Re: Valve frequency multipliers

Martin Brown <'''newspam'''@nonad.co.uk> wrote:

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?

This is a bit tricky because my oscilloscope only goes up to 50 Mc/s and
I also have to take account of the loading effect of the probe.  The
waveform on the grid of the 'x5 multip[lier' triode is like an
assymmetric 15 Mc/s sinewave with the top of each positive half-cycle
squashed and flattened a bit.  The tops are just above 0v and the
bottoms go down to -25v.  (The ECC91 cuts off with around  -5 to -8
volts on the grid, depending on anode voltage.)

There is no voltage on the cathode, which is earthed and is common to
both triodes.  The waveform on the anode is a bit meaningless because it
would be outside the range of the 'scope but I have investigated it
three other ways:

1)  With an "Edometer" Grid Dip Oscillator.  This is a rough check that
the parallel-tuned circuit is actually resonating at about the right
frequency.  With the HT disconnected it gives a substantial dip at
around 75 Mc/s when pointed at the anode tuned circuit.  With theH.T.
connected, it gives a dip but with a wiggle in the middle, corresponding
to a signal at 75 Mc/s

2)  The anode of the 'x5 multiplier' is connected with a 1000pf
capacitor to the grid of the 'x2 multiplier', which has a 33k grid leak
to earth.  With a 120k resistor as a probe extension (to minimise
capacitive effects) and the anode of the 'x2 multiplier' disconnected,
there is a small dc offset on the grid due to grid current.  Tuning the
75 Mc/s circuit and starting and stopping the oscillator has almost no
effect on the DC level - I would have expected a change due to
grid-cathode rectification if the signal had been more than 500 mV
pk/pk.

3)  I have a measuring receiver which was intended for checking cable TV
signal levels.  It has a 75-ohm input and I have made a 1-turn probe on
the end of half a metre of co-ax.  Slipping this probe over the 75 Mc/s
coil produces a signal. which can be peaked-up in the normal way by
turning the ferrite core - but it is only a few millivolts.

 
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.

It is rated to give more than 1W output at 200 Mc/s.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk