[Amps] first time questions

Carl km1h at jeremy.mv.com
Sat May 16 19:30:54 PDT 2009


To add to that IMD also gets worse as Ep is increased especially when it 
is operated above its published ratings for any class of operation.

Carl
KM1H



----- Original Message ----- 
From: "Jeff Blaine" <keepwalking188 at yahoo.com>
To: "Ulf Tjerneld" <star at frizon.org>
Cc: <amps at contesting.com>
Sent: Saturday, May 16, 2009 8:59 PM
Subject: Re: [Amps] first time questions


> Ulf,
>
> To make the pi calculations, you should consider using the LOADED 
> plate
> voltage.  Idle plate voltage is not a factor.  The difference, 
> however,
> is not very much.  Maybe 10-20%...
>
> And on #3, you are right.  The voltage and current are unimportant 
> when
> considered SEPERATELY.  Unfortunately, there is not very much 
> emperical
> data published on the Russian tubes so picking the optimal B+ is hard 
> to
> do.  Generally speaking, higher B+ is better as the efficiency and 
> gain
> is better.  But in my experience with the GI7b, at a certain voltage
> point, the efficiency of the amp starts to drop.  At low B+, the PO is
> fine - the trouble is the Ig gets high due to the low gain of the tube
> at that lower B+.
>
> 73/jeff.ac0c
>
>
> Ulf Tjerneld wrote:
>> Hi all,
>>
>> I'm collecting parts for an amp-project. And I have som newbie
>> questions:
>>
>> 1, Maybe this question is silly: When calculating the Pi network for
>> an amplifier with  GI7-B's I'm aiming for an unloaded plate voltage 
>> of
>> 2500 volts. With one tube this means that the plate current would be
>> 350 mA for an input power of about 875 watts and not dissipating more
>> than 350 watts. If I'm instead going to  use a pair of tubes, this
>> means that I would input 700 mA in the Pi network formula. Right?
>>
>> 2, I have a perfect capacitor for the Pi-tank in the above example.
>> But is rated exactly 2500 volts (Johnson something...). Would that be
>> to little for an unloaded plate voltage of 2500 volts?
>>
>> 3, Most people that builds amplifiers based on the GS35B use a plate
>> voltage of around 3500 volts. This would mean a plate current of 
>> about
>> 700 mA for 2500 watts input. But let's say I only have 2500 volts, 
>> but
>> enough iron in the transformer. Would I achieve equal results if I 
>> let
>> the the tube draw more current? I.e. let the tube run with 2500 volts
>> and a plate current of 1 A? And to extrapolate further, even go to a
>> plate voltage of 2000 volts resulting in a current of 1.3 A? To
>> summarize: does it matter what combination of voltage and current I
>> use as long as it is within the specs of the tube?
>>
>> All the best
>>
>> Ulf/SM0NOR
>>
>>
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>
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