Carel,
When the amplifier is running in any class other than A, (except in one case
which I'll deal with later), the tank circuit 'rings' with each current pulse,
and 'fills in' the voltage swing at the anode. So if you just had a wideband
transformer, you'd only be putting current in for around half a cycle - maybe
for 220 degrees in Class AB. Now the exception for that is if you run in
push-pull, as the solid state amplifiers do, and as audio amplifiers used to do
with valves.
So there comes a point as Q is reduced where distortion will start to rise.
Also, the power output will start to drop: exactly where that point is, I don't
know, but my feeling is that going below a working Q of around 5 will probably
show this up.
Untuned valve amplifiers in Class AB use push-pull, distributed type
amplifiers: for just one band, that's unnecessary. They are also very
inefficient in terms of power in for power out. The other problem with a
transformer is the shunting effect of the output capacitance of the valves:
this will make the anode load reactive and mean the valve is seeing an
elliptical load line, which in turn produces some phase distortion: that phase
distortion can lead to degraded IMD.
73
Peter G3RZP
========================================
Message Received: Nov 12 2012, 02:40 PM
From: "Carel, pc5m"
To: amps@contesting.com
Cc:
Subject: Re: [Amps] low Q output matching
Hi Peter (and Gerald)
Basically my aim is to get the simplest output match circuit possible
(simplest = lowest value of C's, because high valued, high current, high
voltage variable C's are difficult to get and/or expensive). I don't mind
even 0 dB of harmonic suppression.... because that is easily solved
afterwards with fixed valued C's/L's lowpass filter. My question is
basically are there any other parameters I am overlooking (beside the afore
mentioned harmonic suppression) if I go for this ?. In essence, could I just
a high power 3kW broadband 50 to 1700 Ohm RF transformer and the tube would
be happy ?
Gr, Carel.
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of peter chadwick
Sent: zondag 11 november 2012 9:32
To: amps@contesting.com
Subject: Re: [Amps] low Q output matching
What is the value of the lowest Q?
What harmonic suppression are you aiming for? It should be 43dB + 10 log P,
without exceeding 50dB. If there's a tuner on the antenna, that may well
mean you don't need the low pass filter.
73
Peter G3RZP
========================================
Message Received: Nov 11 2012, 07:46 AM
From: "Carel, pc5m"
To: "'AMPS'"
Cc:
Subject: [Amps] low Q output matching
Hi, I was wondering what else, besides bad harmonic suppression, will happen
when I design my (single band) output matching network (PI-L) with low Q
(lowest Q needed to get the match done) ?
When using low Q design I will get
- much smaller (variable) capacitors, especially for the bigger load C, so
easier to get components
- broader no-tune range
-smaller RF stress on the components
As an example for a 160mtrs amp, with Rload of 1700Ohm and intermediate
impedance around 330Ohm:
With Q of 10:
CLoad =1200pF
L-l=10uH
Ctune=293pF
Lpi=33uH
With lowest Q:
CLoad=400pF
L-l=10uH
Ctune=132pF
Lpi=33uH
When you plot both matching networks in a Smith diagram you see immediate
the difference. Low Q is much more straightforward.
To get the needed harmonic suppression a lowpass filter with fixed
components is easy to construct (first harmonic at 3,8MHz needs 25dB and
second one 20dB to get the same values as higher Q design). But maybe I have
overlooked something . Thanks for your ideas 73's Carel, pc5m
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