On Mon, 30 Mar 1998 17:10:39 -0700 John Lyles <jtml@lanl.gov> writes:
>I apologize to you ham amplifier designers as your frequency range is
>soooo
>broad. Bound to hit a sneak resonance if you're not careful, not to
>mention
>a lot of losses across your plate circuit. There have been a number of
>segmented schemes which seem to work in a fashion. Seems to me that
>using a
>GDO is like setting your timing in your automobile engine by blinking
>your
>eyes fast.
C'mon now John....what do you think the commercial companies used back in
days of old? My old Measurements Model 59 GDO came from National Radio.
The engineers used that on military amps and it worked just fine. Works
for me too.
I would love to have a probe for my HP-4815A or access to your toy but
with a little experience the GDO is adequate. BTW, before the EPA clowns
got involved I could time just about any engine by ear..from full race
flathead Fords to 427 Chevy's.....guess I blinked real fast. Had the
timing light but that was just to get you into the ballpark...each
race/track was different. The tools are fine but experience brings home
the trophies.
I suppose that a signal generator and a scope (or RF probe) could be
adapted to get an idea of actual attenuation in lieu of reactance.
It sure helps to know if you've got a series or parallel
>resonance nearby. You can short the leads and measure one, open them
>and
>measure another. It's all very artistic. And a lot of trial and error.
>The
>enclosure affects it too.
Keeps you out of the bar rooms at night.
>
>One old way to overcome the limitations of too low a Z and too low a
>resonance is to use a banked winding. This is done by some of the pros
>in
>large SW transmitters. Take a few turns, then back over a few, then
>wind a
>few more, then step back on the form, then a few, then back over, and
>so
>on. It's hard to picture in words, but the thing looks horrible. The
>effect
>is to reduce the turn to turn capacitance, yet get a lot of
>inductance.
>With reduced capacitance, the self resonance may be moved higher, and
>yet a
>lot of inductive reactance realized (lot of L) over the desired range.
> I'm
>surprised that hams haven't tried it (or maybe they did.
Now, that is a VERY interesting idea !
Bill Orr,
>did
>they?).
>
>At the DC side of the choke, we didn't skimp on the bypass capacitor.
>It's
>a big expensive ceramic thing rated for a lot of current with low
>inductance. That way, if we get a harmonic banging through, it won't
>go
>anywhere but to ground. I've seen them destroyed by using a bad choke,
>as
>someone alluded to today on this forum.
Yep, and a clunky non-RF rated doorknob at the base of the choke does not
help either. It is pretty easy to measure the series resonance of disc
ceramic caps with varying lead lengths and then chose which is best for
your own circuits proclivity to VHF nasties ( 470 to 1000pf are the most
likely values). This should be added in parallel with a nice .005 to .01
so the HF frequencies are adequately bypassed. The biggest problem is
finding a .01 at over 3500VDC so paralleling .0047's is often necessary.
I also noted your use of real fat choke forms John. While OK for your
frequencies they really pile on the winding C rapidly.
A 1/2" diameter is ideal for 80-10M and maybe even 160M. The SB200-220
choke is only 50uh but the first series resonance is around 45MHz. I have
not tried extending that form factor to 100 + uh but it might bear some
testing.
When I did have access to a functional HP-4815A back in pre 1986 days at
Wang Labs, I found the 3/4" diameter to be the most forgiving with #24 to
#28 wire sizes. IOW, lots of combinations worked just fine for the 9 ham
bands.
Also back in the 80's there was a HRM article mentioning the use of a
ferrite rod inside a choke form...the reference was a 5KW commercial
application with no series resonances 2-30MHz. Later on B&W used that
concept on their BBC-5K choke.
I wonder if anyone here has tried that? My one and only attempt resulted
in smoke but that was over 10 years a
_____________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]
go.
73 Carl KM1H
>
>John
>K5PRO
>
>
>
>
>--
>FAQ on WWW: http://www.contesting.com/ampfaq.html
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>Search: http://www.contesting.com/km9p/search.htm
>
_____________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|