5 mA, so the conduction period is about 10%.
>>>
>>>A 75 Mc/s parallel-tuned circuit in the anode circuit is giving so
>>>little drive to the following stage that I can't see any change in the
>>>average grid voltage of that stage caused by the drive.
>>
>>Spice it!
>>
>>LT Spice has tube models.
>
>If Spice was of any use in RF ham gear, the amateur radio guys 
>would have been all over it three decades ago.
>

RF design is still in the ancient days of load pulls and Smith charts
and slide rules. I expect that Qspice may change that. Everything
interesting is nonlinear.

>I've modeled known-good valve power cctry only as a curiosity, to 
>see if spice could come anywhere close to practical results. 
>Curiously the nowhere-near-common valves used already had models, 
>so I'm pretty sure somebody else had already made a run at the 
>identical application. (Tek HV oscillator).
>
>RL

Spice is great for modeling mosfets and phemts. Why not tubes?

Probably because few people use tubes any more.