Centron cm-u tuned input

[W6BDM]

I am having trouble getting proper swr back to my rig (1.8:1) on all bands. In the instructions it says to vary the lenghth of the cable to the amp if driving problems occur. Right now I am using a 2 foot cable to the amp and a 3 foot cable from the radio. Does that cable need to be longer? I thought I would ask before I start chopping or making a whole new cable.. I wonder if I have a bad unit.
73, w6bdm

[K9STH]

The Collins 30S-1 linear requires a 20.5 foot long piece of RG58/U between the exciter and the linear. Now this was for best IMD performance. When the 30L-1 was designed it originally required the same 20.5 foot input cable. However, Art Collins instructed the design team to "fix the problem" and the unit was redesigned.

Glen, K9STH

[W6BDM]

I have a Dentron Clipperton L running with a kenwood ts-570d. I will try a 20 footer. I have to make it first!

[KL7AJ]

Originally Posted by K9STH

The Collins 30S-1 linear requires a 20.5 foot long piece of RG58/U between the exciter and the linear. Now this was for best IMD performance. When the 30L-1 was designed it originally required the same 20.5 foot input cable. However, Art Collins instructed the design team to "fix the problem" and the unit was redesigned.

Glen, K9STH

You droll person, you!

[K5FH]

You didn't specifically say, but it sounds like you're reading that 1.8 SWR on the TS-570's meter. SWR meters in rigs are notorious for their inaccuracy. However, the SWR between rig and amp is probably over 1.5 which is causing the protective circuitry in the 570 to throttle back the output.

What I would suggest is using an outboard SWR meter between rig and amp and then retuning the amp's input circuitry for minimum SWR. Do this into a dummy load, obviously, and try to do this with the same length of rig-to-amp cable that you are going to use in normal service. The Clipperton-L has pretty much achieved antique status and, as a result, the input network probably needs a major overhaul due to its age. Fixed capacitors do not age well.

[KL7AJ]

Originally Posted by K5FH

You didn't specifically say, but it sounds like you're reading that 1.8 SWR on the TS-570's meter. SWR meters in rigs are notorious for their inaccuracy. However, the SWR between rig and amp is probably over 1.5 which is causing the protective circuitry in the 570 to throttle back the output.

What I would suggest is using an outboard SWR meter between rig and amp and then retuning the amp's input circuitry for minimum SWR. Do this into a dummy load, obviously, and try to do this with the same length of rig-to-amp cable that you are going to use in normal service. The Clipperton-L has pretty much achieved antique status and, as a result, the input network probably needs a major overhaul due to its age. Fixed capacitors do not age well.

You have to remember that the INIPUT impedance of a grounded grid amp is highly dependent on the load impedance and tuning...as well as DRIVE level, to a certain degree.

[W6BDM]

The 20 footer doesnt work. I tried 3 different lengths all get the same swr.

[AF6LJ]

Originally Posted by KL7AJ

You have to remember that the INIPUT impedance of a grounded grid amp is highly dependent on the load impedance and tuning...as well as DRIVE level, to a certain degree.

One of many reasons why I don't care for GG amplifiers.
Their only advantage is their simplicity.

[W6BDM]

The cm-u unit is not adjustable unfortunately.

[KL7AJ]

Originally Posted by AF6LJ

One of many reasons why I don't care for GG amplifiers.
Their only advantage is their simplicity.

Well, actually, there are a couple other nice things about them. Any excess drive power is passed on through to the output. And you don't have to neutralize them. (I guess that goes in the simplicity category, eh?)

Eric