[AMPS] G2DAF and super cathode

[sm5ki]

Before talking down the G2DAF circuit I may ask has anybody measured the
distorsion  in it? Where and when was this published?

There is much about the G2DAF in Pat Hawkers TECHNICAL TOPICS SCRAPBOOK
1990-94 and in other RSGB technical publications such as their older RADIO
COMMUNICATION HANDBOOK. The  new 1999 edition is just send out - as soon as
I receive my copy I will let you know more about it. RSGB = Radio Society of
Great Britain.

The super cathode version of the grounded grid has been discussed by W6SAI
and WA6BAN in QST 7/67: SEMI and SUPER-CATHODE-DRIVEN AMPLIFIERS. The same
authors analyzed the GG:  THE CATHODE DRIVEN LINEAR AMPLIFIER in the QST
6/67. Very interesting basics. 

They gave an example of a 4CX300A with the grid tied to the cathode and
screen grid tied to ground. The no signal grid voltage was of course = 0 and
the no signal anode current = 15 mA. With 75
watts of drive they got 375 watts out at -46 db 3rd order products. Screen
voltage was 330 volts peak delivered by the driver.  They did not publish
how
much anode dissipation, but it should have been low. Small fan needed then?

What is the practical use of such a low gain circuit? Well, in a single
frequency linear amplifier, where the tune up of  many tuning circuits is no
problem, you could use a number of cascaded super cathode stages for a real
simple low distorsion linear. Very clumsy solution but the distorsion
figures are impressing, are they not?

I also remember also an old CQ article by a guy running a 4CX1000A as a
super
cathode linear. All electrodes had  normal DC voltages except maybe the
screen grid voltage which had lower DC voltage because the rf cathode
voltage added.
The grid was tied to the cathode with a couple of noninductice caps. As far
as I remember the tube was run in AB1? In order to get a large output, you
had to use up to 150-200 watts driving. And
that is exactly what you have in some modern transceivers. As far as I
understand, and I may be wrong, this circuit should have very low
distorsion. If you then drive it with a transceiver with 28 volt final 
transistors, you get lower distorsion there too than in one with 12 volt
transistors in the final? You never need any external ALC and best of all:
you have not to bother about any grid
current  because there aint any, hi ( the 4CX1000A has a v e r y   l o w 
max
grid dissipation )! 

Nothing new under the sun then and again: let us scan old articles on
linears into a CD disc or let it be  avaiable on the net. A good start would
be if someone please sends me the email adress of Reid Brandon/ EIMAC to get
permission to copy  EIMACs  Amateur Service News Letters to be distributed
on the net. I send a mail to Bill W6SAI but no answer so far.

Now, waiting for the storm to break out from the G2DAF haters club,hi!

Hej daa de Hans SM5KI


>Till: "sm5ki" <sm5ki@algonet.se>
>Ämne: Re: SV:SV:SV: [AMPS] Tetrode Amp Help
>Datum: ons 15 dec 1999 15.47
>

>Hello Hans:
>
>I wanted to respond to your question publicly, but since I am new and
>brought up one matter, I did not yet want to approach the two subjects you
>mentioned.  Give me a few days.
>
>The G2DAF is a very interesting amp and deserves proper discussion.
>However, there has been a general condemnation of it by certain indivuals
>who claim to be competent.  Yet, they cannot cite any empirical tests.  They
>generally claim that the textbooks claim that screen voltage must be
>regulated - and since it is not in the G2DAF, it must therefore be so bad
>that it does not deserve discussion.
>
>Had Tom Edison followed the same logic, we would still be using candles.  A
>number of questions arise regarding the G2DAF.  The first question is
>"regulated against what?"  We know that if the screen voltage goes down when
>the signal increases, we will have distortion.  That is because the gain
>decreases as the drive increases.  But, what if the screen voltage increases
>with signal.  If the voltage varies directly with drive, then it truly is
>more regulated to the grid voltage than a fixed screen regulation.  In
>relation to the grid, the G2DAF is regulated and the standard configuration
>is not.  How is this different than grounding the screen grid with the
>control grid and driving them with the same signal as many fine class B
>modulators did.
>
>In short; I believe if someone offhand dismisses the G2DAF, it is an
>admission that one cannot think clearly and is afraid to let others think
>for fear of finding out they are stupid.  Unless results can be cited,
>amateurs especially should be open.  After all, it was the amateurs that
>proved that the bands below 200 meters were not entirely worthless.  It is
>very interesting to see that some amateurs that are convinced it will not
>work are already questioning any empirical testing by trying to prove that
>tests are not dynamic so that even if you do get good test results they
>should not be believed.
>
>Since the super-cathode design uses degeneration, it should be better than a
>normal amp.  However, since it takes more drive it may not be popular as it
>makes it harder to get 5 kw.
>
>I will wait for a few days to pass before approaching the subject and then
>hope we can get some proper discussion of it.
>
>73,  Colin  K7FM
>
>ps, my mother was a Swede (Anderson), so perhaps the logical thought process
>came from that lineage.
>
>

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