Well, you have a certainly started a string here.
A couple of suggestions for you.
Use the 4CX350FJ. May as well start with the lowest distortion.
Don't try to employ RF feedback across more than two stages.
Use one of the old TRW - Motorola - M/ACom class A bricks for the first stage.
Since you are operating narrowband, consider the feed forward configuration.
If you start with a -50dB distortion amp, you should be able to slice another
30dB off that figure for a -80dB system.
I doubt if your exciter, at any level, is clean enough to drive the above FF
Fortunately, a clean 80M SSB generator is easy to construct compared to the
balance of this undertaking.
73 & Good afternoon,
-- "Tom Cathey" <K1JJ@comcast.net> wrote:
Thanks for the replies.
Let's address a few comments. I'm trying to get the right direction to
follow, then do some serious, focused study in the right areas.
One comment: Why bother to get better linearity?
I think for most bands, the standard GG open loop 3-500Z type amplifier is
FB. You can hide in the 40M BC crap, the 20M loud crowd, 160M noise, etc.
But, notice my intended amp is a 75M mono-bander. The 75M DX window is a
unique situation where extremely clean signals are required often - due to
the low noise floor at grayline or late afternoon when weak DX signals begin
coming in S3, etc. The window is maybe 15kc wide. [3785-3800] If a local
is S9+50 over, even if he is 50db down 5kc away, that is still going to
cover an S3 DX station. And, if the quality is worse, the window gets pretty
tough to use.
So, I am looking for ways to improve upon a few exisiting GG 3-500Z's, an
8877, 160-10M amplifiers as well as build up a new, dedicated 75M
monobander that can give IMD numbers higher than what is achieved with open
loop GG amps, etc to address this unique 75M window problem.
Let's see if we can sort out some NFB techniques here.... I don't want to
pit one guy against the other, so I will not mention names from private
emails or archives, just opinions... so we can discuss things without egos
Richard, you mentioned here to try 100 ohms in the cathode of my GG amp. I
believe it would put the amp into cutoff, since only 10 ohms drops the idle
by 1/2. It was suggested it would require a choke across this resistor to
idle the amp and eliminate the DC drop and bias variation during ssb
eaks - and keep the RF feedback working. There is talk in the archives of
using this choke, while others strongly disagree in it's use.
Here's an email I recieved from an RF engineer addressing the subject of
using a cathode resistor in a GG amplifier. He is addressing my question the
other day of using this cathode resistor for GG amps. [printed with his
permission, but I opted to leave the name out - unless he comes into the
" His logic is WRONG! Adding a CATHODE RESISTOR to a GROUNDED GRID (Cathode
Driven ) amp does NOT increase negative feedback! The drive and the output
are IN PHASE. His logic applies to GRID-DRIVEN amps, where the CATHODE and
SCREEN are at/very near R.F. ground. (Usually TETRODE or PENTODE amps)-With
such as he described,(Grounded-Grid) he will need to add resistance IN
SERIES with the R.F. Drive, NOT the cathode ground return."
So there you are.
Another NFB technique - Some, as above, have suggested to put a resistor in
the grid circuit. I see mention of it in the archives - BUT, some say it
will destabilize things - an old Orr mistake. I would like to try that
technique too and will add one in series with the driver coupling cap as
suggested in a private email. I'll publish results..
These two techniques sound like a great way for the average guy to slightly
improve the linearity of his GG linear. So it's worth talking further about
For my 75M mono-bander, I may go the Harris design route. I am leaning
towards building something like a 6CL6 driving a 4CX-350, driving another
4CX-350, driving a 4CX-3000 with heavy feedback back to the 6CL6. 100mW in
and 1500+ QRO out as clean as I can make it. We'll see.
BTW, [Tnx for the correction of using"m" mW instead of "M', MW, mega vs:
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