Paul Christensen wrote:
If you could list the top few important parameters to low distortion
tetrode sign, what are they? I think most of us know the importance of
well-regulated screen voltage. Other top factors?
First of all, do everything that you would for a triode: give it a
reasonably stable heater voltage, and an anode voltage that remains
reasonably stable at maximum current demand.
Both tetrodes and triodes require a stable G1 bias voltage (relative to
the cathode) but this is done in different ways, each with its own
distinctive set of advantages and pitfalls. GG triodes create negative
grid bias by taking the cathode positive with respect to ground, using
the familiar zener diode or "electronic zener" to create a constant
voltage drop. This is very simple because no external bias supply is
required, but the challenge is to maintain a constant voltage drop when
the cathode current is varying strongly.
Tetrodes do it differently, but they always require a separate negative
bias supply for G1, so that adds some complication. Voltage
stabilization is much easier because very little current is involved,
but it's important that the G1 bias voltage remains constant if the tube
is driven into small amounts of either positive or even negative grid
current. If the grid voltage varies when current flows, that will
produce a sharp kink in the transfer characteristic, possibly leading to
severe high-order IMD.
Another feature of the G1 supply is that most tetrodes have a very small
grid dissipation, so they require some kind of protection against
excessive grid current. In the normal setup for class AB1, my boards
will generate an ALC or warning signal at about 100uA of grid current,
and will trip the whole amplifier at about 3mA (which is well below the
threshold of physical damage to the grid).
As for the screen supply, the obvious point is that tetrodes do require
another separate power supply, with good voltage regulation for both
positive and negative screen currents. Protection and fault recovery
adds yet another layer of complication. Screen supplies are covered at
much greater length in my QEX article:
<http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode-3.htm>
And then you have to think how to switch between RX and TX. If full QSK
is not required, I strongly favor switching the screen to ground (in the
typical DC-grounded cathode setup). This has the advantages of
definitively cutting off the anode current in the RX condition, leaving
the screen grid safely grounded and the screen supply safely
disconnected. Protection in the RX condition is very important because,
when you think about it, this is how an amateur amplifier actually
spends most of its operating lifetime!
The problem is how to switch these relatively high screen voltages
RELIABLY at full QSK rates. MOSFET switches have been tried, but are
destroyed by the first HV arc that comes along... so that means
mechanical relays, which have their own different set of problems
including a limited mechanical lifetime. The best scheme I've been able
to come up with is to use electronic grid-block keying for QSK while
*also* switching the screen relay with a short hang-time to avoid it
switching on individual dots and dashes. This gives the advantages of
both switching methods but requires some careful sequencing.
To summarize, tetrodes require much more support circuitry than triodes
do... at least, they do if you're intending to get everything right.
Well, that was the short(er) version, and before the second cup of
morning coffee I'm sure there are other things I left out...
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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