Hi Kevin, Bob, W6PO, spent many years in the development lab at
Eimac/Varian and we enjoy the benefits of his work even today. No doubt he
learned
early on about zener bias and failures; hence the use of a 50w rated device
instead of an on the edge of destruction 10w device.
Yes, some of us have/do use the pass transistor/small zener combo and it
does work very well. The really great part of this is you can get everything
needed at your local Radio Shack store in case of a failure and need for a
speedy repair. Keeping a couple of good sized pass transistors and a
selection of small zeners on hand allows construction of a bias circuit for
most
any tube/project.
There is an added consideration for some of the tubes we use: the bias
voltage needed is up in the 20 to 60 volt and even higher range. Using cathode
bias then involves a dissipation in the 20 to 60 watt range and this is not
exactly trivial. Attention to mounting and cooling is a requirement, not
an option.
Some of the larger tetrodes, like the TH347 we use at 432 and 1296 MHz,
need bias voltages on the order of 85 to 100vdc. Forget a cathode bias circuit
in this case. I use a pass transistor/small zener between the cathode and
grid (ground), allowing the B- to rise above ground by the amount of bias
needed.
Since there is no grid current without drive, there would be no bias
voltage generated in this scheme while the amplifier is in the idle condition.
To keep it all working, a positive voltage somewhat higher than the needed
bias is connected through a 1K or so resistor to the B- to keep the bias
circuit "alive". The pass transistor then has to dissipate the small "keep
alive" current plus grid current.
My TH347 amplifier requires about 95vdc bias and uses this circuit with a
pass transistor having a voltage adjust pot in the base. a 10W zener
regulates the voltage at the pot and is loafing along dissipating maybe a half
watt. The zener polarity was chosen to allow it to be bolted right to the
chassis. The pass transistor is a PNP configuration, allowing the collector to
be bolted to the chassis as well.
This same circuit would also work for triodes but the cathode zener is the
usual choice. The grid metering circuit is slightly more complex but no big
problem; it connects between the bias circuit and B- instead of the usual
ground to B-.
Surge protection can be provided by a single (or multiple) diode from B- to
ground as the bias polarity is compatible with this simple circuit. I use
multiple diodes, at B-, cathode and metering just in case, You know the
drill: a belt, suspenders and velcro design approach.
Yea, I know this is more information than anyone wants but may be useful to
someone.
73,
Gerald K5GW
In a message dated 10/20/2010 5:25:43 A.M. Central Daylight Time,
knormoyle@surfnetusa.com writes:
Good info Ian.
I was looking around trying to learn more about the various bias circuits
in use.
maybe people can provide historical views on some things I was curious
about:
-This 1971 Eimac paper on a 8877 triode for 144Mhz, had a 1N3311 50 W 12V
zener for the bias.
Makes me wonder why R. Sutherland decided on 50W for that diode.
http://www.landfall.net/Radio/8877-1.htm
-The junction to case C/W is much better for the zeners in the larger
DO-5 stud package..just 2 C/W max (1.5 C/W typical)
compared to 10 C/W (typical) for the DO-4 stud package. I wonder if
that's what's interesting...not that it can
tolerate 50W but that it has lower junction temps to start with, for a
given DC dissipation..because of the
better/larger package. Note that it doesn't matter how good your heatsink
is, if the junction to case thermal resistance
is the problem.
-The ability to tolerate power spikes depends on junction temperature rise
in the zener, right?. So starting from a low
junction temp adds margin?
-rfparts.com has the reverse polarity 1N3306B (50W 7.5V) for a reasonable
price, and claims to have the mounting hw. So
I ordered a pair. It seems easier to drill larger 1/4" holes and use the
stud diodes, then use say the flatter TO-3
packages and say TO-3 heatsinks or something like that. So I ordered a
pair of those to try.
It appears that some amps do use the TO-3 packages though?
-The 1971 Eimac paper mentioned using a smaller zener with a 2N3055
transistor on a heatsink, instead of the 50W zener.
I thought that was funny, because some web sites I've read mention
something similar, like it's a new idea.
Also saw people mentioning using zener plus mosfet, at least for audio
amps...
I guess you could size a pretty large power transistor? How come we don't
see more of these simple lower power
zener+transistor circuits? or do people do this?
Ah, I'm reading Ian's Triode Board manual. I guess that's an example of
taking that to more extremes.
-kevin
AD6Z
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