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Re: [Amps] tube replacement ?

To: r@somis.org, W.Posselt@maco.at, Amps@contesting.com
Subject: Re: [Amps] tube replacement ?
From: Dennis12Amplify@aol.com
Date: Tue, 18 Nov 2003 16:41:37 EST
List-post: <mailto:amps@contesting.com>
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.!
 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 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. 
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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