>In a message dated 11/18/03 5:22:11 AM Central Standard Time, r@somis.org
>writes:
>
>>
>> ** The 4-400A and the 3-500Z use the same 5V/14.5A cathode and have the
>> same peak-emission (c. 1.3A) , thus, the output Z of a 4-400A and a
>> 3-500Z are pretty much the same if the anode supply potentials are equal.
>>
>
>
> Rich,
>
> He's running the 4-400's grounded grid, so the peak emission would not be
>the same without a MUCH higher cathode drive level, and then only if that
>maximum cathode emission you refer to could actually be obtained at all
>without the
>presence of a screen supply to increase the transconductance factor of those
>tubes.!
** Applying screen V to a tetrode with both grids RF-grounded does not
change the amplification because screen-amplification only occurs when
the the grid's RF potential differs from that on the screen.
> If he has less than 300 watts of drive available, (say only 50 to 100
>watts), his power output would positively increase with 3-500Z's being
>used
300W would drive 4 - 5, 4-400As in g-g with a 3500V anode supply.
> in that
>amp instead of using thosen 4-400 tetrodes operating as triodes.
> His amplifier's input impedance would also be closer to 50 ohms with the
>parallel 3-500Z's than with the 4-400's, so he would get better power
>transfer
>from his exciter into the amplifier.
> He would also be able to sustain a few hundred more watts of ICAS input
>power due to the increased plate dissipation of the 500 VS the 400 watt
>tubes.
> This is of course assuming that the power supply is adequate, the plate
>tank
>circuit can tune to the lowered plate impedance, and the tubes can be
>adequately cooled, etc...., but you have to agree that it's an easy
>experiment and he
>shouldn't do any damage to his amp or the tubes by trying it.
>
> I'm not knocking the 4-400's, they are good tubes and because of their
>lower
>surplus prices I have often used them in the past, (even in grounded grid
>applications in SB220s after directly grounding all three grid pins at the
>sockets because at the time I didn't have enough money to buy 3-500Z's); I
>just
>prefer to run them grid driven as they were designed to be run.
>
> By the way: with the 4-400's in the SB220 I could bypass the bias zener
>because it wasn't needed any more, (unless I increased the plate voltage
>significantly), and the amplifier never had any stability problems running
>that way.
With = anode voltages, a 4-400A in g-g has less ZSAC than a 3-500Z
because a 4-400Ain g-g has more Mu.
>Probably because of the lowered power gain and improved shielding from
>running
>the 4-400's as triodes instead of tetrodes.......
>
> Regards,
>
>Dennis O.
>
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