I spent 40 years maintaining Broadcast transmitters. It was always my
preference to get the final tube(s) close to their operating points
before working my way back to the lowest power drivers. Keeping the
screen voltage low, or keeping the grid voltage near or beyond cutoff
while using a sensitive voltage or power meter to find the matched
output network settings resulted in a much reduced stress level..... for
both me and the tubes. Cathode over-current relays were set at 1/3rd to
1/4th typical operating conditions until relatively stable operations
were achieved. We always worked from the highest powered stages
backwards to give the lower powered stages a load to work into.
Lowering filament voltages during tuneups to below operating levels also
helped to keep the "bang" factor low. W4DNR
On 2022-09-12 6:49 pm, Joe Subich, W4TV wrote:
So, where does that drive power go while the driver(s) are being
tuned, and the final is on standby?
Drive power is relatively low and typically dissipated in the grid
circuit of the succeeding stage. For example, the BTR1 (pair of 4-400
modulated by a P-P pair of 4-400) - one of which is in my garage - has
a single 6146 driving the grids of the 4-400s probably less than 50W
carrier drive for 1 KW carrier output.
Based upon that description, when WLW was running a full 500KW, I'd
assume the engineer would set the carrier around 125KW, and then 100%
modulation would peak at 500KW.
Nope ... that 500 KW was carrier power as is typical for AM broadcast.
The RCA BTR-1 is spec'd for 1 KW AM and will certainly do 5 KW PEP
(125%
positive modulation) if one were using a scope to look at the output.
73,
... Joe, W4TV
On 2022-09-12 6:06 PM, Donald Fox via Amps wrote:
One of the things I see that is different, as I have watched videos of
the bigger transmitters firing up, is that they can bring the driver
stages online, and have them drawing plate current, tune them, and
then engage the final. Obviously, there is not tx/rx relay for a plain
transmitter. So, where does that drive power go while the driver(s)
are being tuned, and the final is on standby? There are plenty of such
videos out there that are exciting to watch!
FM of course, does not have any 'swing", so you crank the carrier
where you want it to be, and it just sits there. AM on other hand of
course, is set for about 25% of max carrier, and them modulation takes
it up and down from there. Based upon that description, when WLW was
running a full 500KW, I'd assume the engineer would set the carrier
around 125KW, and then 100% modulation would peak at 500KW.
Any way you slice it, that is a bunch of RF up the feedline!
Don N8ECH
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