[TMC] Anyone using a PAL-350 (any letter suffix)

[John Vendely]

Hi Meir,

Despite what the manual suggests, the PAL-350 is completely capable of 
developing 350W PEP or 350W average (e.g., under single-tone, or carrier 
conditions, where PEP=average).  Same with the PAL-500, which will 
develop 500W PEP or 500W average.  The two RF amplifiers are essentially 
identical.  However, installing 4CX350s in the PAL-350 won't increase 
output power, due to power supply limitations, but I don't know why it 
should misbehave as badly as yours does.  Bias for the 4CX350 will be 
different, which you have already taken care of.  Substitution of 
4CX350s may improve linearity, however, as this tube was designed 
primarily for linear service, is inherently better in this respect than 
the 4CX250.

The best way to tune the the PAL-350 is to load for 350W output, single 
tone, then set exciter drive to limit output to 350W PEP max under voice 
conditions.  Alternatively, load to 350W PEP with a two-tone signal, 
which under these conditions should produce an average power of 175W.

The symptom you describe is totally abnormal, and could be due to a 
number of causes, but I would first check to be sure the driver stages 
are properly aligned, have the requisite gain, and are capable of 
developing the required final grid swing.  This is important, because if 
final grid RF swing is too low, the natural tendency is to load the 
amplifier too heavily in a futile attempt to make rated power.  This 
typically results in very high plate current, and low output power and 
efficiency, kinda what you're experiencing.

A clue to the output loading conditions is the relationship of screen 
current to plate current at resonance.  If the amplifier is loaded too 
lightly, the screen current will be too high, and will tend to be 
"touchy" and critical during adjustment.  If the amplifier is loaded too 
heavily, screen current will tend to be lower than normal for the output 
power.  BTW, these amps will normally show negative screen current under 
some conditions.

Monitor both screen and plate currents, and tune for a dip in both, 
favoring the screen dip over plate dip if you find they don't quite 
coincide.  If they're widely divergent, there may be a neutralization 
problem.  Note that the PAL series amplifiers (including PAL-1K) tune 
differently from hobby-type amplifiers with tetrodes, and no negative 
feedback.  Those amps will typically show maximum screen current at max 
output, nearly coincident with plate dip.  It should be possible to get 
350W on carrier, with plate efficiency around 60%, and 350W PEP with a 
two-tone, with plate efficiency about 30-35%.

I would strongly recommend not using the ALDC.  TMC did so many things 
"right", but they really blew it with that ALDC design, which doesn't 
work properly and adds copious amounts of intermod distortion.  To see 
for yourself, try loading the amp to full rated power and linearity, and 
with a two-tone signal applied, monitor the envelope with a scope.  Dial 
in some ALDC, and watch what happens to the waveform.  It ain't a pretty 
sight. Therefore, dial back the ALDC completely to prevent any gain 
reduction, and prevent overdrive by control of the baseband audio 
amplitude.  If you find overall RF gain is low, it could even be due to 
maladjustment of the ALDC pot, or a fault in the ALDC feedback loop.

Hope some of this helps.  These are beautiful amplifiers, highly 
reliable and capable of excellent linearity, far better than amateur 
equipment.   Let us know how it goes...

73,

John K9WT

On 8/4/2018 10:39 PM, Meir WF2U wrote:
> Folks,
>
> If anyone in this group using any version of the PAL-350, I'd like information regarding the maximum power output on CW (or carrier with no modulation) and the plate current associated with it.
> I'm testing a no suffix PAL-350 which the previous owner upgraded from the original 4CX250:s to 4CX350's. The bias is set as per the later version which came with 4CX350's from the factory. I'm driving it from an SBE-3, connected to the high power input.
> With maximum plate current (340 ma), before the overload circuit trips the PSP-350 supply, the power output is about 110W into a dummy load. The amp is tuned properly (dip with Tune, etc.). This is less than 20% efficiency, certainly just abot half of the power specified for carrier only. The plate voltage is around 1900 V at full load, with 117 VAC primary voltage. This amp came to me from someone who used it in the same setup, I even heard it on the air (I don't know what the actual power output was then).
> I have a second unit, an -A model, with its power supply, that I got in an unknown working condition. I hooked up that amp to the other supply used with the first amp, I got the same results.  Next test will be with its original -A suffix power supply.
> This low efficiency baffles me. Any ideas?
>
> Thanks,
>
> 73, Meir WF2U
> Landrum, SC
>
> ⁣