[AMPS] sweep tube amplifiers and the Knight T-175 Linear Amp

Skip S Isaham nospam4me@juno.com
Sat, 04 Jul 1998 03:38:55 EDT

From: w3am@juno.com
Subject: Sweep tube amplifiers

:What you've got there is one of the great 'ol time CB linears, sold
under cover of :being a 10M & 6M ham amp.  I knew quite a few CB'ers back
in the late 60's and :early 70's that used them.  

In the late 70's and early 80's, I had one on a Hamtronics 6 meter
repeater on Mt Vaca that served for years. A properly designed and
operated sweep tube linear was a cheap way to get a higher power level.
These amps were legal, made by many companies as amplifiers to the
25-54MHz bands. Other brand names of common sweep tube amplifiers were
Sonar, Kris, Lafayette, Pride, Swan, Palomar, Hygain, Allied and Knight.
Yes, they were also designed as CB amplifiers a lot of the time.

When I worked for the Motorola Service Station in the late 80's, we
pulled a slew of Sonar 4 tube jobs from Multitone paging systems in the
SF area. They had been in continuous 24 hour paging service for at least
6 years or more and still had pretty good output.

I know, most of you raise your eyebrows at sweep tubes, but they were a
true experimenters tube for cheap RF power. And we could find them in
older tube TV's in the trash behind TV stores. I also used the
transformer. My Icom 551D needed the gas with only 10 watts output. Your
friendly Dentron GLA-1000x and Amp supply models also used the sweep

:(Back when CB'ers used call signs and had licenses!)  I've yet to meet
an actual ham :that used one. 

That's me, I still have a few sweep tube amps around. Some are home brew,
some are some of names I mentioned above. Heck, one's stacked on top of
my 30-S1 right now. Look in the old ARRL repeater directories for N6BPK
on 52.74 in California. We could "work'em even if we couldn't

:The amp was considered to be a slight cut above the usual sweep tube
junk, in that :it was one of the very few amps in that class that had
variable grid bias so you could set it for AB1. 

Bias was set by a variety of methods, per brand. Most were zener diodes,
some were resistive dividers from a supply. One design had the mentioned
novel resistor network with voltage from the heater supply. I'll
reproduce it and have it up on my web page as soon as I'm done getting
the other basics out of the way.

:Of course, if you did that with AM you had to beef up the cooling air
and talk quickly!  :Back then the 5 minutes on, 5 minutes off rule
protected the sweep tubes more than :anything else.  The tubes are indeed
in parallel and run with an AC grounded control :grid, with a variable DC
bias supply attached. 

Some were grid driven, some were cathode driven, some were super cathode
driven. The better ones were super cathode (IMO).  The grid driven ones
needed a neutralizing cap unless they were designed really well. Typical
AM ratings were 1 to 3 ratio for key down and cooling. 

The majority of companies  were counting on SSB operation or a severely
reduced AM operation. Most sweep tubes had about 30 watts diss, but would
take many times that in low duty cycle (SSB operation).  A quartet of
sweep tubes would put out about 300 watts of power with no problem.

: The screen and suppressor are directly grounded for "triode" operation.
:I have a schematic, operation and assembly manual for it if you need a

Another sidebar: Sweep tubes have a bad rep for high plate C vs other
tubes. I never had to many problems with them on 28 and 53MHz, the output
would drop about 25 to  40% on 53MHz vs the typical 21MHz output. GE
designed and sold the 8950 specific for RF service. I did plenty of Swan
(three-drifty) 6HF5 to 8950 conversions for the local hams.
Standard Pi output networks worked well.

Later I started trying  better/hybrid tuned input designs which really
helped the six meter output...   Talking to Rich (AG6K) one day on 40
(uncle bill land), he mentioned placement of a small carbon resistor
(about 10 ohms in each tube) in the cathode lead, that would help
linearity. He was right on the money, not only did unwanted products
drop, the amp was easier to drive. Problems with un-even tube currents
were also reduced.

Sweep tubes (along with large tubes, with and without handles) amplifiers
are a favorite of mine because they were a true "poor mans tube".... and
I was really poor back then.  I collect and restore them now as a hobby.
It's quite fun to use them on 40 for the "big SSB signal". My home brew
6JE6/6LQ6 amplifier see's more operation time than the Collins or Heath
gear I have.

I have a large file cabinet of original commercial and home brew
amplifier circuits. A lot of them are sweep tube designs. Feel free to
email me if your looking for something (licensed amateurs only please). I
also collect any diagram I don't have if you'd be so kind. It is
interesting that the ARRL Handbook had a 1/4 gallon sweep tube amplifier
circuit one year. Much older handbooks had "that 4 tube job" for multi
band HF operation. Not only was the output section un-conventional, but
the method of bias could have been better designed for cut-off in receive
(also a Dentron GLA Failure). 

OK, I'll wipe the nostalgia tear from my eye and get back to the 3-500z
and 4CX.... starting point.  Wheel out the sweep tube chassis and fill
the space with the BTI again.

Skip May   wv6f

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