[AMRadio] 807's triode connection question...

[Bob Bruhns]

Hi Vince,

Use lower modulator plate voltages, and/or set up the mod
transformer taps for relatively high impedance to the tube
plates.  If you raise the plate to plate impedance, it will
cause increased screen dissipation in tetrodes and pentodes,
so be careful.  If the plate-to-plate impedance would differ
greatly from typical values, you should reduce the modulator
plate voltage and use ordinary impedances instead.

This sort of thing is the main reason for any concern about
impedance in a classic power amplifier.  The impedance
presented will affect the current that the amplifying
devices will draw.  There are limits to maximum voltages and
peak currents, so in most cases the game is to set the
impedance such that you are near the optimum power point for
whatever plate voltages are used in the audio amplifier.

If the plate to plate impedance is too high, the tubes can
not deliver their maximum available current when the plate
pulls down to the minimum voltage, so you get less power
output.  In multigrid amplifier tubes, the screen current
will be higher because the plate voltage is lower during
conduction.  However, it is much better for the tubes to
present a somewhat higher-than-optimum load impedance.

If the plate to plate impedance is too low, the tubes run
out of cathode emission and can not deliver enough current.
They then flatten out (and distort) at high current at
relatively high plate voltage (they can't pull it down
enough).  Plate dissipation is high because of the high
voltage-currrent product, and also the cathodes are
exhausted in the overloaded peak condition, and the usual
protective cloud of excess electrons is not present at those
instants, so the cathodes are exposed to ion bombardment
damage because there is still considerable plate voltage
present at those times.  Oxide cathodes really don't respond
well to that kind of treatment.  All in all, not the way to
go.

  Bacon, WA3WDR

------------------------
Subject: Re: [AMRadio] 807's triode connection question...
   Date: Thu, 4 Jul 2002 10:21:57 -0400 (EDT)
   From: <ka1iic at prexar.com>
     To: <amradio at mailman.qth.net>

...BTW... I've got an VM-3 mod iron that I want to use...
how could one limit the power from the mod tubes to keep
within the power limits of the tranny... Don't care to take
chances because it took me too long to find a good one I
plan to use std chokes for a mod reactor to keep the DC off
of the secondary....

any ideas on this one???

73
Vince
ka1iic
(PW stationary)