I added a 10 ohm current shunt to the anode circuit of V20, the 20 MHz
amplifier, and used it to test all of the tubes. The anode currents
varied from 4.8 to 5.7 milliamps, the gate voltages varied from -10.3
to -12.6 volts, and the screen grid voltages varied from 72.2 to 76.0
volts except for one 7587 which had a screen voltage of 63.5 volts for
some reason.
That one also had the highest anode current but not the highest grid
voltage. I don't know if that is significant.
I can get 2N5950 JFETs and MPSH11 NPNs cheap at the moment so I will
start off with them.
On Wed, 02 Jan 2013 22:06:30 -0000, "Ed Breya" <edbreya@...>
wrote:
You may want to try swapping tubes from the other spots too, although it seems they're all biased for the same operating conditions, so should wear at the same rate - but if they are only on when needed for their specific function, then it all depends on the history of use. I think the only one that's always on then, would be the 10 MHz oscillator, so it should have the most wear.
I looked up the 7587 and see that it's rated for 20 mA max cathode current, but there were no charts showing what it should be near zero grid bias - unless the grid current gets pretty high, it doesn't seem that it will make much into 100k. It would be interesting to see what it is if you measure it and the actual plate currents.
I think your cascode version with a JFET and an NPN would be best, since you can then assure cutoff by pulling the base to ground. The double-JFET form won't positively cut off unless you can pull the gate negative, making it more complicated. You will have to watch out for the voltage ratings of the NPN to make sure it can handle the cutoff conditions.
I'd recommend around +15V or so for the bases (via small R so they won't oscillate on their own), and about +25V for the collector supply. Those should work with many types of VHF JFETs and small signal RF NPNs. For parts I'd recommend something like 2N5109 or MPSH10 for the NPN, and a J300 with around 15-25 mA Idss.
Ed
--- In TekScopes@..., David <davidwhess@...> wrote:
On Wed, 02 Jan 2013 17:55:38 -0000, "Ed Breya" <edbreya@...>
wrote:
It looks like the required RF power levels aren't out of reach for JFET substitution of the 7587 tubes. The tricky ones will be the 500 MHz multiplier and the 100 MHz driver for the 200 MHz diode doubler.
Coincidently those are the stages that have been giving me trouble. I
have to break out one of my faster oscilloscopes to see what is going
on with them. It is possible that the 500 MHz multiplier just needs
tuning but so far the output from the 200 MHz diode doubler has been
low and not real symmetrical. Swapping V40 and V30 made no difference
but maybe both are weak.
The diodes in the doubler do not test as obviously bad in circuit but
I may try and find higher performance replacements.
None of the tubes use any cathode degeneration, and are only biased by grid current through the grounded 100k resistors, so dropping JFETs in their place means that they'll just run at Idss. For the 500 and 100/200 MHz circuits, the more, the better - in this case, high gain and nonlinearity are your friends. If you get a bunch of JFETs like U300, with Idss near the top of the range (>40 mA), they should be OK in those spots. If you come up short on power, you can try paralleling two devices with the same Idss for each spot. The other three tube spots have more modest power requirements, so lighter JFETs can be used there. For drain supplies, the +12V is the only other positive supply already available, which should work OK in place of the original +125V plate supplies.
I guess I should go through and measure the operating points via the
grid voltages and maybe plate currents to get a better idea of what is
going on.
For the drain supplies, I was thinking of just adding a low quiescent
current series regulator to the 125 volt plate supply and dropping it
directly to a voltage compatible with JFET operation. Normally I
would eschew such a wasteful configuration but the original design
does essentially the same thing via the plate resistance and quiescent
current of each stage anyway which may be why the design only enables
each stage as needed.
One thing that will complicate matters is the disabling of certain sections when needed. The 100, 50, and 20 MHz amplifiers appear to be cut off when deselected, by grounding their screen grids. You would have to replicate this action in the new circuitry if you want to keep it operating the same as original.
I noticed that every stage is disabled when not in use either via the
screen grid or in the case of the 500 MHz multiplier, both the screen
grid and anode supply. That is one of the reasons I wanted to try
using two JFETs or a JFET and bipolar per stage in a cascode
configuration which would allow the screen grid circuit to disable
each stage as designed.
Cascodes would also offer much higher performance at least as far as
gain at high frequencies. The higher needed voltage is obviously not
a problem in this case.
David Hess