[AMRadio] Push-Pull Parallel Tubes In Modulator

[John Coleman, ARS WA5BXO]

	Carbon resistors or placed in the plate wires of PP and
especially PPP circuit as a preventative for VHF and UHF oscillations.
When placed in parallel, triodes have a tendency to oscillate because of
the high plate to grid capacitance.  The short lengths of wires that are
used to connect the grid, plates, and filaments as well, may become high
Q VHF resonate circuits and it is easy to have a tuned grid tuned plate
(TGTP) oscillator when the tubes or wired in direct parallel.  Placing a
carbon resistor in one plate wire unbalances the circuit and lowers the
Q of the plate resonates hopefully to eliminate TGTP oscillation.  Some
folks place a resistor in each plate wire.  I have heard pros and cons
about this.  I lean towards the theory of one resistor in series with a
single plate on each side of the XFMR and the other plate connected
directly to the XFMR.  These resistors may be any where from 10 ohms to
100 ohms (what ever works).  They are generally carbon or some type of
non reactive resistor.  It is the skin effect that makes it really work.
The resistors have a lot more resistance at the VHF frequencies than at
audio frequencies.   
	Resistors in series with the screens of each of the four tubes,
is a good idea.  This provides a self balancing inverse feed back to
help equalize the gain of each tube dynamically.  Some one has expounded
on the virtues of screen equalizing resistors here on the reflector some
months ago as to the increase linearity even with just two tubes in push
pull.  I'm sorry I don't remember who it was.  I'll have to look it up.
Resistors should not be used in the control grids of a circuit where
dynamic grid current will flow.  That is grid current that is not linear
with the input signal.  I would think this would include most all class
B and AB2 circuits.  We have enough trouble keeping the signal voltage
up on the control grids with out placing resistor there.

There I go again with another long lengthy gol-darn explanation.

Sorry bout that.
John, 
WA5BXO 

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net] On Behalf Of
brett.Gazdzinski at mci.com
Sent: Wednesday, December 03, 2003 7:36 PM
To: amradio at mailman.qth.net
Subject: [AMRadio] Push-Pull Parallel Tubes In Modulator

Don,
When running tubes in push pull parallel, you can add a small amount of
resistance to the plate, value depends on plate voltage and current.

On screen grid tubes, you can add resistors in the screen and/or plate
circuits.

To much resistance will cause problems, as will tubes that are much
different.

In my ppp 100TH mod deck, I have something like 25 to 50 ohm 2 watt
resistors in line with each plate, I don't know the exact value as they
have
heat
shrink tubing over them.
All the tubes glow with resting current, and light up more under
modulation, and all are equal.

The modulator runs 2000 volts, 120 ma (30ma per tube) resting current.
You can likely discount peak current, as its short, and does not add
much
to the resistor dissipation.

Push pull parallel using triodes is not a great way to go.
It works, but is never as clean as something running AB1.
Even driving the grids with a very low impedance source presents
such a variable load on the driver, very hard to keep clean.
With AB1, its only voltage, so you can drive lots of grids without
problems.
I don't think you gain much with triodes, substantial driving
power is needed, at a low impedance, harder to do than a stiff
screen voltage supply in AB1.

With high power solid state driver devices, the triode method likely
works
very well.

Brett
N2DTS

>
> I have had this modulator chassis sitting around here that I
> was fiddling with over the weekend. It came to me with no
> tubes. It uses 4 tubes in push-pull parallel. The mod
> transformer is a UTC VM-3 rated at 125 watts. The VM or
> "Varimatch" series of UTC mod transformers was sold pre-war
> and evolved into the later "CVM" series that is more commonly
> seen today. This chassis may have come from a commercial
> amateur rig because the construction standard is strictly
> professional.
>
> Through some artful guesswork using only the finest
> assumptions, I have concluded that the modulator tubes should
> be Taylor TZ-20's. The TZ-20 is (quoting Taylor) "a high-mu
> triode designed for zero bias class B audio operation". The
> Taylor catalog even suggests this modulator design under the
> TZ-20 listing, saying "4 of them in push-pull parallel
> operation will form a most economical 140 watt modulator",
> though no actual design for such a unit is given.
>
> Fortunately I have some TZ-20's (though I can use a few
> spares if anyone wants to part with a couple). But I know
> nothing about running tubes in PP-parallel. Are there any
> special considerations? Should the tubes be matched in any
> sense? And lastly, I suspect this should be obvious but I'll
> ask anyway, how critical is the resistor between the 2 plate
> caps of the tubes in parallel?
>
> Comments?
>
> 73, Don Merz, N3RHT
>
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