{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?

Rick WA1RKT
Londonderry, NH
Catch me on 14.300 MHz Intercon / MMSN most days



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