In article ,
Scott Dorsey wrote:
Anyone out there still remember the M-30B? I am dusting off one at a
local station and have a few questions.

There is an "operate/tune" switch which appears to change the screen grid
bias to reduce the output level when tuning, but the directions for alignment
don't mention it.

It appears I can do all of the tuning up until the output stage section
with the switch set to TUNE. I can tune the output section (plate and
antenna coupling) and get the power set around 95 on the meter, like
the manual says, with the switch set to tune.

I assume, though, that I want to set the antenna coupling control for
final power setting using the switch on OPERATE.

So questions include: how do I use the switch for tuning, AND once I have
done that, is there any reason not to just leave the switch on TUNE in
order to reduce power and extend the life of the final if I don't mind
the reduced range?

I'm not familiar with that particular transmitter, so my response will
be in general terms.

From your description, it has tetrodes in the final, since you refer
to screen voltage. With tetrodes, power output is controlled by
screen voltage. Putting a variac in the screen supply primary was a
very standard method of doing this for things like day-night power
reduction. You might tell us what tubes are in the unit for
buffer-driver and final stages.

Yes, a final tweak when you go to full power is a good idea, but you
should not need to move things very far to get right back in the
center of the sweet spot.

As to running on the TUNE position permanently for reduced power, it
depends. Assuming the TUNE-OPERATE control does operate by lowering
screen voltage, how does it do this, and how many stages are affected.
You may be doing this through resistors, with a setup intended only
for short duty cycle operation in the TUNE mode. As a Variac in the
screen supply implies, best method is to use an effective change in
the turns ratios in the devices between the power in and the screen
supply rectifier input.

Not only that, if I set the antenna coupling for reduced power, is that
going to extend the life of the finals?

I don't see why it would. You don't want to detune the output tank on
a Class C amplifier---it gets its efficiency by pulsing the tank at
the right point in the cycle. And if you're decoupling by reducing
mutual inductance between the tank and the antenna feed, yes, you'll
reduce power going to the antenna, but may not reduce average plate
current by much.

As I've said, with tetrodes, you'll control plate current with screen
voltage/current as a throttle. No real need to reduce plate B+ if the
plate and screen supplies are separate. Also watch for upsetting
things in prior stages if you lower plate and screen voltages to them.

Hank