[TMC] SBT-1K

Wed Dec 8 00:09:51 EST 2010

To add to what John said: the 8295 is rated at 1kw plate dissipation. So you
would subtract your output power from the input power to find how much plate
dissipation you are using. At 1kw output at 65% efficiency you would be
running around 1535 watts input. Subtracting 1kw from 1535 watts leave you
with only 535 watts plate dissipation. Well under the tubes rating. 

When tuning for AM you should tune up for cw at 1kw output and then reduce
the carrier to 250 watts output (ideally). This will leave the amp with the
1kw headroom to develop the 1kw pep of the 250 watt am signal.

At 250 watts output when tuned for 1kw out, the efficiency of the carrier
will be around 1/2 of what it is at full power or around 32%. So at 250
watts carrier your input power will be around 780 watts for the carrier.
Still well within the plate dissipation rating of the tube.
As full modulation is applied (100%) the pep output will reach 1kw and the
input power will reach 1535 watts pep input.

The efficiency with carrier at 250 watts is ideally 1/2 the efficiency at
full output but is dependent on the zero signal idling current of the tube.
The higher the zero signal plate current the lower the efficiency will be at
reduced powers as the tube is now approaching class A operation from class
AB at full power out.

As to the 500 watts output on AM, I have seen older equipment specified at
higher than 25% carrier power in some cases. AM will work at 50% carrier but
you will not be able to achieve 100% modulation without going into
distortion as the amp would be overdriven by the peak power and will
flattop.

However I have also seen some equipment specified and operated this way and
it works ok as long as the ALC is set up properly. The ALC will reduce the
drive level as maximum pep is approached allowing 100% modulation without
overdriving the amplifier. This will cause the carrier to pump up and down
with modulation. It operates like controlled carrier but in this case the
carrier swings down with modulation rather than up. If it is not overdone it
can operate quite satisfactory in this manor. Pep is limited to 1kw.

As to plate current dip and screen current dip being at the same point, if
they are not then the neutralization and or feedback is not quite right. It
should not matter if the load the amplifier is attached to is reactive or
not as that reactance should be canceled by the pi network tuned circuit.
There should be no reactance seen at the plate of the tube.

With a tetrode operating normally without feedback, as the plate current
dips the screen current should peak and the grid current should peak (if
operating in the grid current region) all at exactly the same tune point of
the plate tune capacitor if the tube is properly neutralized.

When RF feedback is employed around the tube, when plate current dips the
screen current and grid current should also dip slightly at the same point
of the plate tune capacitor. This is because as the plate is tuned to
resonance the plate RF voltage rises and that sends more feedback voltage
back to the grid lowering the drive slightly.

If the amp does not tune like this then the neutralization or feedback are
not adjusted quite right. This may vary from band to band slightly as it is
difficult to maintain these across a wide frequency range.

Bottom line, if you have a wide swing in screen current as you tune the
plate tune and a rather large dip in plate current then the amp is not
loaded heavily enough.

73
Gary  K4FMX

> -----Original Message-----
> From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net] On
> Behalf Of John Vendely
> Sent: Tuesday, December 07, 2010 10:03 PM
> To: tmc at mailman.qth.net
> Subject: Re: [TMC] SBT-1K
> 
> 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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