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



OK guys, let’s get some more practical RF theory injected here.

 

For one thing, Don Jackson, a professional transmitter and receiver designer and author for our Signal Magazine , wrote two very good articles in the Signal (Q3 2013 and Q4 2013) about that cable and its impact on IMD and Stability. You should read those. That work involved three professional engineers and a lot of work and many 30L-1s. While there is a stability issue on the 30L-1, it is by no means a bad problem in practice, and if it was, Collins would have fixed it long ago. That problem was NOT related to the input characteristics, but rather involved a spurious output in the 700 to 900 kHz region that appears in each and every 30L-1. Since it is low level and then attenuated by tuner and or antenna characteristics at that low frequency, it has never been detected on the air as far as we know. The only instability noticed in the upper bands during all of this testing was not related to input termination or cable characteristics, but rather to questionable load characteristics.  Into a good antenna, and properly tuned, the 30L-1 is well behaved on all bands.

 

For those interested in this spurious output in the AM band, read the article, look at the modeling work that was done, and there is a simple fix that was tested on 5 30L-1s which all became clean as a whistle. The  instability that was found, and analyzed and dealt with in the Q4 2013 article was a “tuned grid, tuned plate oscillator” that runs in the broadcast band. After modeling the circuit and duplicating the oscillation theoretically, Don made a change in the L3 Grid Choke to 56 uHy which fixed the problem. He found a particular choke that had the suitable inductance and a very high (75 MHz) self-resonant frequency, and this totally eliminates the spur should you so desire. The choke is used in parallel with resistor to eliminate the spur.

 

Don also comments in his articles about some of the W8JI work and I will let you read his comments about this “work”.

 

Re the loss of the longer cable helping with stability.  Hmmm..  The loss of a typical 9913 type (RG-8U) cable is less than one db total PER 100 FEET at 30 MHZ.  The amount of loss injected into the stability picture at the input by using a 20.5 foot cable instead of the typical 4 to 6 feet of cable is mice nuts – much less than one db.  I suspect strongly if loss was the motivation for the recommendation to use the longer cable, the length would not have been set at 20.5 feet. 

 

In fact, Warren Bruene wrote an article about the use of the 20.5 foot cable in the 30S-1. He found that it did help with phase distortion and was based upon the principal that with any cathode driven  amplifier. phase distortion can be minimized by using a cable length that has a phase shift in the band of interest that is either zero or some multiple of 90 degrees.  Don duplicated Warren’s analysis of the 20.5 foot length and found that it satisfied this criteria on average for all of the then ham bands except one. Warren also acknowledged that phase distortion was a small component of the total measured IMD which consists of both phase and voltage distortion.

 

Gene Senti was the designer of that amp. This amp “inherited” the 30S-1 20.5 foot cable recommendation as far as we can tell. The smaller amp was expressly designed to be a Low Cost solution to a table top linear and one can only conclude that it WAS VERY SUCCESSFUL..  There are way more 30L-1s made, in use today, and surviving, than any of the other flavors. Some of the “compromises” in the design of the 30L-1 are due to cost, and some are more subtle.  While- for instance- the grid grounding could have been improved, that would have made the ALC and the metering much more challenging and Gene chose the circuits and component values that he used for a reason. I will repeat something I often say..  You have to look at the entire system and the design goals before you start changing or commenting critically on one component or technique. I personally run 3 30L-1s here on a multitude of bands and have never seen a problem. I discussed that cable with Gene one afternoon, and he pooh poohed the need for it and said it was a vain attempt to get the IMD even lower than it already was and that it died when Art looked at the need for it and told him “If there is a problem, solve it, and if there isn’t a problem, get rid of it.”  They got rid of it.

 

I suggest you all read the two articles involved and then draw your own conclusions. While there is a theoretical basis for using the 20.5 foot cable and Don did confirm the theory, he also concluded, as did Art, that it was unnecessary and lost in the “noise”.

 

Enjoy your 30L-1. It’s a great little amp.  You cannot argue with success. And finally, no, the VSWR on the line is not a function of cable length, unless of course you make the dang thing so long that it starts to look really lossy, and then it appears to help – but you have thrown the power away anyway. There is no free lunch. And, as concluded last time after the two great articles by Don, don’t worry about that 20.5 foot cable. Make it what fits and if you have an input stability problem, start looking for another culprit.

 

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)

 

 

 

 

From: Mike Waters [mailto:mikewate@xxxxxxxxx] 
Sent: Friday, May 29, 2015 12:47 PM
To: Scott Johnson
Cc: Bill Carns; collins@xxxxxxxxxxxxx
Subject: 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 <http://www.w0btu.com> 

 

On Fri, May 29, 2015 at 12:18 PM, Scott Johnson <scottjohnson1@xxxxxxx <mailto: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 <mailto:collins-bounces%2Bscottjohnson1> =cox.net@xxxxxxxxxxxxx <mailto:cox.net@xxxxxxxxxxxxx> ]
On Behalf Of Mike Waters
Sent: Friday, May 29, 2015 9:32 AM
To: Bill Carns
Cc: collins@xxxxxxxxxxxxx <mailto: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 <http://www.w0btu.com> 

On Fri, May 29, 2015 at 8:38 AM, Bill Carns <wcarns@xxxxxxxxxxxxx <mailto: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 <tel:512%20618%202762>   (Cell)
> 512 847 7010 <tel:512%20847%207010>   (Home)
>
>
>
> -----Original Message-----
> From: Collins
> [mailto:collins-bounces+wcarns <mailto:collins-bounces%2Bwcarns> =austin.rr.com@xxxxxxxxxxxxx <mailto:austin.rr.com@xxxxxxxxxxxxx> ]
> On
> Behalf Of Mike Waters
> Sent: Friday, May 29, 2015 8:14 AM
> To: collins@xxxxxxxxxxxxx <mailto: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 <http://www.w0btu.com> 
>
> On Fri, May 29, 2015 at 6:44 AM, Joe <jmarino@xxxxxxxxx <mailto: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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