Silly Signal Shifter Saga

Robert Groh rgroh at SWBELL.NET
Wed Mar 17 23:50:06 EDT 2010

I'm loving your struggle with the old girl!  I used to have an urge to own one of those at one time. I did have the thought that you might document what you find out and put it on the web. You can measure caps fairly easily these days and the inductors will probably come fairly close using handbook formula's.  All valuable information that should be captured so the next sucker .... oops, I meant 'the next expert' will know how to fix and tune one.

Seriously your trip is worth while.  


From: Wilson Lamb <infomet at EMBARQMAIL.COM>
Sent: Wed, March 17, 2010 4:06:26 PM
Subject: [BOATANCHORS-TEMPE] Silly Signal Shifter Saga

This is going out to everyone (I think) who helped me with Shifter info or
stories.  No doubt Boatanchors will generate some dupes, sorry.  If you
don't want to hear any more about shifters, let me know.

Well, I've reached a plateau, I think, after a slightly difficult climb.  It
wasn't hard to get the SS started, just new filter caps.  Then the fun
began.  It seemed to be working (40m) into the dummy load, but when I fed it
into my little MFJ tuner and the long wire, confusion started.  I had a few
watts output, but high (5+) swr, no matter how the tuner was set.  Terry
wisely suggested there might be harmonics going to the tuner, which would
cause reflected power because they were not on the rig frequency, where the
tuner was set.  That was such a good idea I decided to put the scope on the
output and "see what's happening".  WOW, there was a lot of 3.5MHz coming
through, even though the 6L6 was supposed to be doubling.  At various tuner
settings, I could get more or less 3.5 energy, but the scope often showed
bizarre waveforms, making me not want to go on air!  Looking at the plate of
the 6L6 and the output on the link were equally discouraging.

I messed with that problem quite a bit, even reading up on doublers to see
if there were tricks I'd forgotten, or even never known.  I then determined
that the grid of the 6L6 doubler was not being driven very hard.  I had
adjusted the oscillator plate tuning on another and and thought it would be
OK on 40, but, guess what, it was WAY off.  When I peaked it I had a couple
mA of grid current to the 6L6 and much more output!  Still, the output
waveform did not look good and it changed radically with adjustment of the
tuner, now feeding a dummy load.

I checked the output tuning of the 6L6 and found it to be very broad.  I
could get no dip when coupling a Grid Dipper to the plate tank of the 6L6.
I took the dummy load loose, found a strong dip, and set the tank to the low
end of 40m.  Things looked better, but I still had the bizarre waveforms.  I
connected the tuner and end fed antenna, but there was little difference in
the scope views.

It finally hit me that I was being really dumb and that I was seeing
reflected energy coming back from the tuner, which added to the outgoing
signal and made the weird scope pictures.  When I moved the scope probe near
the antenna, to look at the signal getting through the tuner,  I got a fine
picture of 40m RF!    Not strong but quite clean and at 40m!

The last hurdle was a serious chirp.  I found that when the output was
peaked there was a LOT of chirp, far too much for even CX use.  It was due
to the 6L6 grid loading the osc and pulling it off frequency!  When I
detuned the osc plate tank until I lost 10-15% of the output, I also lost
the chirp.  It's clear that the electron coupling of the osc (The osc is the
grid of the 6F6, electron coupled through the screen to the plate.)  is not
good enough isolation to prevent loading.

So here I am, on the air.  I've made several QSOs as far away as MI, OH, NJ.
I still show a high SWR, which I don't yet understand, and I don't know the
output power because the tuner probably doesn't read properly at high SWR.
The output link is six turns, on a sixteen turn coil, so I think that's part
of the problem.  They did not design for 50 Ohms.  I'm going to whip two or
three turns on there and see if it behaves differently.

That's probably enough time spent on a 1942 VFO that I won't use much!
Hopefully I'll find some coils or get the energy to wind some before long,
but that's about it for the Signal Shifter.  Actually, 80m may be the only
other band I try, but maybe 20, who knows.  I'll build a driver stage so the
SS can drive my old wooden chassis final and that will be that for a while
too.  The driver chassis is already in the rack, so it won't be much
trouble.  I'd like to use an 801, but Ebay audiophools have driven their
price to unreasonable levels.  Maybe an 807, but I'd hoped to stay with mid
30s triodes...

Please let me know if you have Shifter lore, literature, or problems to
share.  I'd like to find out the values of all the tuning caps, which are
not in the book, but I sure don't want to set up to measure them!  And I'm
still really wanting an original 1938 version, to be a closer age match to
the old rig.

Also let me know if you want off this list.  I know you may have better
things to do than read about shifters!  I'll issue an update when something
big happens, like getting the big final going or solvint the SWR mystery.


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