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[Amps] HELP

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Subject: [Amps] HELP
Date: Fri, 16 Oct 2009 11:39:52 +0200
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GS35B PA design.......

I am busy with the build of a HF amplifier for contest use with a pair of 
Russian GS35b valves.
With a PI calculator,I have calculated some C1-L-C2 values by means of the 
virtual load LOAD RESISTANCE with the use of my power supply voltage and total 
anode currents of both valves.
Please tell my,what I am doing wrong around ??????????
The power supply I will use is full loaded still steady 3111volts DC by a anode 
current of 1.2A each GS35b valve (totally anode current will be 2.4A) 
By means of those values I arrive by a load resistance of  693 ohm.(Ra= 
Ua/Ia*K=1.87) like the formula tells
Some guys told my that it is not possible to obtain a good tuning with such low 
load resistance,in this case you have to use other LOAD filter configurations 
instead of a simple classical PI-filter !!

The problem I have for the moment is that by means of the PI-measurement 
configuration (a load resistor connected from anode to earth and a SWR analyser 
on the AMP output)

- I can only make some hambands perfect in ressonance from 14Mhz to 3.5Mhz. 
- Some bands is it not possible to tune(21Mhz and higher not possible and no 
L-coil anymore in circuit).
- I  most of the time are both capacitors C1 and C2 totally turned open (they 
bring pratically no capacitance into the circuit),what is the reason for this.

I now there are some parastic capacitor values but they could not play a big 
amount in HF frequencys I think and hope so !!!
We are not playing with frequencys in order of a few GHz !!!!

What also a possible reason could be,is that from the calculated PI-tabel,that 
C1 must be in order op 1200pF for 160M use.
For 10M band I use the same big capacitor from 1200pF witch I really do not use 
many only a few "pF" what is normal on the higher bands !!!!
But tuning on this way is very difficult at the end of the capacitor range of 
such big capacitor and not safety at all ??????????

Some guys tell me that such small numbers of turns for the both coils in series 
for the whole hambands ,it is not possible to tune on a good way.
They told my that I need a lot of more coil turns and that the C1 value of most 
amplifiers is maximum 300pF not 1200pF like mine ???????????????     
Taking a look on different website design C1 is also mostly only 250pF maximum 
but I need more from calculation  for 2 tube sof course !!!
The calculated LOAD RESISTANCE should be a lot of more higher than mine !!!!
How many tubes you put together how lower this value will be,how higher the 
value of C1 will be  ????????????? 
I am already building a few years on this design,and I hope you can my help 
further on the way to finish this nice peace of gear ..
Most people talking about amplifier building but could not give my any solution 
on it t'ill today.
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