[AMPS] Suppressors, measurements, and acrimonious blather
Thu, 3 Aug 2000 08:08:14 -0700
>> It appears that the suppressors that Wes tested are not the same
>> suppressors that Rich provides today. Rich advises that the length of
>> wire in today's supressors is the same as the length of wire used in the
>> unit Wes tested. But the one Wes tested was wound about a resistor,
>> whereas "today's" suppressors apparently are hairpins.
>Wes' goal was to prove if nichrome made the suppressor better at
>VHF. He measurements show it does not, but then that's what
>anyone who understands parallel R/L circuits would already know.
Are you making this stuff up, Tom?. Anyone who reads Wes' numbers knows
>If you look at the page where Wes graphs the results, the bottom
>sentence concludes that a simple slight re-adjustment in resistor
>value would make the two suppressors be IDENTICAL at VHF.
VHF Q always decreases when L-supp increases. However, as L-supp
increases, dissipation in R-supp increases exponentially on 10m. If
R-supp burns out, trouble could ensue. As I see it, the advantage of
resistance-wire is that it decreases the dissipative burden on R-supp.
Wes is right in that the VHF Q can be reduced similarly with a
copper-wire suppressor by simply increasing L-supp. The trick is to keep
from destroying R-supp.
>The only difference is nichrome had a lower HF Q.
Please read Wes' measurements to see if Tom is blowin' smoke.
>Rich's hairpin, as you pointed out, is not the suppressor tested.
>The hairpin would have less inductance, and thus higher system Q,
>when installed in the anode system than a conventional suppressor.
A virtual Porcine Bubblebath. Hairpin/U-inductors can easily have more
inductance than a coil inductor.
>I sit around chuckling at the people who increase VHF Q, and
>think it somehow increases VHF stability.
At 100MHz, the copper-wire suppressor exhibited a Q of 2.2. The
resistance-wire suppressor exhibited a Q of 1.2.
(see Wes' measurements)
>Nichrome suppressors do have an advantage in PA's that have
>tubes that oscillate at HF or just above HF. That is the reason they
>used nichrome in the 20's.
>In the pre-WWII era, many tubes were unstable at 10-50 Mhz...very
>near the operating frequency. With poor layouts, thin long leads
>and coarse grids in the tubes, and floating plate tuning capacitors
>in link coupled tanks...HF and lower VHF stability was a severe
>problem. The systems needed de-Q'ed at HF and lower VHF.
>If you are using 100TH's or even 250TH's, a wooden or bakelite
>chassis, and a tuning cap floated from ground through bypass
>capacitors and a RF choke, nichrome suppressors are absolutely
>Nichrome or other distributed loss systems can be very helpful
>when the PA oscillates near the operating frequency, and you have
>some gain and power to waste.
In G-G service, I know of no tube that has enough feedback C to oscillate
at HF. The grid acts as a pretty good shield until grid resonance is
reached. For the 3-500Z, this frequency is around 85MHz.
- Rich..., 805.386.3734, www.vcnet.com/measures.
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