Re: {Collins} 30L-1 Input SWR High on 21/28 bands



Here's what W8JI said about that cable, FWIW:

"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."

Just FYI. I've never owned a 30L-1 and so never had a dog in this fight.
:-)

However, depending on how much loss that piece of cable has on 10m, it
could certainly lower the SWR due to loss alone.

Does the 30L-1 normally have a 2:1 SWR? In wouldn't have mattered if it was
driven with a 32S-3 or KWM-2A, because the pi-net in those transmitters
would have matched that with no problem.

73, Mike
www.w0btu.com

On Fri, May 29, 2015 at 12:18 PM, Scott Johnson <scottjohnson1@xxxxxxx>
wrote:

> According to the writings of the big W.B., it was done exclusively to
> improve the already good IM performance, and, correct me if I am wrong
> Bill,
> he wasn't adamant that the 20.5 ft. be adhered to for amateur operation.
>
> Scott W7SCJ
>
> -----Original Message-----
> From: Collins [mailto:collins-bounces+scottjohnson1=cox.net@xxxxxxxxxxxxx]
> On Behalf Of Mike Waters
> Sent: Friday, May 29, 2015 9:32 AM
> To: Bill Carns
> Cc: collins@xxxxxxxxxxxxx
> Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands
>
> Hi Bill,
>
> As I understand it, the 20.5" cable length that Collins recommended in the
> manual was more about improving stability on 10/15 meters, than IMD. I
> forget all the details. I thought it might help, but it sounds like you've
> already tried it.
>
> 73, Mike
> www.w0btu.com
>
> On Fri, May 29, 2015 at 8:38 AM, Bill Carns <wcarns@xxxxxxxxxxxxx> wrote:
>
> > Input coax will NOT affect the input VSWR.  The cable length thing is
> > way overrated anyway regarding IMD.  Been a number of good threads on
> > this.  I use what fits.
> >
> > Bill
> > Bill Carns, N7OTQ (Trustee K0CXX)
> > Past President, Collins Collectors Association Founding Board, Collins
> > Radio Heritage Group Editor, Signal Magazine Wimberley, TX
> > 512 618 2762  (Cell)
> > 512 847 7010  (Home)
> >
> >
> >
> > -----Original Message-----
> > From: Collins
> > [mailto:collins-bounces+wcarns=austin.rr.com@xxxxxxxxxxxxx]
> > On
> > Behalf Of Mike Waters
> > Sent: Friday, May 29, 2015 8:14 AM
> > To: collins@xxxxxxxxxxxxx
> > Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands
> >
> > How long is the input coax? The 30L-1 manual recommends 20.5 feet of
> > RG-58C/U.
> >
> > 73, Mike
> > www.w0btu.com
> >
> > On Fri, May 29, 2015 at 6:44 AM, Joe <jmarino@xxxxxxxxx> wrote:
> >
> > > ... check the input swr with my MFJ-259B and the lower bands are
> > > down at
> > > 1.3 but 21 and 28 are >2.0 with the slugs turned all the way in.
> > >
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