I am running two linears with a pair of GS35b tubes myself. You can see them
on my homepage: www.hoeppe.name
Your calculations are right. The more tubes you choose, the lower the plate
resistance...if you want full drive for both tubes! I would not calculate
1,2A per tube, as you will get limited by too high grid current with your
anode voltage. I calculated 1A per tube for full output, also using 3,1 KV
under load. Driving more will reach the Ig limit. You need 100W per tube to
get that values! If you only have only 100W input for both tubes, you have
to calculate the same as if you only use one tube. I made 2 calculations,
one for 100W input for 2 tubes, and one for "bad boy use". You will see that
you get troubles with the fix add-on capacitor on the load when you want to
cover both "modes". You need a load C with very high variation (high value),
or 2 different add-on C's! I have only one fixed C, as we only have a 100W
TRX at our clubstation. Why 2 tubes, you might say. It is much more
relieable, cooling is better, and with 2 tubes the gain is much
higher....the gain saturates very early at GS35b's with conservative plate
voltage around 3 KV.
It might be possible that you get troubles on the higher bands, because the
minimum capacitance of your plate-C is too high. You have the anode/grid C
of 2 tubes, plus the C of the large coolers against the wall, plus some
straw capacitance (plate choke...) plus C min. The total minimum capacitabce
might just be too high! I use Cs with a minimum capacitance less then 10 pF.
So you need a vacuum capacitor, they have high variation. If you drive it to
the limit, you get low plate resistance, which demands higher values for the
Cs. Higher valued Cs also show high values for the minimum C....causing
problems on the plate side as described. Sometimes it is a good idea to use
add-on Cs on the plate to get a lower minimum capacitance. If you don't find
a way to get C min low, you can use higher Q on the high bands (e.g. Q=20
instead of 10-12). However, the losses increase, and the stress on the
components rise dramatically. I = Q x Ia.....your coils, bandswitch e.t.c.
must be able to withstand that, even on 10m where skin effect is already a
I use PIEL from tonnesoftware. You can play around. If you know your Cmin,
you can play with the Q to see what you need on 10m. You also get the
currents in the system....possibly a strong argument to change the plate C!
Good luck with your project!
Tom, DJ5RE in Bavaria.
----- Original Message -----
From: "BEHIELS JEAN-PIERRE" <ON4AEF@base.be>
Sent: Friday, October 16, 2009 11:39 AM
Subject: [Amps] HELP
> 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
> 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
> - 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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