>G'day Amps Champs
>
>This is just a quick note about Nichrome suppressors. Yeah, I know they've
>been done to death a number of times here and elsewhere. I just want to
>relay my experience.
>
>I have a homebrew amp which runs a pair of 3-500z triodes. I built it quite
>a few years ago and since day one, it has had stability problems,
>particularly on the higher hf bands.
>
>I don't know if the cause was voodoo, parasitics, poor layout, my ugly face
>or any combination of the above. Whatever the cause, it was a pig of an
>amplifier and it only ever got used on 40m with an occasional careful
>outing on 20m.
>
>I've worked on that amp a number of times changing layouts, bandswitches,
>tank coils, coupling and decoupling capacitors etc etc. Nothing would cure
>it's bastard nature.
>
>Well, I've been a member of this mailing list for a few years now and I've
>followed all the arguments with interest. In a last ditch effort to win my
>argument with my amplifier, I sent for the kit from one of the more well
>known subscribers to this list. I got a few mates interested also and we
>got a few more kits at the same time to save on postage costs from the US
>to downunder.
>
>Just as an aside, I don't believe Rich recovers much more than his costs
>from the sale of his items but his service is the best in the world. -
>Thanks Rich.
>
** You are welcome, Alek. The teenagers who pack and send the orders
deserve the credit for doing the work because I, like Bagwan Rajneesh,
am content to ride around in my fleet of Rolls Royces, sipping Dom
Perignon and smoking Cubano cigars.
>Did I say this was going to be quick? Sorry, I'm getting carried away
>aren't I?
>
>Yesterday, I finally found the time to open up the bastard amplifier again
>and I set about fitting the Nichrome parasitic suppression equipment. I
>installed the 4 supplied resistor/Nichrome suppressors and the special dual
>wire suppressor which goes between the plate choke and the plate coupling
>cap. The kit also include glitch resistor, protection diodes, gride fuse
>resistors and coupling caps. I didn't use these as they had already been
>fitted in previous attempts.
>
>I've just returned form my local club shack where I took the amplifier for
>testing into our large dummy load. I am very excited to report that the
>amplifier is no longer a bastard. The change is more dramatic than I could
>have ever imagined or hoped for. It is now stable, predictable and easy to
>tune and operate.
>
** The important thing to remember about vhf/uhf suppressors is that
there is no magic in Ni-Cr resistance-wire. Compared to same-gauge
Cu-wire at 50MHz, resistance-wire provides a Q-reduction of c. 5.5. The
net effect on the parallel-equivalent R (Rp) of the anode's vhf-resonant
circuit is roughly a 50% decrease in Rp as well as a 50% or so reduction
in vhf gain. Half is not a lot, but it seems to make a difference in an
amplifier's ability to oscillate above the grid's resonant-freq.
However, the same 50% decrease in vhf Rp of a Cu-wire suppressor can be
achieved by increasing the L of the suppressor inductor (L-sup) -- but
with the tradeoff of increasing the dissipation in the suppressor
resistor to the point that it may burn out on 29MHz. The reason that
increasing the dissipation in R-sup is not practical is that resistors of
c. 12nH are needed to build an optimal vhf suppressor and no one
presently builds such resistors that will dissipate the needed wattage.
Thus, the advantage of Ni-Cr is that it has more R than Cu, so it shifts
some of the dissipative burden from R-sup to L-sup. The disadvantage is
that Ag/Sn solder and acid flux are needed with N-Cr,
>After extensive testing on all bands into the dummy load, I ran the amp
>into real life antennas on the 80m to 10m bands. I also went up and down
>each band on high and low power. The result was the same, stable and smooth
>as silk whatever I did to it.
>
>I now have a very useful amplifier. As to what the science or magic is in
>the Nichrome suppressors, I don't give a rip. I'm just happy they work and
>I'm quite happy to recommend them to anybody.
>
>73, Alek
** Thanks, Alek. Say to our aunt Minnie for me.
cheers
>VK6APK
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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