[Amps] Russian tubes GS35b et al.

Ian White, G3SEK G3SEK at ifwtech.co.uk
Fri Feb 7 09:28:07 EST 2003

Sorry for the delay in replying to this...

Eric wrote:

>     I have been thinking about your original response to my request,
>and realize that I failed to see a good point that (I think) you were
>making. I wish to amend my original response as follows.
>      I stand by my judgement about needing actual plate curves to make
>the final calculations and judgements about amplifier performance using
>a given tube.

Mostly; but see below.

>However, I have just realized that the basic
>inter-relations of PS voltage, voltage swing, and output power are
>really the same as with solid-state designs. That is, V-squared/2R
>still applies. The only difference is that our tubes can not be driven
>into hard saturation but only down to a few hundred voltages. As B+
>increases, this becomes a small percentage of the potentially available
>voltage swing, so we can still hope to achieve close to V-sq./2R.
>In summary, for a given power supply voltage and target output
>power, R is inherently fixed as V-sq./2P.

The ARRL Handbook gives a formula that seems to work rather better:

RL = Vp/(K*Ip) where K is a variable that replaces the "2" to account 
for the larger conduction angles in class B, AB or A; about 1.6 often 
seems to work for class-AB2 triode amps. You can estimate Ip from Po, Vp 
and an assumed efficiency.

With a little help from the tube data - but without any characteristic 
curves at all -  these approximate methods will get an amp on the air.

>The only thing the tubes
>curves will tell us is that the tube is actually capable of doing it -
>that is, it can be driven to near-saturation. So as a useful rule of
>thumb for estimating a potential amps performance, I fully agree.
>      My comments about power gain still stand, however. There are no
>basic rules about the power gain, grid current, etc. These are all
>unique to the physical design of each tube and must be all be
>determined from the documented tube specs. If they are not available,
>it becomes voodoo (oh-oh, there is THAT word again.)

No; it simply becomes a case for experimentation. The last I heard, that 
was still a scientifically valid approach :-)

Even if you had a full set of characteristic curves, they still wouldn't 
tell you the optimum locations for the two ends of the load line (zero 
signal and instantaneous peak signal). Pappenfus, Bruene and Schoenike 
have a nice little diagram showing all the conflicting factors that have 
to be balanced using "engineering judgement". The curves will help you 
stay out of the high current / low voltage area where the curves 
compress and bend, but that's about all the useful help they can 

You can only get *all* the necessary information - and especially the 
information on IMD - by actually testing the unknown tube in an 
amplifier... which is exactly how the tube manufacturers come up with 
their recommended operating conditions.

Having adjusted the input network for optimum VSWR, and having adjusted 
the output network for the optimum loading (balancing the conflicting 
requirements of efficiency and linearity), then the amp is switched off. 
The input and load resistances are found by connecting small trimpots 
from cathode to ground and from anode to ground. These trimpots are then 
adjusted to give a 50-ohm match at the other side of the input and 
output networks - and then the ohmmeter gives you the design values for 
the cathode and anode load resistances.

Fortunately all this has been done for the GS35b: with 3.5kV on load, 
and expecting 1500W output, 1900 ohms is a good design target for the 
load resistance, and the optimum input resistance is a touch under 50 

>I have also heard
>the 50 ohms Rin  rumor for the GS35b,
>The Russian data sheet shows grid current
>curves which imply close to 50 ohm at lower power levels (<500W) but at
>1A. of peak plate current, Rin is dropping like a rock.

It certainly does change, but that's true of many tubes AFAIK. The 
normal practice is to optimise the input match at full drive power, and 
let the driver operate into a mismatched load only at lower output 

> but the power gain figures I hear
>are all over the place.

Well, that depends on the competence of the experimenter. If someone can 
only get 600W out of a GS35b, it isn't the tube that has a problem.

One of the main pitfalls is the facile assumption that a GS35b is a 
"Russian 8877". Both tubes are triodes with a 1500W anode dissipation - 
but there the resemblance ends. The 8877 is a high-mu triode that works 
well at low anode voltages, and requires only a low bias voltage and 
therefore quite low RF drive. The GS35b is the opposite in every 
respect. It *demands* high anode voltage (it's useless below 3kV) and it 
also needs high bias voltage and plenty of drive. If those requirements 
are met, it goes well.

Its main practical drawback compared with the 8877 is that, for 1500W 
output, the GS35b needs most of the drive power that your transceiver 
can provide. That means you don't have so much opportunity to improve 
the IMD performance of the transceiver finals by backing-off the RF 
drive. Even so, you can expect typically 12dB gain at 1500W/3.5kV, and 
1-2dB more at higher voltages and/or lower outputs (and those figures 
are verified up to 432MHz).

All of that information has been derived experimentally without 
reference to the characteristic curves - but you can now pretty much 
infer what the curves must look like!

I really do feel that dismissing solid experimental data as "rumor" 
comes awfully close to "not invented here". There are many hundreds of 
GS35b amps working well in Europe, from Top Band to 432MHz and even 
above. G0RUZ has built a dozen VHF/UHF amps, all with similar 
performance, and the GS35b and its smaller brother the GS31 are to be 
found in commercial amps covering HF, 50MHz and 144MHz.

Don't worry about the lack of characteristic curves - if you build it, 
they will go.

73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'

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