Mike Silva wrote:
The only difference in capacitance I see is Cgp, 0.24 pF for the 6146A
vs. 0.22 pF for the 6146 and the 6146B. Doesn't sound like any
difference at all, unless maybe the average values are quite a bit
different for the 6146B vs. the other two (but what would cause that?).
I don't question those who say there's a problem with some rigs, but
on paper all three tubes do seem to be the same (at the lower screen
voltage the 6146 is designed for). Apparently RCA thought the same
thing.

No conclusions, just ruminations...

73,
Mike, KK6GM


Hi Mike,

Neutralization is more than just interelectrode capacitance. It is
also affected by plate to everything outside the tube capacitance, and,
of course, the layout of the transmitter finals cage.

The plate of the 6146B is bigger than the 6146, or 6146A. It has about
33% more plate dissipation as a result. There is more coupling between
this bigger plate, and the outside world. As was pointed out by another
poster, it also requires a higher screen voltage.

In order to accomodate the 6146B, Collins completely redesigned the
neutralization circuitry on the S-Line transmitters, and the KWM-2(A).

The old circuitry had a 8-50pf PA neutralization capacitor, the new
circuitry has a 1.8-8.7pf PA neutralization capacitor.

If you put 6146B's in a KWM2(A) with the 8-50pf neutralization cap, you
will be replacing all of the neutralization circuitry in the near future.

-Chuck

"Chuck Harris" wrote in message
...
gil wrote:
Just recently got a HW-101, with low power out,I can get a few more
watts by adjusting the bias slightly above the 20ma mark, how safe is
it this?
Since I dont have spare 6146's what should I be looking for that could
be a problem other than the finals?

Thanks in advance...Gil


Sorry, but the finals are the usual reason for low output. You will go
through many sets of finals before you wear out the driver.

Make sure that your replacement finals are 6146, or 6146A's, never B's!

Note, 6146W's can be all three types. The later 6146W's are B's.

Don't get creative with the bias. It has nothing to do with the power
output, but everything to do with balancing tube life with low distortion.

When your bias is low, your tubes are cut-off on opposite cycles, and you
get
cross over distortion.

Huh? The tubes in the HW-101 are in parallel, not in push-pull!!

BFoelsch wrote:


Don't get creative with the bias. It has nothing to do with the power
output, but everything to do with balancing tube life with low distortion.

When your bias is low, your tubes are cut-off on opposite cycles, and you

get

cross over distortion.


Huh? The tubes in the HW-101 are in parallel, not in push-pull!!

A senior moment, and audio habits, made that phrase come out. Sorry!

A resonating tank circuit makes how you think of Class A, AB, B, and C
operation somewhat different, though.

(A = 360 degree plate conduction;
AB1 = less than 360, but more than 180 degree conduction, no grid current;
AB2 = less than 360, but more than 180 degree conduction, w/some grid current;
B = 180 degree plate conduction;
C = less than 180 degree plate conduction.)

The tank "free-wheels" through the areas of non-conduction in the RF signal,
effectively taking the place of the second tube in a push-pull amplifier,
thus making AB, B, and C modes of operation usable with one tube.

But the tank does nothing to improve amplification linearity.

The correct amount of bias is still chosen for the same reasons as in the
push-pull amplifier: so that the stage remains suitably linear under normal
operation, and tube life is adequate for the mission. In this case, the
correct bias puts the stage in the Class AB1 region.

The OP was running a pair of 6146's, at 20ma bias, and commenting on how
turning the bias up a bit made his too low output power rise a little.
I assumed he meant 20 ma each tube, as the usual bias for a pair of
6146's (ala Collins) is 40ma, 50ma if you are running into a linear.

If he was indeed running 40ma to a pair of 6146's, and his output power
was still too low, the usual reason is he has a used up pair of finals.

And the thread went (some would say downhill) from there...

-Chuck

Hi,

The bias should be set to 40 mA with both finals installed.
Set the bias by using push to talk mode in SSB, no audio.
If you have another receiver, listen to the carrier output and adjust
the internal notch controls for minimum carrier
with no audio.

Running the HW-101 at 20 mA bias would reduce the output, and worsen the
distortion on SSB.

You can check the balance of your finals by running one at a time,
and setting the bias for 20 mA with a single tube.
(remove the other tube completely, don't just disconnect
the top cap). If the tubes aren't within 5 mA of each other,
I'd look for around for a better balanced set of finals.
I'd do this check on 80m, because by removing one tube,
you are upsetting the neutralization.

73,
Ed Knobloch

gil wrote:
Just recently got a HW-101, with low power out,I can get a few more
watts by adjusting the bias slightly above the 20ma mark, how safe is
it this?
Since I dont have spare 6146's what should I be looking for that could
be a problem other than the finals?

Thanks in advance...Gil