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

[ka1iic at prexar.com]

Tnx Bacon for the info...  I'll print this out and post it over my work
bench until my old mind can absorb it for later recall hi hi...

Hey!  Maine is under a tornado watch right now.... hmmm  hope it is just a
watch and no wind ehhh???

73
Vince
ka1iic
-.--.


On Thu, 4 Jul 2002, Bob Bruhns wrote:

> 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)
>
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