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

By the way (with 50 ohm cable coupling the driver to the PA in a transmitter), I should have added that when you do the math on a 20.5 foot cable and take an average velocity factor for our cables of around 0.7, that cable is really about 14 feet long electrically. This equates to 4.26 meters or a wavelength of 17 meters...  So, it you were to use that transmitter on 17 meters or even 15 meters, with a 50 ohm coax connected between the driver tube (which presents what amounts to an open circuit - - 20,000 ohms terminating a 50 ohm cable), that would translate to a short across the plate of the driver tube at RF frequency (the active load line) and that tube would be very unhappy.


-----Original Message-----
From: Collins [] On Behalf Of Gaylord Hart
Sent: Saturday, May 30, 2015 3:03 AM
To: 'Steve Beveridge'; collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands

I'm with you Steve.  I don't think the cable length makes a bit of difference, and if anyone here has actually tuned and loaded the XMTR and AMP combo and looked at actual IMOD performance (before and after) on a spectrum analyzer with "specified cable length" versus short lengths, I'd love to see the data.  I'm a big believer in hard data, and without it, not much matters.  Theories are wonderful, but every physicist will tell you they have to be validated with real-world confirmation from observed measurements.   Quite honestly, the measurement tools we have available today by far exceed the tools available when the equipment we are discussing was designed.  If anyone wants to spend the time to make the measurements, and has the right equipment to do so, this can be put to bed once and for all.  I have the equipment and expertise, but am not inclined to spend the time here.  I believe Steve is right when he indicates Collins dropped the issue after realizing it was a non-issue.

If you follow the logical "theoretical" conclusions presented by some here of the need for a measured coax between driver and amp to reduce distortion, then this should apply to the driver and finals in the transmitter itself, and I've never seen a transmitter that had a 20' coil of coax between these two tubes on any of my XMTRs.


-----Original Message-----
From: Collins [] On Behalf Of Steve Beveridge
Sent: Friday, May 29, 2015 5:28 PM
To: collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands

Hi folks,

Appreciate the technical analysis Don.

Taking a more simple line, one only has to look at the more recent versions of the 30L-1 manual to find that any  mention of the magical
'20.5 ft' cable is deleted entirely.

For example, looking at the 7th edition manual (1965), a 4 ft RF input cable (RG58) is provided by Collins. Not much attenuation there! Here's the link:

Steve VK2LW
CCA member

On 30/05/2015 8:41 AM, Don Jackson wrote:
> Hi All,
> Ok, I'll chime in here. First, an extra dB or so of loss in the cable 
> is highly unlikely to have any affect on 30L-1 stability. My studies 
> didn't show any significant affect. If the real question concerns the 
> function of the 20.5 foot cable, here are two quotes from Warren 
> Bruene on the subject. This design approach was originally intended 
> for the 30S-1 in an attempt to obtain a few more dB of IMD performance.
> From “SSB Principles & Circuits”, Bruene states:
> “The RF coupling network between a linear amplifier stage and its 
> driver should have a total electrical length of either zero degrees or 
> some multiple of 90º. This is necessary to avoid phase distortion due 
> to a nonlinear load on the coupling network.”
> From Bruene’s QST article “Inside GG Amp” he states:
> “...nonlinear grid-current loading causes flat-topping and becomes the 
> major cause of intermodulation distortion at large signal levels. The 
> effect of this nonlinear grid loading can be reduced by providing a 
> low driver source resistance. This is the reason for the special 
> length of coax specified to connect a Collins KWM-2 to a 30S-1 
> amplifier, for example. This special length, plus the phase delay in 
> the KWM-2 output network and that of the 30S-1’s cathode circuit, 
> approximately equals some multiple of 180º on each band. This provides 
> a low source resistance, which reduces the effect of nonlinear 
> screen-current loading in the 30S-1’s 4CX1000A tetrode, which is 
> cathode-driven and operates in class AB1. The length of connecting 
> coax is therefore important for minimizing IMD.”
> For my money, this is the final word on the function of the 20.5 foot 
> cable. My article on the cable contains further analysis, but reading 
> it is only an option if you wish to completely understand how the 
> cable does what it was meant to do.
> Don, W5QN
> -----Original Message----- From: Mike Waters
> Sent: Friday, May 29, 2015 4:45 PM
> To: Bill Carns
> Cc: collins@xxxxxxxxxxxxx
> Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands
> I don't have time to read all this now, but so far, so good, and I've 
> learned a few things. Thanks.
> However, I did notice that you refer to RG-8; but Collins specified 
> RG-58C/U with more loss. Just food for thought. I didn't calculate 
> anything.
> 73, Mike
> On Fri, May 29, 2015 at 4:24 PM, Bill Carns <wcarns@xxxxxxxxxxxxx> wrote:
>>  [snip]
>> 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. ... 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.
>> ... 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”.
> [snip]
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