The TUNED INPUT and older amplifiers.
The complete solution page for
modifying older antique amplifiers and new solid state radios.
Well it is about time I revisit
I have added some comments and offer
you 811A / 572B lovers something to think about.
You probably know
someone who recently bought a newer solid state radio and has
connected it to an antique amplifier. I am referring to those amps
that did not come with a "tuned input". Or one of the units that was
designed poorly like the FL-2100B or those Palomar "sweepers" (OMG).
You may already have one of the "ego maniac" radios that cost
thousands of dollars and does everything but make your morning
coffee. Now you decide to "save some money" and you are going to
"hook" up your second mortgage to a Dentron Clipperton,
Hunter Bandit or a Swan
Mark II. Well Bunky, that might not be the smartest thing you have
The "grounded grid" amplifier was designed back in
the days of CW communications and these amps were simple cheap and
dirty. I mean real dirty. We make fun of the CB amplifiers that came
out in the 70's and 80's, but in reality, our first attempts to
mangle a decent RF signal was not much different. Problems are
evident today when they are "hooked" into a modern solid state
radio. First we deal with the unbalanced impedance issue. These
amplifiers used ".01" capacitors as the input connection to the
cathode. The grids were grounded or biased with a low voltage and
were run damn near on the edge of self-oscillation. This was done
because it took high amounts of much drive to push the TRIODE (or
TRIODES) into a state of amplification. All the while the CATHODE
IMPEDANCE bounced around like the Easter Bunny on steroids. This was
all Ok when we had PI NETWORK RF stages driving these animals, but
not by today's standards. The broad-band solid state finals don't
work like that and they are "as designed" BROAD BANDED.
Even if you have a "tuned input" amplifier from the 80's you should take
time to download and review the ZIP file I have added...HERE.
This is an important file to keep on hand because it
will help you understand the solution to using an antique ".01"
input designed amplifier like the Gonset GSB-101. There are several
of these ".01" input amplifiers on the market and there are still a
lot of HOME BREW units out there that use this design. Well they do
not function well with modern solid state radios. They are suspect
to HARMONICS and IMD (distortion) that renders splatter when
over-driven. They have ALC (AGC) circuits that should never be
connected to your modern solid state radio. Some of these units have
no ALC voltage at all...and who really cares? But this is not about
ALC, this is all about the HARMONICS and the IMD. These problems
come from the lack of tuned circuits ahead of the amplifier, or
behind the radio, what ever.
Those old PI NETWORK finals using 6146 or 6JB6 tubes, as an example,
were capable of driving the ".01" amp simply because the PI NETWORK
is a tuned output or input depending on how you look at it. Actually
it was a TUNED CIRCUIT (period) and was designed to match the final
with the load. It was and remains that today, an IMPEDANCE matching
device. What we did wrong when we became "appliance operators", we
threw out the TUNED CIRCUIT for a much simpler designed low
impedance matching "broad band amplifier". We satisfied the bean
counters too...just what you wanted to hear. But I digress. This
reality came true when amplifier designers managed to add the tuned
circuit "back into" our lives, this came just about the time we
learned all about AUTOMATIC ANTENNA TUNERS. Holy "crap" we threw
them out and re-introduced them as "new " items not more than five
years after the fact.
So here we are...forty years later and we are connecting an antique to
our FTDX-5000...Oh Lord what can I expect now? Well if you still own
a home, you can do one of two things. Build an input circuit, or
draw out another home equity loan and buy an all solid state
amplifier and ATT unit. me I prefer to "build" the tuned input
circuit. It will be cheaper...a lot cheaper in the long run. Dentron
knew the problem existed, they designed the CM-U and sold several of
them to Dentron owners. They were later found in Ham Shacks where
radios like the TS-180S was driving a Hunter Bandit or the HA-1
Heath Warrior. The CM-U is still available at swap meets or on Ebay,
but they are expensive.
If you downloaded the ZIP file from
the link above you should open it now and look at the list of files.
The first is the CM-1 unit which was an add on unit for Clipperton
amplifiers. This device aided the NON-TUNED input circuit for use
with modern solid state radios. It was a compromise, but it
worked...to a fashion. The CM-U unit was a true "tuned input" unit
and it worked great. However it does not allow for the WARC bands
and certainly does not offer broad band operation for the SSB user
on 75/80 meters. However this is a good place to start.
The .PDF file shows the basic schematic (no rocket science here)
and it also gives you the building information. You can find the
toroid cores at the
Corporate website. The wire is also available there
and you can wind the input cores based upon the data sheet for the
It must be mentioned that
CATHODE impedance is not consistent through out the entire tune up
process. That means it will not be stable throughout the operation
cycle. Impedances are related to iP and eP values. For instance the
iP of an 811A at 180mA with an eP of 1300 VDC, the impedance for one
tube is 320 OHMS give or take 10%. That is the same basic impedance
of a folded dipole fed with twin-lead. Try tuning that with your
IC-7800 without using the ATT. The radio will shut down (period). Do
the math and find out that four 811A tubes will be some where in the
neighborhood of 80 OHMS, but only if the amplifier is tuned LINEAR.
If you miss-tuned the amplifier you could be off into no-where land
and not much input will reach the cathodes of the amplifier, let
alone your antenna. You can see that no matter how much plate
voltage you have on the tubes, it is LINEAR OPERATION that maintains
the correct CATHODE impedance. Out of tune means non-linear and
non-linear means a miss-matched impedance at the CATHODE. So in
order to correct the input impedance, during tune up and operation,
we place a TUNED INPUT at the amplifier. This maintains the 50 OHM
impedance that the radio sees. Tune the amplifier properly and
everything will be "hunky-dory".
In the paragraph above...I mention a
single tube 811A amplifier with 300 ohm impedance. That is the
correct load resistance for one tube. Two is obviously 150 OHM
impedance. Using four, the impedance is around 60 OHM...so let me
mention tube failure in an amplifier (AL811H) when one filament goes
dark. Do the MATH and you can see that your input SWR will rise
dramatically if one of the tubes in your amplifier fails. Problem
is, you can't see that from the operator's chair. You should always
monitor INPUT SWR (and grid current) when using a multi-hole
By now you probably know me enough to understand that I have a lot
of problems with 811A and 572B tubes. First of all these tube
amplifiers require neutralization. The manufacturer specifications
of these tubes is very inconsistent from one to another. Finally why
use four tubes when one will outlast and out perform the same. The
AL80B sells for 400 dollars less then the AL572B...these are
identical amplifiers! They are so identical that most of the
components used in one are used in the other. The difference...the
AL80B uses a 3-500Z. The top voltage for a set of four 572B tubes is
around 2900 VDC; while the 3-500Z will easily handle 4000 VDC on the
plate. The 572B is capable of 175 watts of dissipation. In a four
hole setting that is close to 850 watts. The 3-500 is capable of
1000 watts, with a true dissipation of 500 watts DC (not PEP). A
single 3-500Z can be operated safely at 4000VDC and 500 mA (2000
watts). Now, you tell me why
the AL-572B deserves to be 400 dollars higher then the AL80B??? If
the 811A and the 572B are interchangeable (as everyone, but me,
claims) why is the four hole 811A, 1000 dollars less MSRP??
This has to be a scam...What
is really interesting...four 572B tubes in an 811H (with some
modifications required) will perform almost as well as the AL-572B.
And, the guy on the other end will never know the difference.
Radio Shack is about to go out of business, their stock is less than
3.00 a share (3/1/14). They could rejuvenate their sales by simply
adding a line of linear amplifier kits for Amateur Radio Operators.
A simple 500 WATT solid state kit would add real "boom" to the
inevitable "bust". How simple is that...Radio Shack who was the
moron that thought you could compete with Verizon, Sprint, ATT or
all the other mobile stores. You made a huge mistake taking radio
hobbies (SWL, RC, Amateur Radio) out of your retail philosophy.
Thanks, The RFampGUY.
For more reading on this subject I suggest:
HOME PAGE ~
LINKS PAGE ~ CONTACT PAGE ~ BACK ONE