The heaters are always on though so I am not sure how much difference
having the tubes in cutoff most of the time would make.
I have a full set of characteristic curves which show just above 20mA
at Vg1=0 volts and the data I have about biasing suggests that grid
biasing will yield about 10mA but nothing matches the precise
conditions Tektronix used so I will just have to measure it.
Thinking about a dual JFET cascode gave me a headache. On one hand,
the top JFET cannot be in cutoff without negative bias and on the
other hand, the bottom JFET is hardly going to conduct with a Vds of 0
volts. As Vds on the bottom JFET increases, the top JFET will have
negative bias. At worst I think this is one of those cases where even
though the current will not drop to zero, it will be close enough for
engineering purposes. Calculating it is pretty straightforward but it
depends too much on the individual transistors. Using an NPN on top
neatly solves the problem and is what I originally had in mind
although I will test it both ways.
I will have to be careful about the Vce on the NPN since RF
transistors tend to have relatively low ratings. That will ultimately
limit the supply voltage. On the other hand, the cascode
configuration relaxes the transistor performance requirements and
allows them to operate in the best case configuration.
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
David Hess