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Re: [Amps] tube replacement ?

To: Amps@contesting.com
Subject: Re: [Amps] tube replacement ?
From: Dennis12Amplify@aol.com
Date: Wed, 19 Nov 2003 05:04:01 EST
List-post: <mailto:amps@contesting.com>
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? 

> > 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'm saying that he would need 25% more drive to obtain 30% less output 
power using 4-400s than using 3-500s.

^  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.

> 
> 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.

^ 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.

^ 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?

^ Regards,

^ Dennis O.
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