Post by KB3IBT Dan KHello All,
I was told by a ham during a QSO that if I buy a matched pair of 6146b's,
I would not need to neutralize them?? Is that true?
Thanks KB3IBT
I've read where certain resistors in the hybrids served double-duty as
fuses. What gives?
Usually the resistors, if you overload your rig for extended periods! <g>
As I've told lots of other people, the first thing I check when I unbutton a
rig is the cathode and screen resistors. That gives me an immediate
indication of whether the rig has seen some abuse during tune-up or
operation. Kenwood made the cathode and screen resistors in the final
section just large enough to do their job, with a little room for beginning
operators learning to tune it up properly. Before any real damage can occur
to the expensive items, like the power transformer (priced one lately at
kenwoodparts.com?) or the final tubes, the cheap resistors will give it up.
Cheap, simple, and quick to replace, they have saved many, many Kenwood
hybrids from the bone-pile. See my further description HERE.
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How can I tell when my cathode resistors are cooked?
The easiest way is to measure them, with a zero-adjusted ohm-meter. From
cathode to ground, it should read 5 Ohms, +/- 5% (4.75 to 5.25 Ohms). You
can see tell-tale signs without opening the case, however. The plate
current metering is read from the cathode voltage, and the higher the
cathode resistance, the higher the plate current reading. Also, the lower
the output power. If you read high plate current and low output power,
those cathode resistors are probably climbing in value! The plate current
is actually lower than what you are seeing, and the output power is just
confirming it. Time to order some replacement resistors!
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How do I know when I have bad final screen resistors?
The final screen voltage directly affects your output power. It is the
basis of most QRO mods, just raise the screen voltage beyond the Kenwood
design values, and viola! More power!. Since we already know we probably
don't want to RAISE the screen voltage, what we are concerned with here is
too LOW of a screen voltage. Again, the failure mode of resistors is to go
UP in value when they are getting cooked, and that will also lower the
screen voltage in the Kenwoods. If your fresh finals are not providing an
honest 100W output, and if you have plenty of drive, it's probably a good
time to check your screen voltage, and the value of the 100 Ohm and 470 Ohm
resitors in series with the screens. There can be other causes, but let's
go with the most common, and check those resistors first.
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How can I tell when my finals (6146's) are getting "soft?"
Tubes go "soft" when their cathode emissions begin to drop. That means that
for a given set of circumstances, plate voltage, screen voltage, drive
level, etc., fewer electrons make it from the cathode to the plate. When
this happens, plate current begins to fall off, output power starts to drop,
and we start to increase the LOAD control and re-dip to maintain the power
output. As we increase the LOAD control, and more heavily tax the finals,
the tuning starts to get broad, with a much less pronounced "dip." This can
occur gradually over time, so we tend to overlook it. Finally, we're seeing
70W out on 20M with very broad plate tuning - It's time to replace the
finals. Fortunately, for Kenwood, this can take over 35 years, depending on
your operating habits, as I get lots of rigs with the original S2001 tubes
in place, almost as good as new!
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When should I replace my 12BY7A driver tube?
The ALC metering directly measures the performance of your driver tube.
Usually, when the tube starts to go soft, it will be hard to obtain full ALC
swing on the highest bands. 10M will always go first. As long as you have
sufficient drive to get well into the ALC range on the bands that you work,
the tube is fine. When ALC drops to zero on bands of interest, it's time to
replace it. You need ALC indication for the speech processing and other
features to work properly, so don't operate in the grey areas ouside of
normal ALC readings.
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In your Tune-Up procedure, you intimate that 225ma is a sort of "Magic
Number" for plate current. Why is that?
When we're adjusting the plate and load controls during tune-up, what we're
actually doing is attempting to match the output impedance of the finals
(the two 6146B's in parallel) to the 50 ohm load. Maximum power transfer
always occurs when we match source and load impedance, and for a pair of
6146's in good condition, with nominal plate and screen voltages as found in
the Kenwoods, that turns out to happen at around 225ma indicated plate
current. 800VDC X 225ma also turns out to be 180W DC input, right at
Kenwood's design goal. If you watch your output power as you increase
loading while re-dipping, you'll see increasing output power until Ip
reaches about 225ma, then it starts to come down again. Plate current will
be higher, but power out will be lower.
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How can I Neutralize my Finals?
Neutralization is the process of compensating for the inter-electrode
capacitance of the final tubes. It helps prevent the tubes from
oscillating, and allows them to perform as a stable amplifier. Here is a
simple sequence for neutralizing your finals:
(1) Tune up normally into a dummy load on 10M, perhaps 28.1 MHz
(2) Turn your SG (Screen Grid voltage) switch OFF. This effectively turns
off your finals, and will prevent them from putting any power into your
measurement setup.
(3) If your dummy load has external connections to monitor the RF it is
seeing, use those - Otherwise, remove the dummy load and replace it with a
50 ohm resistor (47 or 51 Ohms, 1/2W will work fine) that is mounted onto a
PL-259 Plug. During neutralization there is no significant power in the
load, so use what's handy.
(4) Using an RF millivoltmeter or oscilloscope with at least 20MHz
bandwidth, connect it across the small 50 Ohm test load. A 20MHz scope will
display 28 MHz adequately, although it won't accurately measure the RF
voltage present. That's OK, because when we neutralize, we're simply tuning
for minimum output.
(5) Key the rig and observe the output voltage on your RF VTVM or Scope.
This is the drive power "leaking through" to the load with the finals
inoperative. Our goal here is to minimize this leakage, and in doing so, we
will ne "neutralizing" the inter-electrode capacitance.
(6) Using an INSULATED tool, either plastic or a carved wooden dowel, tune
the neutralization capacitor for minimum output. You should be able to get
down to 10 millivolts or so. There is LOTS of voltage present, so don't
even THINK about using a metallic tool!! You will spark and arc and create
all manner of scarey fireworks if you do - Not to mention perhaps joining
the ranks of the SK's . . .
Don't forget to return your SG switch to ON for normal operation.
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Why Can't I Successfully Neutralize my 6146A's?
The Kenwood's were designed around the 6146B, which was significantly
different from previous 6146 and 6146A versions. You should not use the
6146, 6146A, or 6146W built before the mid-1960's in the Kenwoods. After
the 6146B was introduced in 1964, the 6146W's gradually were migrated to the
6146B design, and later versions of those tubes are fine. The
neutralization circuit employed in the Kenwood hybrids was designed around
the 6146B, and may not be able to successfully neutralize the older
versions - That can lead to instability in the finals.
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What should I do after changing Finals?
Whenever you change finals, you must read and understand the following
information whether they are NOS U.S. or of foreign manufacture.
Absolutely every time you change the finals in your Kenwood, you MUST do two
things: (1) You must adjust the bias for the new tubes, and (2) you MUST
neutralize the new tubes. Simple stuff, but you have to take these two
steps to avoid problems.
Whenever you install new finals, the following steps for adjusting the bias
MUST be followed. Install the new tubes. Turn the bias pot full CCW (No
bias). Turn on the rig and heaters. Turn your MIC control full CCW (no
gain). Set your BAND to 14 MHz (20M). Set your MODE control to USB.
Flip the SEND switch to TRANSMIT and adjust your bias for 50 ma.
Why is this important? All tubes from different manufacturers or eras show
differences in operating characteristics. You have to tune the factory
provided adjustments to compensate for these changes when you swap finals.
It may take a few minutes, but it is well worth the effort. Your new tubes
will be operating safely, and will then provide many years of peak
performance.
For neutralization, see the FAQ question a few steps above this question.