[Amps] tube replacement ?

[R.Measures]

>In a message dated 11/17/03 7:46:29 AM Central Standard Time, 
>W.Posselt at maco.at writes:
>
>
>> Hello!
>> 
>> I have an amplifier with 2x 4-400 which has a really big power supply and a
>> very rugged design.
>> The only thing I am missing is the appropriate output power.
>> This lead to the idea to replace the tubes by something different.
>> The two 4-400 are run as triodes in parallel.
>> So what has to be considered when changing the tubes to let's say 2x GS31b ?
>> 
>
>Werner,
>
> If I were you I would either add a screen supply to those tetrodes to get 
>the transconductance back again, or (since you said the 4-400s were 
>running with 
>grounded grids), try just plugging in a pair of 3-500Z's and see what 
>happens.
> In either case you will probably have to modify the output matching network 
>to match the 50 ohm load to the lower impedance plate circuit. 

** 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.

>(Try spreading 
>the turns of the tank coil a bit further apart and hope you have enough 
>range 
>on the plate tuning cap.)
> By the way......, I have seen power supplies that appeared huge, but didn't 
>perform very well under load. You may want to load test the power supply 
>first, before you do anything else.
>
>Regards,
>
>Dennis O.