[Amps] How do I determine Class

Bill Fuqua wlfuqu00 at uky.edu
Wed Sep 1 17:19:19 EDT 2004


At 09:59 AM 9/1/2004 -0700, R.Measures wrote:

>On Sep 1, 2004, at 8:22 AM, Jim Isbell wrote:
>
>>  This is EXACTLY the answer I needed.  The amp uses a pair of 4-125As 
>> and the screens are at "350V - 400V"   Plates at "2000V - 3000V" (but no 
>> terminating resistor) so I guess that makes it a class AB1 linear.
>
>Jim --  Class AB1 amplifiers always have a grid-terminating R because the 
>grid draws zero current.  .

Not always. You don't  have to have a grid-terminating resistor for AB1 
there is always a parallel resistance due to the loss in the grid tank 
circuit.  Q= R(parallel)/ X. The tank circuit is loaded by the drive 
impedance also, so it is not totally unloaded Q.

Many of the old designs were for maximum gain so that very low power 
exciters could drive them. Here you can vary the drive quite a  bit by 
changing the link coupling.


>>The screen grids have several bypass caps to ground but no resistor.
>
>A shunt resistor is needed to sink reverse screen-I.  Typically, about 25% 
>of normal screen-I is shunted by this resistor.

The 4- series does not have problems with the negative screen current like 
the 4CX series. I just checked the data sheets. It is a serious problem 
with the 4CX tubes. However the bleeder is desirable to maintain regulation 
if you don't have regulators.


>>The signal is fed to the control grids through the MB-40-L and the plate 
>>circuit is prety normal..
>>
>>Just what I needed for my vintage shack linear.
>>
>>R. Measures wrote:
>>
>>>Jim -- If the tetrodes are being operated at roughly half of their max 
>>>screen-V rating, they are likely being run in a non-linear mode.
>>>If they are being operated at their max. screen-V rating, and there is a 
>>>grid-terminating R, they are probably operating in Class AB1 linear service.
>>
>>
>>
>Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
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