Finals going bad in Heathkit SB-401 transmitter?
David
that exhibits a couple of peculiarities during tune-up:
1) First, on 80 meters (and only 80 meters) when I tune the final
amp’s plate circuit, the plate current dip point does not correspond
to maximum output.
2) Second, I never read any grid current during tune-up. However, the
manual states that I could see as much as 0.5 mA during tune-up.
I’ve check the components in the final amp, to the extent I can
without un-soldering anything, and found no problems. So I’m
wondering if I have flaky tubes in the final that can’t be neutralized
(problem 1) and draw negligible grid current (problem 2). The tubes
are 6146s and one slightly glows blue during key-down in CW.
This really isn’t problem--just a nag. I monitor my on-air signal with
an SB-610 monitor scope and the output looks fine.
-Dave, K3WQ
Kenneth Scharf
The tank circuit on most rigs for 80 meters tends to be low Q which
means it will tune broadly. If the the plate current dip and max power
output doesn't match it could mean a miss-match between your antenna and
the rig, IE: high SWR. The tank circuit on the '401 is designed to
match between a 50 ohm load, stray too far from that and the tank
doesn't tune correctly, 80 meters is the worst in this regard due to the
limited range of the variable caps on that band.
No grid current? check your bias setting, driver tube OK?
David
>
> A pale blue glow can be a normal ionization glow, doesn't mean gas.
> The tank circuit on most rigs for 80 meters tends to be low Q which
> means it will tune broadly. If the the plate current dip and max power
> output doesn't match it could mean a miss-match between your antenna and
> the rig, IE: high SWR. The tank circuit on the '401 is designed to
> match between a 50 ohm load, stray too far from that and the tank
> doesn't tune correctly, 80 meters is the worst in this regard due to the
> limited range of the variable caps on that band.
>
The maximum-output-not-at-plate-current-dip problem occurs when I use
my 50 Ohm dummy load.
Incidentally, the 401 is specified for output (antenna) loads between
50 and 75 Ohms. I suspect this is why the LOAD control has little
effect on the tuning.
Honestly, it's the no-grid-current problem that has me more puzzled.
I've checked the grid meter circuit and found the resistors to be OK.
Just for kicks, I may open up the 401 and check the them again. The
meter works fine for other measurements (plate current, peak output
voltage, etc.), so I don't think it's the problem.
-Dave, K3WQ
bill
The blue glow on the glass is from electrons striking the glass. That is
not a problem. If the glow is inside the plate (between the plate and
grid) that is gas.
In my experience, the tuning problem is from not completely neutralizing
the amplifier.
Bill K7NOM
Antonio Vernucci
amp’s plate circuit, the plate current dip point does not correspond
to maximum output.
R1) If this problem only occurs on 80 meters it means that, although the tubes
are correctly neutralized, some particular effect occurs just on 80 meters which
alters the zeroing of the neutralization bridge on that band.
The most likely cause could be a bypass capacitor having too low a capacity (the
reactance of a capacitor decreases with frequency). For instance, your screen
bypass capacitor may result to have enough capaciity for the 10-40 meters needs,
but not enough capacity for 80 meters. If so, neutralization on that band would
be upset. The same could occur due to a too low capacity cathode bypass
capacitor.
A possibility could be that one bypass capacitor has got "open" (at those times
it was not uncommon to put several bypass capacitors in parallel, so that, if
one capacitor gets open, the total bypass capacity decreases).
So, the simplest thing to do is to try putting a bypass capacitor (say 0.01 uF
with appropriate voltage rating) in parallel with the existing one, on both the
final tubes cathode and screen, and see what happens.
Q2) Second, I never read any grid current during tune-up. However, the
manual states that I could see as much as 0.5 mA during tune-up.
I’ve check the components in the final amp, to the extent I can
without un-soldering anything, and found no problems. So I’m
wondering if I have flaky tubes in the final that can’t be neutralized
(problem 1) and draw negligible grid current (problem 2). The tubes
are 6146s and one slightly glows blue during key-down in CW.
R2) Having no grid current is usually due to the fact that the driver tube does
not deliver enough drive power into the final tubes grid. Although this could
possibly be due to a weak driver tube, the most likely cause is that the driver
stage (or the stages before that) are not well adjusted. You could then try
performing the adjustment procedure described in the manual.
In any case, having no grid current does not cause any problem. Probably you now
get a somewhat lower RF output power than if the final tubes grid would be
driven harder.
73
Tony I0JX
Rome, Italy
David
Tony,
Thank you! I'll try your suggestion of placing additional caps in
parallel with the existing bypass caps.
Incidentally, I found a spare set of 6146 tubs, installed them in the
SB-401 and tuned-up on 80 meters. Even though I didn't neutralize the
tubes, the '401 exhibited the same behavior I described above. This
suggests the problem is not with the tubes, but with some other
component.
Yeah, come to think of it, little or no grid current isn't anything
much to worry about. The 6146s are pentodes. Their grids shouldn't
draw much power, in any. I think the narrative in the manual is
misleading.
-Dave, K3WQ
Antonio Vernucci
Thank you! I'll try your suggestion of placing additional caps in
parallel with the existing bypass caps.
Incidentally, I found a spare set of 6146 tubs, installed them in the
SB-401 and tuned-up on 80 meters. Even though I didn't neutralize the
tubes, the '401 exhibited the same behavior I described above. This
suggests the problem is not with the tubes, but with some other
component.
Yeah, come to think of it, little or no grid current isn't anything
much to worry about. The 6146s are pentodes. Their grids shouldn't
draw much power, in any. I think the narrative in the manual is
misleading.
-Dave, K3WQ
Hi Dave
when you'll have done the test, please let me now as I am curious to know
whether the extra capacitor helps.
The screen capacitor is the most critical one because, if the screen does well
shield the plate from the grid, the neutralization bridge gets obviously
altered.
Yes, there should not be much grid current. Anyway, if you get the RF output
power you are supposed to get, you may forget that grid current issue and you
would be all set.
73
Tony, I0JX
Rome Italy
Kenneth Scharf
output loading capacitor. On 80 meters the three gang variable loading
capacitor isn't big enough so the band switch adds an additional fixed
cap in parallel. Maybe that cap has gone bad or the switch is not
making contact on that band. Try putting a 500pf 500v cap in parallel
with the output loading capacitor and try it on 80 meters.
Antonio Vernucci
news:ivqmqc$92t$1...@dont-email.me...
> If the problem is ONLY on 80 meters then the issue may be with the output
> loading capacitor. On 80 meters the three gang variable loading capacitor
> isn't big enough so the band switch adds an additional fixed cap in parallel.
> Maybe that cap has gone bad or the switch is not making contact on that band.
> Try putting a 500pf 500v cap in parallel with the output loading capacitor and
> try it on 80 meters.
Should the output Pi network capacitance be too low (bad additional fixed
capacitor or faulty switch), one would just see a dull plate current dip and get
low RF output power, due to the Pi network overcoupling to the antenna. I see no
reason why it should also cause the final stage to become non properly
neutralized (as evident from the fact that the plate curent dip does not occur
as the same plate capacitor setting as for maximum output power)
Kenneth Scharf
out of neutralization on 80 meters. The grid / plate capacitance is
simply too low to cause feedback on this band.
The Heath SB series used an unusual pre-selector / driver tuning
circuit. On 80 meters a single coil with a coupling link is used in the
tank circuit on both the input and output of the driver. It is tuned by
variable capacitor, and tracking is adjusted with a variable powered
iron slug. On the other bands additional slug tuned inductors are
switched in parallel with the 80 meter inductor. These inductors are
connected in series and are shorted out one by one between 40 to 10
meters by the band switch. On 80 they are disconnected from the
circuit. IF the 80 meter inductor were open the circuit would not tune
correctly on that band, but the series connected inductors might still
resonate close enough on the the other bands, on 10 meters the open 80
meter inductor would not even be noticed. Since this circuit is part of
the neutralization bridge it would be worth checking.
Antonio Vernucci
to the Pi-network loading capacitor.
As to the other issue you have raised, it is not easy for me to fully
appreciate - without actually looking at the diagram - whether an open inductor
could really yield some change in the neutralization bridge, and not just a loss
of drive signal, but it could well be.
Anyway, it now comes to my mind that also the capacitor byassing (though not
completely) the cold side of that inductor to ground could be responsible for
the noticed effect. As a matter of fact, in typycal diagrams, the neutralization
partitioning circuit is just formed by that capacitor and by the high-voltage
low-capacity variable capacitor directly connected to the tube plate.
Yes, on 80 meters the grid / plate capacitance is probably be too low for
triggering an oscillation, but we are just talking of a modest offset the
neutralization bridge that can only be detected when tuning the pi-network plate
capacitor.
Anyway, let us read the tests results and then comment further.
73
Tony I0JX
Rome, Italy
I had understood that you suggested to check
"Kenneth Scharf" <wa2...@bellsouth.net> ha scritto nel messaggio
news:ivsld5$8qr$1...@dont-email.me...
David
I connected additional bypass capacitors across the cathode resistors
and screen grid, just as Tony suggested. Nothing changed. I also
checked the fixed tuning capacitors in the pi network, just as Kenneth
suggested. Nothing found.
OK, all the caps were fine. So what else was wrong?
Well, I noticed that in CW and TUNE modes, the peak RF output was less
than the peak audio output in LSB or USB. Strange. So I poked
around and determined that the diode-based balanced modulator was not
sufficiently “imbalanced” for CW and TUNE. I think this was a
combination of carbon resistor R402 having drifted higher in value
(it’s specified at 68K Ohms) and a bias supply voltage running a bit
low (should be +250 VDC, but runs at +230 VDC). So I soldered a 220K
Ohm resistor across R402 and now get full power in CW and TUNE because
diodes in the modulator are now sufficiently forward biased.
So why was low power a problem?
Well, the peak-power-at-plate-current-dip rule really only applies at
full output power when finals are running class B. When I was tuning
in CW or TUNE, and didn’t have full power, that rule didn’t apply and
it looked as if the finals weren’t neutralized. However, the apparent
neutralization problem didn’t show up when I tuned-up the SB-401 by
applying a pure tone to the mic input in LSB or USB.
-Dave, K3WQ
Kenneth Scharf
higher than it is at low power, and the PI network does not match the
load to 50 ohms. The Q of the network at resonance is off, I'm not sure
if it is higher or lower .... depends on where the loading capacitor
ends up. Also on 80 meters the range of the capacitors is limited to
begin with....
David Drumheller
>
> Because at low power the effective load resistance of the finals is much
> load to 50 ohms. The Q of the network at resonance is off, I'm not sure
> if it is higher or lower .... depends on where the loading capacitor
> ends up. Also on 80 meters the range of the capacitors is limited to
> begin with....
Yup. I'm sure the difference in load resistance was at play.
Since my last post, I permanently replaced the aforementioned 68K Ohm
resistor (R402) with a 200K Ohm in parallel with a 100K Ohm. The
original carbon composition resistor had a resistance of 380K Ohms--
over five times its original value. That's the largest amount of
drift I've ever encountered.
Also, I now get grid current during key-down in CW and TUNE.
Thanks for all the help, guys.
-Dave, K3WQ
buzz
> I have a Heathkit SB-401 transmitter I refurbished a few years back
> that exhibits a couple of peculiarities during tune-up:
>
> 1) First, on 80 meters (and only 80 meters) when I tune the final
> amp’s plate circuit, the plate current dip point does not correspond
> to maximum output.