I know this discussion started with the no-tune amplifier question. Since
we deviated to automatic tuning, I have a couple of suggestions. For years,
the military sets (trailing wire, HF high power, for aircraft) have used
binary weighted capacitors and small vacuum relays, driven by a
microprocessor. For example, use 10 pF as the least significant value, then
20, then 40, then 80, then 160 pF. With all five switched in parallel, you
have 310 pF total, with increments of 10 pF all the way down to the least
significant bit. A five bit word is used here, so you have 31 possible
discrete values (32 counting zero if you open all relays), enough to tune
the plate or load side of a Pi network for a large variation of load and
frequency.
With no moving contacts like wipers, this works reliabily, as long as the
relays are not switched hot, or are suitably rated for it. I scrapped out a
couple of amplifiers built this way, and got dozens of relays and high
voltage ceramic chip capacitors from them. Once could use a microprocessor,
or even some TTL or CMOS counters (up/down) with suitable direction control
logic, to implement autotuning, based on the direction and adjust signals
from one of the diode detectors mentioned here yesterday. All you need is a
bunch of the little Jennings vacuum relays, the hitch. Pin diodes are also
used in some state of the art stuff, but I don't know much about it.
As for broadband tuning, 10 to 20% bandwidth "should" be achievable using
lumped/discrete L and C low pass networks but it should be calculated for a
real tube case, to see if it is really possible. I know it has been done
for several FM broadcast amplifiers on the input network, to cover 88-108
MHz without tuning or with one less knob. Transformers certainly play a
part, with quarter-wave lines doing transformation. Broadbanding the input
to a cathode driven triode is common.
I must have slept through the eighties, who made no-tune tube power
amplifiers? Did they work? How well?
John
K5PRO
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|