Comments about CE 20A modifications:

quote from WD5JKO email:
I did a few things to my 7591A tube based 20A today, and I kind of lucked into a big gain today..

So, I was figuring that the presence of the audio phase shifter when running AM, only screws up the frequency response. Running square waves, or triangle through the phase shifter gets all screwed up..only a sine wave between 200-3000 cps gets through unscathed. So why do we need this when running AM when only one audio channel is used?

First, here is phase 1 of what I did several years back:

1.) At the audio driver 6U8 (I use 6MU8), make a series R-C consisting of 470K and .01uf, and place between pins 1-9. Then remove C4 in the same area. This NFB really cleans up this stage without sacrificing too much gain.

2.) To open up the top end of the phase shifter to about 5 Khz, make another series R-C of 390K and 180pf, and place under PS-1 socket at pins 2-9. Then bridge R15 at the balance pot with a 1.8K. Then re-do the phase shifter setup the usual way. You will see as ripple starts to emerge at about 3 Kc, it then diminishes going up and then at 5 Kc it starts to lose the shift like it did stock at about 2500 cps.

Then last year I lost an audio modulation transformer. After putting in a new one, a used a 22K 1W load resistor for the 12AT7  drivers going to LV B+,and capacity coupled the audio paraphase style with 1uf polypropylene coupling capacitors. No more DC current in the modulation transformer primary.

So here is what I did next:

1.) There is an unused set of contacts on the mode switch. The common is all the way against the chassis side, but with a small iron it can be soldered to. Connect a twisted pair of wires to the mode switch (shorts on AM only), and route to wires to under PS-1 socket, and connect to pins 3-4. This effectively bypasses PS-1 when on AM!

2.) Install a 2.2 meg resistor at audio preamp tube (12at7a) between pins 1-6. This adds about 6db NFB to the preamp stage. More importantly this raises the overload point when driving from line level audio. I never had enough to use the other audio jack, and I always had to watch the incoming level for overload signs. No longer a problem.

3.) Magic eye tube. I replaced mine years ago with a 1629 after making the switch to an Octal socket, and add a 1/2 wave voltage doubler for the filament + a small 12V fan to cool the PA tubes. I was out of eye tubes except for some Russian 6E5C’s I got several years ago on Ebay. The 6E5C has a 6.3v filament along with an Octal base. The pinout is almost the same as a 1629..Had to move a wire and resistor at pin 4 to pin 6. Works!

So with these changes, using a crystal D-104 Mic, the audio level for full output is about 2 O’clock on the knob…So gain is fine.

More important, the AM mode sine wave frequency response at -3db (.707 volts) is 40 hz to 10 kc after the NFB and getting rid of PS-1. Playing with triangle and square waves is much improved..Not perfect but damn good midband.

Running SSB it is as before except the overload issue with the preamp is gone.

Then the big discovery…On AM, the tendency to overmodulate into DSB is greatly reduced. I took out my diode-resistor limiter..not needed. I have no idea why. Dumb luck I guess.
End of quote from email.

Jim
Wd5JKO