Re: {Collins} Tuning the 30L-1

Read some of my past comments on tuning the 30L-1. There have been a ton of
strings on this. I would suggest that at 80 watts drive and 700 watts
output, you may be above the point where the amp is linear. It will go
there. If you want to find out quickly:

Rough method. - run drive up from about zero and watch output rise and you
will see/feel the linearity between input and output. Stop and go back and
note output at the point right before you notice the knee start to occur.
More rigorous: Add a watt meter between the driver and the 30L-1 and
actually plot Pout vs Pin and you will see a definite knee and the drive
point is usually down around 60 watts or less.

That amp will make way more power than it will stay linear at as will most
amps. One of the most common mistakes operating a 30L-1 is to overdrive it.
The IMD suffers first and then the amp itself.

That's my two cents worth for the evening.

Bill Carns, N7OTQ
Trustee K0CXX
President, Collins Collectors Association
Editor, Signal Magazine
Wimberley, TX
512 618 2762  (Cell)
512 847 7010  (Home)

-----Original Message-----
From: Collins [] On
Behalf Of Rick Poole WA1RKT
Sent: Saturday, November 01, 2014 3:58 PM
To: collins
Subject: {Collins} Tuning the 30L-1

I have two 30L-1 RE amps that have been in use for a couple of years, one
driven by a KWM-2A and the other by a Heathkit Marauder.

I have never had any meaningful success following the process in the manual
for tuning.  Instead I have had good luck tuning for maximum output while
keeping the output at or below 700 watts PEP and paying close attention to
the duty cycle to keep from stressing the tubes.  Then I increase the
loading a notch or so to hopefully keep the grid current under control (gee,
thanks, Art, for the grid current meter... NOT...), re-peak the tuning and
there they sit.  Load control usually ends up between 3 and 4.  Both amps
seem happy with that.

The "Tune" meter is nowhere near zero when I finish... usually it's around
.1 or .2 below zero.

Tuning duty cycle is:  10 seconds on and 20 seconds off, repeated three
times, then a full minute off, lather, rinse, repeat... although usually the
amp is tuned before the end of the second 10-20 cycle.

I had one of the amps (with four older RF Parts 811A tubes) on the bench for
some unrelated work, and connected it to a dummy load and peak-reading
SWR/power meters on both input and output.  Tried the "book" way of tuning
and was able to get only about 400 watts peak out with 100 watts peak drive,
the Load control ended up at about 9, and the tube plates would start to
turn color within just a few seconds.  I didn't dare leave it on long enough
to check the plate current but I'm sure it was very high.

Reverted to "my" way and got 650 watts peak out with 80 watts peak drive and
600 ma plate current, the Load control ended up at about 3, and the tube
plates stayed black for the full 10 seconds (didn't try it for longer than
that).  SWR was 1:1 at the amp's output (to the dummy load) and 1.1:1 at the
amp's input.

My question is... why are the "book" method results so obviously far away
from anything reasonable?  Do I need to go through all the components to the
left of the tubes in the schematic, and check them for value?  Is that
circuit a common failure point in these amps?

Londonderry, NH
Catch me on 14.300 MHz Intercon / MMSN most days
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