[TMC] Report #4

[telegrapher at att.net]

Fired up the beast today around noon and let it cook for a while before 
doing any testing.  Transmitting was done into 50 ohm dummy loads, one a 
heathkit Cantenna and the other is a motorola air cooled unit.  I worked 
with first one and then when it got warm i would switch off to the other 
so i didn't cook anything.

I find tuning this thing is like the PAL-500 amp, it can be touchy and 
unless the meter readings are understood, you could get into trouble. 
For one thing the screen current Isg fluctuates around depending on the 
action of the Tank tuning control and it would/can be distracting to 
watch both the plate current and the screen current readings at the same 
time.  It seems that if the tank is not tuned just right you could/can 
run the screen current off scale real easily.  The book says readings 
below 35-40 on the meter are allowable.  With a dip of the plate current 
and the loading control not properly tuned, max or min screen current 
does not jive with plate current dip.  As i've never played with 
anything like this power wise i'm kind of a novice at what is happening. 
  At any rate, i was able to tune it on 40 meters to around 300W 
indicated out on the Bird wattmeter.  this was with around 1v rf drive 
and a single tone imposed on the driving signal.  It was real easy to 
trip the screen overload breaker and for that reason i am thinking that 
the tube might be a bit on the soft side.  Isg drifted up with to much 
drive applied and with to little rated power output couldn't be 
achieved.  At 350 ma Ip the overload breaker would operate.

Now on 80 it was a different story, sort of.  300W out was easily 
obtainable with 300 ma Ip and around 20-25 on the Isg meter reading. 
When i removed the tone from the rf drive (HP 8640B) i was able to 
increase the drive to about 1.5V rf and with it tuned properly i 
actually saw 450W out with just 300 ma of Ip.  Isg was very reasonable 
around the 20 mark on the meter.  I would suspect that it should have 
been higher but i didn't push it, again thinking that the PL-172/8295 
may be soft.

All in all i think it's going to work out fine and if it operates 
continuously at these values, i'll probably reduce the output under AM 
conditions to 250W as i think it will handle this easily.  The air 
coming from the exhaust port on top of the cabinet never did get very 
warm indicating that either i  wasn't working the tube very hard or it 
wasn't able to do any more than what i was getting out of it.

Not knowing what value of Isg is acceptable as the book only gives you a 
"reference" point for operation over a range of frequencies, my thoughts 
are to keep that value as low as possible yet obtain the most power out. 
  Specs say that IMD products of -40 db are obtainable at some 
frequencies.  Again without test data or having the equipment to make 
these measurements or knowing just how to adjust the transmitter for the 
best IMD value we're kind of shooting in the dark.  I'll put a Heathkit 
SB-610 on the output and look at it on there.

The beast lives!  And i'm thrilled.  More work to be done to fully 
understand what all is going on but if it sounds good and i have a clean 
signal with 200-250 W output on AM, i'll be happy but also looking for a 
new tube.  This would be a really ideal RTTY transmitter as it'll never 
overheat no matter how long i keep it keyed down.  Wonder if the Dipole 
or a beam will handle it?  Hi Hi....  Certainly some of the FSJ-50 or 
other hardline coax i have around here will handle it.

Any thoughts or suggestions or comments, please send them along.

received a new choke for my VOX power supply today so that will be my 
next project, get the VOX operational so i'll have a VFO that belongs 
with the rack.

Thanks for listening.