Re: Valve frequency multipliers

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