In a grid-driven amplifier, using tetrodes, one bypasses the filament leads
for RF, to ground. It is printed in every book I've seen. Some even go into
more detail, like the Philips RF Heating book by Dittrich says to be sure
and decouple the cathode/filament connections in an RF heater/oscillator,
to prevent damage to the filament by RF. I spoke with my coworker who is a
reputable high power RF engineer going back many moons in the big tube
business. What I heard surprised me. He suggested that one only should put
bypasses there to prevent RF from getting to the filament transformer. Yet,
(and I showed him the quote) Seimens and Philips both say that a good RF
capacitor should bridge the filament connections below the socket, and
Eimac, ARRL, Terman, you name it, all show the typical 0.01 uF across the
filaments and from one leg to ground. Of course that one is mandatory if
you have a center tapped filament XFMR as the RF should get a path to
ground besides through the winding. What my friend said is that the
filament has "RF all over it inside the tube" and that you don't need to
worry about it on the leads (differentially), but only the common mode
stuff needs to be bypassed. Now, the European books say to be sure and kill
all differential RF on the two connections, with good low L capacitor.
Where does this lead me?
I know from experience with industrial and home oven magnetrons that
capacitance is required to prevent RF backheating in these tubes from
shortening the filament life.
I spent the past two days measuring the filament impedance differentially
on a tetrode, a 250/300 KW device. Using the trusty HP 4193A impedance
meter, it was swept over and over (Crimson and Clover) from 1-100 Mhz or
so. By installing the right amount of shunt C across the socket, sure, I
can parallel resonate the filament. Where to park this resonance? I settled
on between the fundamental and the second harmonic. On frequencies below
the resonance, one sees the inductance of the filament structure, and above
it, one sees capacitive. About 10-15 MHz or so, there is series resonance,
at which time the phase goes back inductive, (the caps themselves resonant)
and this continues all the way to 100 MHz (inductive).
I tried the following mounted radially around the filament connections in
the socket:
combinations of silver mica leaded caps, 3900 pf
combinations of ATC 100E high power ceramic chip with ribbon leads, 2200 pF
each
combinations of the above, 4 ATC, 4 silver micas, soldered in parallel
combinations of 0.01 ceramic disk, 3 KV
combinations of Centralab 585, or High Energy HT58 1000 pf 5 KV ceramic
knobs (too big to mount there)
Cornell Dublier CDM 272 mica cubes, 0.02 uF, rated for 1 MHz (NASTY
RESONANCE APPEARS AT 40 MHZ)
My tests ran from A-I last night and A-D tonight, so 13 combinations and
values were swept, with the tube in the socket, upside down, ass end
sticking up, the socket sitting on top. It takes three of us to move the
tube without straining.
All caps had their charateristics, and the best (cleanest, low inductance)
was the ATC 100E series. Similar to the Murata and Unilator/Matroc ribbon
leaded high power ceramic caps. They cost about $8 each, and I got 50 in my
order. So I was happily soldering and punching the ribbon leads, etc. What
I did was to parallel two in a sandwich, put it under the screws between
the F and K connections on the socket. There are 6 metric screws in this
tube socket (itself costing $10,000), and I used one pair of caps between
each set of hot and ground screws. After it was built a few hours ago, I
swept it, with the filament leads connected, and now it doesn't change
whether i connect or disconnect the 300 amp transformer. Aha! I consider it
decoupled. The impedance at my operating frequency and the harmonics is
around 3 - 5 Ohms. F is 2800 KHz for this amplifier. I have 0.0022 x 12, or
0.0264 uF. So, I am happy that I did bypass the leads for the filament, and
am working to make sure K to ground is also low enough impedance for the
RF.
I was wondering if some of your parasitic-obsessed contributors out there
care to comment on your experiences bypassing filaments. While the ARRL,
and others have treated it mundane and ho-hum, for many years, as has my
friend, who has many successful cavity amps under his belt (not literally),
I could imaging really tearing up a filament in a big tube if one casually
placed the standard 0.01's across the leads, and parked a big ol parallel
resonance on the operating freq or a low harmonic. The inductance and the
lossy nature of the filament do a heck of a job de-Qing the resonance, so
I read about 30 Ohms at the peak. But it's still there. Grid dipper was
used to verify, yessir-ree. I could imaging developing some RF potential
across 30 Ohms. Would this also be on the filament structure, which is up
to 6 inches away inside the tube, at HF?
John
K5PRO
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