[Top] [All Lists]

[AMPS] A "no tune" amplifier output section/operation question.

To: <>
Subject: [AMPS] A "no tune" amplifier output section/operation question.
From: (Richard W. Ehrhorn)
Date: Tue, 7 Apr 1998 12:24:26 -0600
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 

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 []
Sent:   Monday, April 06, 1998 9:31 PM
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.

You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at
Or call Juno at (800) 654-JUNO [654-5866]

FAQ on WWW:     
Administrative requests:

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>