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?

Not only that, if I set the antenna coupling for reduced power, is that
going to extend the life of the finals?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
(Scott Dorsey) wrote:

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?

Gee, and I get called an old fogey...I haven't seen tube-type gear in
ages! But seriously, to answer your question: there might be a problem
doing that. The purpose of the "tuning" mode is to limit stage operating
parameters to "safe" levels until proper tuning is either achieved, or
is good enough to allow safe operation of the tube.

You have to ask yourself if the unit, properly tuned, might be better in
terms of efficiency and component longevity in "normal" mode than in the
parameter-limiting "tune" mode. It's a distinct possibility.

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

It could, assuming you don't kill the efficiency of the output stage.
One of the arts tuning up any transmitting device is finding the sweet
spot in terms of plate current, grid drive, and stage coupling. The
Harris MW-5 transmitters were notorious for having a narrow range of
final plate current and drive. Operate outside that range, and your tube
life drops precipitously, even though from a raw tube-spec point of
view, everything was well within limits.

--
John Higdon
+1 408 ANdrews 6-4400
AT&T-Free At Last

John Higdon wrote:

You have to ask yourself if the unit, properly tuned, might be better in
terms of efficiency and component longevity in "normal" mode than in the
parameter-limiting "tune" mode. It's a distinct possibility.

That is, in fact, why I was posting. It looks like they are just changing
the screen grid bias, so I could imagine some situations where that could
be the case.

I am assuming that if I really wanted to reduce the power, the sane thing
to do would be to reduce the B+ voltage to the final?

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

It could, assuming you don't kill the efficiency of the output stage.
One of the arts tuning up any transmitting device is finding the sweet
spot in terms of plate current, grid drive, and stage coupling. The
Harris MW-5 transmitters were notorious for having a narrow range of
final plate current and drive. Operate outside that range, and your tube
life drops precipitously, even though from a raw tube-spec point of
view, everything was well within limits.

Yup. I was hoping to find someone old enough to remember setting these
things up for regular use. I dragged this one out and started cleaning it
up because the 160 MHz auxiliary channels are totally unused in our area,
and the 950 Mhz stuff is very crowded. And you know, this thing doesn't
sound bad. Down 3dB at 8 Khz which may be a transformer issue.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
(Scott Dorsey) wrote:

I am assuming that if I really wanted to reduce the power, the sane thing
to do would be to reduce the B+ voltage to the final?

Of course, there are numerous ways to trim the output stage power.
Adjusting plate voltage is one, but to avoid over-dissipating the
screen, both screen and plate voltage should be adjusted together.
Another way is to adjust the loading, as you mentioned earlier. Due to
those "sweetspot" considerations that I mentioned, the loading should
only be used for minor power adjustments during operation. As I'm sure
you are aware, stage tuning and loading go hand in hand.

Yup. I was hoping to find someone old enough to remember setting these
things up for regular use. I dragged this one out and started cleaning it
up because the 160 MHz auxiliary channels are totally unused in our area,
and the 950 Mhz stuff is very crowded. And you know, this thing doesn't
sound bad. Down 3dB at 8 Khz which may be a transformer issue.

We're using the 160MHz channels in our area as well, since everyone else
went to 450MHz. But I've only had to deal with RPU gear in the past ten
years, so I missed the joys of Marti tube gear. FWIW, the very first
piece of auxiliary broadcast gear I ever laid hands on nearly forty
years ago was a Moseley PCL-303. It was solid state, of course, and back
then transistors that could operate at 950MHz were still considered
magic.

--
John Higdon
+1 408 ANdrews 6-4400
AT&T-Free At Last

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

John Higdon wrote:
We're using the 160MHz channels in our area as well, since everyone else
went to 450MHz. But I've only had to deal with RPU gear in the past ten
years, so I missed the joys of Marti tube gear. FWIW, the very first
piece of auxiliary broadcast gear I ever laid hands on nearly forty
years ago was a Moseley PCL-303. It was solid state, of course, and back
then transistors that could operate at 950MHz were still considered
magic.

So what is the current licensing status on these? Do I need to get an
auxiliary license added to the station license like with the STL, or do
I just call the local frequency coordinator and ask to book the frequency
for certain days? I remember this being covered on the first phone test
but a lot of things have changed since I took the test....
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
(Scott Dorsey) wrote:

So what is the current licensing status on these? Do I need to get an
auxiliary license added to the station license like with the STL, or do
I just call the local frequency coordinator and ask to book the frequency
for certain days? I remember this being covered on the first phone test
but a lot of things have changed since I took the test....

Yes, those must be licensed for routine use. The difficulty in doing
that will depend upon where you are located. Here in the San Francisco
Bay Area, the channels are packed. Even use of a licensed channel
requires spot coordination by anyone planning use that might spill out
of his immediate service area.

The bottom line is it needs to be licensed and you need to coordinate.

Read Part 74.

--
John Higdon
+1 408 ANdrews 6-4400
AT&T-Free At Last