A
single 2C39A Amplifier for 23cm using spring collet anode and grid connections.In
the early 1980's, an article in the RSGB's Radcom gave details of an improved method of providing connections to a 2C39A in a cavity design which yielded a significant improvement in gain.The
basic design of this cavity using conventional finger stock for the anode and grid connections had featured in numerous editions of the RSGBs VHF/UHF Manual but instead of using the traditional fingering, a silver plated springs located in collets are used. The springs provide a lower inductance connection than can be obtained with conventional finger stock, which resulted in lower degenerative feedback and hence more gain.This
version using the springs for the anode and grid connections now appears in the latest version of the VHF/UHF Manual.
The
assembled cavity. The output is to the left, the tuning paddle spindle to the right.
The
cavity with the 2C39A removed, the spring just visible.
A
close up of the collect and spring. The spring must float loose in the collet under it's own tension - it must not be soldered otherwise it will be impossible to insert the tube.
A
bottom view of the cavity. The cathode line can be seen in the diecast box. The screw protruding into the cavity (the one looking directly at the head in the picture) is the anode cavity coarse tuning screw. The zener diode to provide the correct idle current is visible adjacent to the BNC input connector. The 23cm input is capacitively coupled to the input line, the input tuning screw is visible directly opposite the BNC connector. 
A
internal view of the cavity showing the grid collet (centre), the output coupling loop (left), and the tuning paddle (right). The tuning screw (centre top) can be seen protruding into the cavity.K&S
metals are a convenient source of copper and brass tubing to make the output line, and is obtainable from most good model shops. One advantage of K&S tubing is their range includes incremental sizes which are a sliding fit - one inside the other. The ability to adjust the loop in the cavity is essential for correct loading.For
the line itself, I used copper tube and copper rod of dimensions such as to give a near to a 50 ohm impedance as possible. A small PTFE plug was machined to be a push fit in the end of the inner line closest to the loop which supported the inner line - the other end was supported by the N Type coaxial connector.The
one trick which I did use was to make the output coaxial line an exact electrical half wavelength long. When this is the case, the exact impedance of the line doesn't matter too much - as the loop will see whatever impedance the load is - i.e. hopefully 50 ohms resistive. I think that one problem frequently encountered by builders is they don't use a halfwave line and if the line isn't exactly 50 ohms, then the loop will see some other impedance which will have a reactive component, which may give rise to matching and loading problems, a frequent problem in cavity amplifiers.
The
underside of the cavity top plate. The tube into which the tuning screw protrudes can be seen to the left. This tube and the tuning screw together create a capacitor for the coarse frequency setting for the cavity. Fine tuning is via the paddle.This
plate has the heads of four countersunk head screws soldered into it to provide the fixing for the anode plate.
The
top plate secured to the cavity. The four protruding screws for securing the anode plate can be clearly seen. A thin sheet of PTFE and PTFE bushes provide the insulation between this plate and the anode plate.This
amplifier was initially built as a driver for the UPX4 amplifier, but subsequently replaced by a unit using Mitsubishi RF Modules. Return
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last updated 9/6/2002