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From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Sat, 1 Feb 2025 11:50:11 +0000
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On 01/02/2025 10:50, 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%.

If you can make it generate a 25v 15MHz square wave then you should have 
approximately 8v of 3rd harmonic and 5v of fifth harmonic (as well as 
some even harmonics because of how real valves behave). Slight low pass 
filtering to discourage any higher harmonic content will help here.

What does the output waveform actually look like at 15MHz?

Does the valve have enough gain at 75MHz to cope with a tuned load?
ECC91 sounds like it ought to unless something else gets in the way.

> 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.



-- 
Martin Brown