[TMC] SBT-1K

Tue Dec 7 23:02:34 EST 2010

Hi Larry,

Here are a few observations that I hope will be of help:

The PAL-1K amp in the SBT-1K is designed to produce a maximum output 
power of 1 kW, PEP or average.  In order to stay within the rated 
distortion limits of the amplifier, neither of these conditions may be 
exceeded, whichever is reached first.  Under single tone conditions, the 
amplifier will produce 1 kW average power.  And with this single tone, 
and by the definition of peak envelope power (the power averaged over 
one cycle of the RF output, measured at the envelope peak), the 
amplifier is then also delivering 1 kW PEP.  The single tone case is the 
only one in which PEP and average power are equal (the peak to average 
ratio is 1:1).  This is the situation you have in "key down" CW 
operation, and in FSK (forget for the moment the distinctions between 
CCS and ICAS operation, especially the term "ICAS", which is pretty much 
meaningless, for all practical purposes).

When any modulated signal is applied, the ratio of peak envelope power 
to average power increases (the peak to average ratio is greater than 
1:1).  The amplifier is still limited to 1 kW PEP, but since the signal 
is now modulated, the average power must now be lower.  As you know, in 
DSB full carrier AM, at 100% sinewave modulation, the PEP output is four 
times the unmodulated carrier power, so the unmodulated carrier must not 
exceed 250W.  Average power under these conditions will be about 1.5X 
the unmodulated carrier power.  So, in sinewave modulated DSB 
full-carrier AM operation, the PAL-1K is now considered to be rated to 
deliver 1 kW PEP, but only about 375W average.  Obviously, the amplifier 
cannot operate at 500W carrier power output, and still be modulated 100% 
without exceeding its 1kW PEP output limit.

The final stage is biased class AB, and its plate input power will be 
very dependent upon the combination of its RF drive power, and how 
heavily or lightly the amplifier is loaded.  That is, a fairly wide 
range of input powers can result for a fixed output power of 1 kW, and 
plate efficiency is therefore variable.  In addition, plate efficiency 
is highly dependent on the modulated signal's peak/average power ratio.  
A plate efficiency of around 65% is reasonable for a properly adjusted 
amplifier under  single tone conditions at 1 kW PEP, but it will be much 
lower with modulation at the same 1 kW PEP output.  For example, with a 
two-tone signal at 1 kW PEP, the plate efficiency will drop to only 
around 35%, and the exact value will depend a lot on drive and loading 
conditions.

So the question arises: what is the correct combination of RF drive and 
plate loading?  The power gain of the PAL-1K varies greatly with 
frequency, so predicting the required drive power isn't 
straightforward.  At 7 Mc and below, it may  require as little as 10 mW, 
but at 28 Mc, it may require more like 100 mW.   And, there's 
significant variation from amplifier to amplifier.  So, TMC designed the 
exciter to produce at least 100 mW at its rated distortion value.  
First, be sure your exciter is capable of at least 100mW.

Tuning the amplifier correctly is a matter of obtaining its rated 1 kW 
output and rated intermod levels with the best compromise between plate 
current, screen current, and drive level.  Under single tone conditions, 
you should obtain 1 kW output with plate current of around 450-500 mA, 
and screen current not to exceed around 40 mA.  Plate and screen current 
should dip pretty much concurrently with a resistive load, but they may 
not dip exactly together, particularly if the load is significantly 
reactive.  A compromise setting is usually required,  and it's usually 
best to favor minimum screen current, even if plate current is slightly 
off the dip.  In order to optimize linearity, though, it is often 
necessary to load the amplifier more heavily than you would for best 
plate efficiency, and depending on the individual output tube, it may be 
necessary to run considerably more plate current.  But it should never 
exceed 600 mA max (1800W plate DC input).  If plate current is low, the 
screen current is high and "touchy", and the screen current tends to 
"peg" or the screen breaker trips, the amplifier is probably loaded too 
lightly.  Try increasing loading, and if you have adequate drive, you 
should see an increase in plate current and decrease in screen current 
after adjusting to resonance.  After a few iterations you should 
converge on an Ip of 500-600 mA, Isg of 30-35 mA, and 1 kW output.  
Under 1 kW PEP two-tone conditions, the 3rd order intermod products 
should be at least 35 dB below either tone.  Most properly adjusted amps 
with good tubes will make closer to -40 dB.  If you can't make make 
rated power with similar plate and screen currents, be sure the driver 
section gain is sufficient.  The grid swing on the final tube will be 
about 150V at full power (measured with a scope).  If grid drive is 
good, you may have a final tube with low emission.  But usually, low 
emission tubes don't show the kind of high screen current you are 
apparently getting.

Personally, I wouldn't recommend replacing the 8295/PL172 tube with 
anything else unless you simply have no other choice.  These amplifiers 
are extremely well designed as they are.  I have 6 1kW TMC transmitters, 
all with the PAL-1K amp, with various TMC exciters.  They are stable and 
tune smoothly, easily make 1 kW PEP continuous duty with excellent 
linearity, and are completely dependable.  They are a pleasure to operate.

Hopefully the foregoing will be of some help.  Best of luck with your 
SBT-1K, it's one of the all-time great 1 kW HF transmitters.

73,

John K9WTOn 12/7/2010 6:56 PM, LARRY D GODEK wrote:
> Kind of curious.  The manual for my SBT-1k says that i should get 1KW output on CW and RTTY and 500W on AM (DSB with carrier and low level modulation).  Doesn't compute.  Even if i was to keep it at 1/4 KW on am.
>
> 1 KW output on CW would mean it has to run 1500W min (figuring 66% eff) in order to get 1 KW out.  CW should be classified as ICAS and not CCS.  If i can run 500W output in AM mode (DSB with carrier) then i should be doing 2 Kw input if you use the 50% carrier and 25% in each sideband rule.  But the tube is supposedly only a 1KW tube which would cause you to run the amp at 250W output and not 500.
>
> RTTY would be harder on the tube because you have a tone applied continuously.
>
> Either way the ebst i can do is about 600W out on CW with my 8295A.  I tested several others today and found all of them to be defective dither in the screen or plate circuits apparetly internally.  Would like to find a new tube that i wouldn't have to take out a mortgage on the truck in order to afford it.  Nix on any PL-172 types.  Worse comes to worse i could change it out along with the socket to a 4CX1500B.  Only requires a couple additional parts changes then.. Got several of those NIB.
>
> This things sets here at 250W output and doesn't even have warm air coming out the exhaust vent.  Gotta becareful though cause when you get to the 50 ma Ig mark then it's close to dumping.  Requires about 380 ma Ip to get 600W output in CW mode.  Oughta make some noise..
>
> Larry
> W0OGH
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