Hello Skip & all...
"No tune" vacuum tube power amplifiers: All the practical ones (i.e., NOT
distributed amps with 8++ tubes and efficiency <30%) are really band-pass,
not broadband. Most hams have no use outside the ham bands, so why spend
lotsa $$ & complexity for general coverage? It's very similar to the choice
between single or multiband beams and a log periodic: the broadband
approach gives you less performance, more complexity, higher cost. And you
don't need it.
1. In the early 60's Wes Schum of Central Electronics in Chicago
manufactured and sold at least many hundreds of CE-100V and CE-200V
transmitters, 100W all mode output, 160-10M, which used a pair of high
quality Mullard (8850? not sure) audio tubes in the final. Used switched
single-band output networks - basically a hard-to-build, fixed, overcoupled
double-tuned network for each band, wound on ceramic forms and tweaked
laboriously by hand. Expensive to manufacture, but worked great into 1.5:1
or a little higher SWR
2. There are many thousands of ALPHA 374, 374A and 78 amps (built 1974
through about 1983) on the air working DX & whatever daily. They all use
factory pre-set pi-L output networks for each of the "old" (non-WARC) bands
160-10M. (On the wider (%) bands, some pi-L parameters approximate bandpass
Chebychev filters). The bandswitches also have MANUAL positions for
80-40-20-15M, whereby the operator can tune/load the pi-L nets to suit
himself. During the mid-90's one or another of this classic ALPHA series
appeared on CQ's "outstanding stations" cover series more than a dozen
times.
3. The ALPHA 87A uses a conventional full pi-L output network, bandswitched
for all ham hf bands including WARC. Bandswitching and tune-up during
operation is fully automatic (the amp actually counts the incoming
frequency in a millisecond or two, and then bypasses drive from xcvr to the
antenna for <1 sec while it switches to the correct 100 kHz segment of the
band (40 kHz segments on 160M & 400 kHz on 10M) and tunes itself according
to data stored in EEPROM (digital) memory. Factory "default" data is set up
for >>1500W Po into 50 ohm resistive load, with separate data stored for
each ~100 kHz segment of each band. At least 1500W Po also is available
into SWR >1.5:1, and usually into ~2:1.
The "user programmable" feature allows the user to tweak each segment for
optimum efficiency and Po into the real load in use, up to at least 2:1
SWR. This user-determined data is stored in memory and thereafter it is
automatically selected in preference to factory default data when changing
frequency and/or band.
If any other "broadband" or "band-pass" high power -TUBE amps have been
sold in significant quantity to the amateur market, either I don't know
about it or don't remember. Owners of the 87A understand the significant
advantages it has over "barely or not quite maximum legal power" solid
state amps.
Hope this helps.
73, Dick W0ID ex-W4ETO
-----Original Message-----
From: Skip S Isaham [SMTP:nospam4me@juno.com]
Sent: Monday, April 06, 1998 9:31 PM
To: amps@contesting.com
Subject: [AMPS] A "no tune" amplifier output section/operation question.
Hello again,
I have a question about the No Tune type of amplifiers that are/were
offered for sale to amateurs through the years.
Would one of you take a moment to mention the makeup and considerations
of the "no tune" tube amplifier, I'd appreciate it. I've seen only the
outside of the Alpha, there was also an Amp Supply or Dentron version
using sweep tubes?
I've been told at one time there was also a no tune amp from one of the
older amateur equipment mfgrs, prob that company is now out of business?
I'm more interested in the PA section and actual operation
considerations. What type of design and components were used. One report
of the "older unit" mentioned above was that the tank used a toroid coil?
Would the tank Q be seriously reduced for the bandwidth? Did the mfgr
follow up with a LPF to make up for the reduced Q? Was a pi section the
preferred output circuit, etc... etc... You get the idea. I'm just
curious how the mfgr made the output section play.
Is/was this type of tube amplifier practical or is/was it more a
convenience/selling feature for the operator.
Last but not least, did the tube(s) operate at reduced levels for the
reflected power or mismatch?
Thanks for your answer(s) in advance.
skip
nospam4me@juno.com
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