The low power AM BCB transmitter project that I designed and built which runs directly off of line volatge worked out so well, decided to do something
else.

It'll be a full featured regen that runs right off of line volatage. No power
or isolation transformer required. Not that there aren't plenty of power
transformers 'in stock' here, just want to see if it can be done.

The tube line up will be; 12AY7 regen/1st audio (chosen because it
has a lower mu than most tubes for smoother regen control),
12AU7 2nd audio and phase splitter for P-P 50C5 output tubes.
Three 12AX7's will be used in the 'Select-O-Ject' which is a variable
audio notch/peak filter. Half of one 12AX7 will be used as an AGC
amp (!) and finally a 1629 eye tube and 0D3 VR. Note that tubes have
150 mA filaments.

The unique thing about this design is the filaments will be run off
of well filtered DC via a full wave bridge right off of line voltage.
Very important to keep hum out of the regen detector.
This arrangement will provide approx 173 vdc (123vac * 1.414),
so the series filaments will need to add up to this, or close.

The design is done and construction is underway.
Steve

double triode mu/transconductance (umhos) comparison, at 100V plate:

12AY7: 44/1750

12AU7: 20/3100

12AT7: 60/4000

If you want low mu for the regenerative detector, the 12AU7 triode seems to be a better choice than the 12AY7. In my experience, a high transconductance value is at least as important as low mu. For example, one of the smoothest regenerative detectors I've tried is a triode connected 12BE6 pentagrid (mu approximately 20, transconductance over 7000umhos). The 12AT7, both sections connected in parallel, was also excellent (mu 60, transconductance 8000umhos).

Rob

Thanks for the tip Rob. I did notice that and since the 12AY7 and
12AU7 have the same pinout figured it'd be easy to try either one
and compare performance.

In your experience have you had problems with using a dual
triode as the regen detector and audio amp? I've built quite a
few regens using many different tubes and configurations but not
this one. Have always used a seperate tube as the detector.
Steve

Steve, no problems using a single tube for the detector and an audio stage.

That's a very ambitious regen you're building. I look forward to your progress reports.

Rob

Thanks Rob. Designing and building regens has been an iterative and challenging process. And more than a little addictive!
The last one was a two level breadboard style using scrap
pieces of trianglar pine seperated by wood dowels. It used a 39 detector
two 75's for the audio amps and a 6E5 eye tube along with a 1N270
for a simple AGC. Hope to improve the AGC action with this next one.
Steve

Hey Steve, I know what you mean by becoming addictive. Like all of us, I have tried about 4 different designs for SW regens over the last two years. I am fortunate to have a friend with unlimited parts and tubes to chose from. Thinking like you, my first filament supply was filtered and regulated DC. My final, and best regen, has a 6 volt AC filament circuit with a bypass caps to ground at the tube socket. My DC supply was over kill and was not any quieter. You'd be better off, I think, using a small 12V transformer rather than stringing the filaments together in series. I tried all the detector combinations you have listed, and a few more. I also tried a cascode circuit, and even went so far as to separate the detector from the Q multiplier using a 12AU7. They will all give you a slightly different result, a couple pretty dramatic in a disappointing way.

After all my fooling around, my advise to anyone building a regen is: Design your set around a Hartley oscillator. Winding "tickler" coils on an Armstrong, to me is a PITA. A tap 30% up from ground on a Hartley always works the first time. My personal preference for a detector is a pentode of the sharp cut off variety. Control the regeneration with the screen grid. It's always very smooth with the correct voltage.

I decided on my last receiver that a 6AU6 was the quietest tube I could find. The 12AU6 has the same characteristics. 36mu and a 3900 transconductance at 100 plate volts.

Keep us informed as to what you have discovered. Regens are fun, even with the frustration factor. michael

As far as using a dual triode as a detector and amplifier........ well, that is exactly what they did in the old Knight Kit Space Spanner using a 12AT7. I've also seen these dual triodes used as a detector and RF amp. Grounded grid or otherwise.

Michael,
I've had more fun building all of these regens than anything else in quite
a while. For such a simple circuit there are seemingly limitless way
to improve performance. Or hurt it! Experience and attention to
detail is everything.

Agree with you on the tapped coil design. Found that to be just as good as using a seperate tickler and easier to build. However, finding the best
place to tap the coil takes time and patience. Quite often a tap
position will work well at one end of the band but become 'ragged'
on the other end. Have considered a switch to choose between
two or three taps but haven't tried that yet. The best detector tubes
I've found so far are the 6C4 and 37. And I've tried a wide variety
from compactrons to 01's and even a nuvistor.

Completed the line operated 9-tube regen in my original post
a couple of days ago and it works! Still tweeking the design a bit
but the concept of running a regen right off of the line, which was
the main challenge, has been achieved.

Anywho, keep building, Its nice to hear from others that enjoy this 'sport'.
Steve