Grumpy The Mule wrote:
Howdy,


I've only read about them. G2DAF wrote extensively
on pasive grid linears.

I think the ultimate value of grid resistor will
depend on the grid capacitance and layout parasitics.
The articles I've read use 300 Ohms as the grid swamping
resistor but with a 4CX250 450 Ohms might be possible.

Here is an article you may find useful.
http://www.radioamator.ro/articole/files/291_1.pdf

I have a bucket full of 4CX250B's and would like
to try passive grid someday. Please Let us know
how yours shapes up if you decide to build one.


73,
Grumpy


ken scharf wrote in
news
Anybody here ever build a passive grid linear amp?
That's where you simply terminate the grid of a tetrode with a 50 ohm
resistor and just drive it directly. Svetlana recommended that
configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should
work well with more commone surplus 4cx250's.

I was thinking of using a 4:1 or 9:1 balum and increasing the grid
resistor value to 200 or 450 ohms to reduce the amount of driving power
required. With the 4cx250 a peak grid drive of about 50 volts is
required in class AB1. With a 50 ohm termination the driving power
would be 50 watts, with a 200 ohm termination 12.5 watts, and with the
450 ohm termination 5.6 watts. That's actually just the power sucked up
in the resistor, but the tube requires less than 1 watt of drive itself
in class AB1. Some power might be lost in the balum, so maybe the
actual driving power might increase by a watt or two. Since I wanted
this to be a final for a QRP rig the larger terminating resistance
looked like a better way to go. The only issue is how high can you go
with the termination resistance and keep the tube stable without
neutralization being required?


That's an interesting article. He shows no matching network between his
exciter and the linear so there would be a 6:1 SWR with the 300 ohm
termination, though a tube type transmitter with a Pi network might
still be able to load into it. Deriving the screen voltage from the rf
drive seems a hybrid method of doing grounded grid. I was thinking of
an amplifier with normal bias and screen supplies which would reduce the
drive required.

I have a pair of used 7034's (4cx250's with glass seals) but I have no
idea how much life (if any) are left in them. My idea was to build an
amplifier chassis that could be used to experiment with several tube
configurations. I've found a source for a 400VA toroidal power
transformer with two 550v and two 6.3v (5A) secondaries and a 120/240 v
primary for about $65 postpaid. In a bridge rectifier configuration
with a capacitor input filter using large capacitors it should yield
about 1400 volts under load. This would be a suitable plate voltage for
a single 811A/572B or a 4CX250A. With the bridge rectifier
configuration the center tap of the HV secondary will provide 700 volts
that can be regulated down to the 300-350 volts required for the screen
of a 4CX250. One of the 6v filament windings would power the tube
(unless I find some 4CX250F/G's which would require an extra 25v
transformer) and the other winding could be used with a voltage
multiplier for the bias supply (or drive another filament transformer
bassackwards for that purpose).

So I could build up a power supply and tank circuit that could serve
either configuration, and swap out one of two chassis; one for a single
811A/572B and one for a 4CX250.350/400A. The target amp would be
between 150 to 250 watts output (a nice afterburner for a 10-20 watt QRP
rig I'm thinking about).