Collins 30L-1 help question - please

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Dave Plowman:
Gentlemen
I have a recently aquired Collins 30L-1 amp that I want to use with my S line.
Reading the manual I notice, in several places, that there is a very specific requirement for the coax between the exciter and the amp of 20.5 ft of RG-58CU.
Please explain why Collins specifies that length and does it absolutely have to be RG-58CU and not and not just any good 50 ohm coax.
I am anxuous to get this amp on the air so any help would be greatly appreciated from someone with personal 30L-1 experience.
Thanks
Dave
W4PBU

Clark McDonald:
Read this rather recent thread on the same subject, paying much attention to what W8JI has to say about the subject: 

http://www.eham.net/ehamforum/smf/index.php/topic,68774.0.html

Then use a piece of coax that reaches from exciter to amp and be not afraid.


Tom Rauch:
Quote from: W4PBU on September 05, 2010, 11:10:16 AM

Gentlemen
I have a recently aquired Collins 30L-1 amp that I want to use with my S line.
Reading the manual I notice, in several places, that there is a very specific requirement for the coax between the exciter and the amp of 20.5 ft of RG-58CU.
Please explain why Collins specifies that length and does it absolutely have to be RG-58CU and not and not just any good 50 ohm coax.
I am anxuous to get this amp on the air so any help would be greatly appreciated from someone with personal 30L-1 experience.
Thanks
Dave
W4PBU


The real reason Collins wants that length of cable is the 30L1 is unstable.

It has 4 811 tubes in parallel, and that is NEVER stable without neutralization. Even Gonset, Heathkit, and Ameritron neutralized their amps with 4x 811 tubes.

Worse yet, the grids in the 30L1 are poorly grounded through low value mica caps. This hurt stability also. As a matter of fact Collins had a series of component changes trying to make that amp more stable, including some changes in the grid RF choke system.

A partial fix that results in a bit more stability is to lower the Q of the input system by adding loss, and this is why the long input cable is used and should be used.

The story Collins used was some weird tale about phase shift and feedback having to total a certain number of degrees, but if you actually analyze the phase shift it is nothing like the story claims. Years ago I bench tested a 30L1 and could easily make it oscillate on ten meters with a short cable to a KWM2. Adding 20 or 30 feet of RG58 made it a whole lot less touchy to tune  on ten meters. Adding a 2 dB attenuator pad made things even better.

A fellow with a WA9 call, independently of me, found exactly the same thing.

73 Tom



Jeff Young:
Collins and a few other manufacturers require a 50 ohm termination, but did not design in the impedance for some reason. I use a 50 ohm terminator to help out the design "problem" with some of their test equipment problems...
Course, collins and other manufacturers never make mistakes...

John Raydo:
I use about a 4ft piece of RG8x, just enough to get from here to there.  Works fine on all bands.  RG 58 would also be find.  However do use good quality RCA plugs (my have a metal shield and reasonably good internal connections.  Some I have seen are junk.

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