That's more valuable info, Marv - Tnx again!
A few follow up questions:
If I tap off the FT-1000D 10mW low level point, can I run a long ~ 20' coax
cable to the amplifier CA2XX module's input, or will this cause problems?
This is for 75M only. Maybe there is a way to do this.
I looked at the FT-1000D's circuit that puts out 10mW, just before it goes
into the power amp board. I'm trying to figure if it's possibly as clean
as -55db there. I don't have a spec analyzer. It uses all pnp transistors -
a 2SC2026 base driven, driving another 2CS2026 in emitter follower, driving
a 2CS1973 in emitter follower at 10mW out. There's much more stuff involved,
like the balanced modulator, etc. Is this enough info to make a guess from
your experience of what kind of IMD we are dealing with at this point? This
will have a big effect on what direction I take, of course. Or maybe I
could sample it into a receiver and get an idea using the same relative IMD
procedures I use for a big amp.
OK on the sample amplifier using a 6146 input running reduced voltages and
1W, low power for cleanliness. Guess NFB will not help there.
Last question: Let's say I did put two 4CX-350's in cascade, so had lots of
gain to work with. What is the practical limit for conventional negative
feedback? Is it a matter of running into instability, perhaps? And I take
it from your comments, that you would run feedback from the final to the
predriver, [two stages at a time only] and then where would you run the
second loop to cover the pre-pre driver 4CX-350 and the input 6146, for
example?
73,
Tom, K1JJ
----------------------------------------
Hi Tom,
That '5106 is probably for Cable TV use and likely cuts off around 40 MHz
(on the low end!).
If I recall correctly, some of the general purpose units that go down to
a MHz use numbers from CA28XX group. They come in both single ended and
push-pull versions.
I've seen a few of the 350J's for sale on the web over the years. I
don't believe the demand is very high for those as they have a 26V heater
and as such can't be dropped into a 4CX250B socket. There is also the
4CX600J/JA/JB.
The 6146 shown on the Hughes schematic is run deep into Class A. Note the
low screen & plate voltages. I bet the numbers were just fine running all
of a watt output.
For feed forward, an error amp with perhaps another 4CX350FJ would be
necessary. It is not a difficult scheme to implement but, it would double
the parts count for the project. If you read about the technique on the
web, keep in mind that "they" are typically working with transistor amps
that start with distortion numbers 20dB worse than tubes. Therefore, in
this case, less correction power will be required, the output combiner ratio
will optimally be a bit higher, and power lost from the main amp will be
lower.
The subject line said you wanted "the Cleanest Linear". To avoid
degrading the system, the driver should exhibit distortion specs at least
10dB better than the amplifier. Your FT-1K will still be useful for the
receiver.
73 & Good morning,
Marv WC6W
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