Johnson Ranger 6550 turbo mod distortion problem

[K3ZW]

When I had one of the stock metal modulator tubes go bad. I decided to do the 6550 turbo mod to my ranger.
I have completed the mod. I drive the 12AU7 amp/phase inverter with the stock 12AX7 preamp. I now have fuzzy distorted audio. It is intelligible but not as clear as the original ranger modulator.

Many folks have done this mod to their rangers and I wonder if others had the same problem and how it was resolved.

Thanks

Paul

[WD5JKO]

Paul,

      I would start looking at the DC operating points of all the audio stages. Then I would run a tone up to about 30% modulation and look at the stages from low level up to the 6550 control grids. Use a compensated 10x probe, and AC couple the scope input. Does the phase inverter show good balance to the 6550 grids? Be careful about full power modulation with a sine wave, and if you go there, also look at the final modulated envelope to minimize over modulation.

     One possibility of your problem is the quiesant 6550 operating point is too close to cut off. I recall another conversation on this topic, so at the time I went back to the HLR mod, and it said to bias the 6550's at 100 ma total. The 6L6's are usually cleaner when biased at 50 ma total whereas the 6550's will exhibit some crossover distortion there.

Jim
Wd5JKO

[KA2DZT]

Jim is correct about the resting current for 6550s, 80-100ma for the pair.  Don't exceed the plate dissipation on 6550s or you'll be looking for a new pair.

Fred

[N8ETQ]

  Hey Paul,

      I'm doing the same thing here except I had
a new pair of EL-34's kicking around so I chose to
use them.  I'm not as far along as you are, I just
brought the LV up yesterday and am getting nice
waveforms to the EL-34 grids.

      You mention that your using the stock speech
amp. Is that to say you didn't change it per Tron's
article?  I also wonder what Ranger you have? The
EARLY Ranger without Taps for the bias board on the
power xfmr, or the later model with the "Y" taps?

     I am using the WA3DSP scheme here, which differs
from Tim's in that both half of the phase inverter are
powered from the LV supply and only uses 1/2 of the
12AX7.  The owner of this unit wanted the fully adjustable
output power and the heat reducing items that are
implemented in this article.

http://www.crompton.com/hamradio/JohnsonRanger/RangerRestoration.html


     I made solid state plugins for the rectifier tubes
on this one and have been trying to keep the LV to
around 300V, but as it sits now with the tube fil. at
6.3 VAC the LV supply is just above 440 VDC. Normal
AC line voltage drives the fil. voltage to about 6.8
and the LV is close to 450 VDC. I already have the 5.0
volt winding wired to buck the AC line but need to add
an external job. "I luv my Variac!"

     The VFO crapped out somewhere along the line here
so I need to get that going before I can do much more
with the Mods. This Ranger came here all "Cooked" out
from excessive Line Voltage. It's an "Early" model
without the "Y" taps. I wonder if Johnson added a few
turns to the secondary side of the later production Xfmrs
to keep up with the ever rising line voltage?

     It's quite an interesting Mod. and I am enjoying
doing it. I'll try to get some pix of my waveforms later
today.  I wonder if your distortion is from excessive
voltage. My 12AX7 has Ep 120 and the 12AT7 phase inverter
has Ep 240 triode 1 and Ep 250 triode 2 with my LV
adjusted to 300 VDC. Of course the fils are only at 5
volts. Waiting for MOSFETs for regulator.

GL

Please keep us posted as the situation develops!

Thanks

/Dan

[K3ZW]

Thanks Dan for steering me to the WA3DSP Mod.

I modified the 12AU7 phase inverter to cathode resistor bias and  capacitor coupling like in the WA3DSP circuit.

 This fixed the distortion problem.

Your Ranger looks great. Mine is a Frankenranger.

I operate mostly on 7.290 to 7.295. I live on a city lot and I am unable to put up a good antenna for 3.890.
I am working on a 40m dipole with coil loading for 75m. I think I have the room for that.

Many thanks

Paul

[N8ETQ]

  Hey Paul,

      Thanks for the reply. Glad that took care of it.
Nothing wrong with Tim's scheme but still not sure
if you changed the speech amp. Not sure which Ranger
your dealing with either.  Still wonder what your LV
is at.

73

/Dan