Bill Fuqua wrote:
This has always been an issue. Particularly when high-Q circuits are
involved.
One time while helping the accelerator tech try to find out why our 20
MHz, 10KW bunching amplifier signal was getting into the sensitive
instrumentation I noticed that in haste they only installed every
other screw on a panel shielding a very high-Q tuned circuit from the
outside world. This was a 3 turn ( really looked more like twist)
water cooled coil with the ends of the tuned circuit going to
defection plates used to deflect 5 MegaElectron Volt protons at a 20
MHz rate.
Well, I took an insulated screw driver and put it between the shield
(where a screw was supposed to be) and the chassis. And there was an
RF arc. The there were screws on either side only about 6 inches away
and we were producing an arc half way between them on a 1/8 inch thick
aluminum plate which they assumed was grounded. Needless to say, they
reinstalled the screws and the problem went away.
I have noticed some homebrew amplifiers I have purchased have the
plate tuning capacitor grounded to the front panel. This is bad. Since
all the fundamental and all the harmonic currents from the plate of
the tube flow thru this ground path and can produce a good deal of
radiated TVI off the front panel.
On smaller amps I used to like using the "APL" series of air variables
by Oren Elliott. The reason being is I could order them with extra
spiders (wipers) that you could connect straight to ground on the
chassis pan. These I had put on the aft end of the capacitor between the
last plate and the ceramic end. Actually you can have this added to the
larger air variables they make also. They come standard with one on the
front if no back ceramic end is used. But this wiper has a solder
terminal which can still be connected to the chassis pan and before the
faceplate. Since currents take the closest path to ground, it should go
through this wiper lead to the pan instead of the threaded shaft through
the face plate.
Also, it is bad when the plate capacitor return path to cathode on the
chassis is common to the input circuit. Even though the circuits are
on opposite side of the chassis there can be a good deal of coupling.
Just some thoughts.
73
Bill wa4lav
Best & 73's
Will Matney
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|