>In a message dated 11/18/03 7:13:14 PM 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.
>
>^I don't know where you got your peak emission numbers from, but the
>characteristic curves published by Eimac for both tubes show Ip as high as
>2.2
>amperes.
>
>^ The only difference is that the 4-400A needed 500 volts on the screen to
>do
>it.
>^ (The 4-400 did the 2.2 amps Ip with an Ig of 200ma and an Iscn of 100ma.)
>>
>> ** 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.
>
>^ Wouldn't you consider the RF riding on top of a DC potential of 500 volts
>(or 750 volts) on the screen grid to be at a different RF potential than
>the RF riding on the Grid which is at ground potential or somewhere very
close to
>it?
** With the screen at RF-ground, there is no RF potential on the screen.
RF-ground can be either connecting the screen direct to chassis-gnd
(tetrodes with handles), or bypassing the screen to chassis-gnd with a
low-L capacitor (4cx 600J, 4cx1500B, on and on)
>
>> > 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.
>
>^ Probably more like 6 or 7 if you use the 40 watt Max Sig drive figure
>posted both by the ARRL and by Bill Orr for those tubes running in a
>Ground Grid configuration.
** I tried running 4-400As in a SB-220 at 70W PEP drive per tube.
Anode/plate current was c. 350mA per tube instead of the rated 400mA per
tube. My guess was that it takes c. 3600v to make 4-400As play fairly
well in G-G.
>
>^ I'm saying that he would need 25% more drive to obtain 30% less output
>power using 4-400s than using 3-500s.
>
** With more Mu, less driving potential is needed at the cathode to
achieve the same anode current.
>^ If he only has 50 to 100 watts of drive available he should be able to
>see 1500 watts peak from a pair of 3-500Zs at 3KV. That number would drop to
>less than 1000 watts, (probably more like 750 watts), if the same drive was
used
>on 4-400's running G-G at the same plate potential.
** I doubt this. The main rub with 4-400As in G-G at 0v-bias is that
the ZSAC is not enough unless c. 3500VDC is used. The same problem
happens when a 3cx3000A7 is operated at 1500VDC and 0v-bias.
>
>>
>> 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.
>
>^ I cannot argue that point because I have never seen any published figures
>from Eimac or any other source stating what the Mu was for a 4-400A with
>both grids tied together.
** It ain't rocket science. The higher a triode's Mu, the lower the
0v-bias ZSAC.
>
>^ All I can say is that my own experiences have proven contrary to your
>statements and are reinforced by what I have seen published by Eimac,
>Arrl, and Bill Orr.
** Bill Orr is much revered, but I have a copy of a Bill Orr letter the
ARRL has in its files that is not exactly supportive of his reputation
for electronic knowledge. Most people seem to forget that Mr. Orr was
principally a publisher of other folks construction projects.
>
>^ My personal experiences with replacing 3-500's with 4-400's in grounded
>grid amplifiers have always resulted in significantly less power output for
the
>same drive levels and plate potentials, and I had to double or triple the
>drive level to obtain the same power output I had with the 3-500Zs.
>
>^ If what you stated is true, and the 4-400's had higher gain (Mu) and the
>same plate voltage and peak Filament/Cathode emission capabilities, please
>explain to me why Heathkit (and everyone else) switched to the more
>expensive 3-500Z tubes?
>
** OK
1. New 3-500Zs were cheaper at the OEM price because they do not have a
screen.
2. 4-400As require at least another 600VDC to achieve the needed ZSAC,
which would mean that: 2-more 450V filter caps would be needed, a more
costly/higher-V bandswitch would be needed, a wider-spaced Tune-C would
be needed, which would have required a Tune-C padder to cover 80m, which
would have required another bandswitch section.. In other words, Heath
had an OK SSB linear amp kit at the best price in town.
cheerz, Dennis
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