Atwater Kent model 55 or 55c
Modified 27 driver tube circuit to Replace Defective Interstage Driver Transformer
The following is a contribution from Norman Chamberlain, Huntsville, Arkansas.
Norman was so kind as to draw up schematics to share with other collectors that have experienced a common problem with the Atwater Kent 55 and 55c.
February 2003, Norman writes:
"Enclosed is the modified circuit using the 27 driver on the AK 55 as a phase inverter to drive both grids of the 45 tubes when the interstage coupling transformer goes bad.
It works pretty dog-gone good and the result is spectacular when you should also have to replace the old output transformer having some shorted turns in it,
using a late high fidelity transformer. You ain't going to believe the frequency response is all the way... from about 2 cycles per second all the way out to the outer limit
of the AM broadcast band spread, which is about 8 kilocycles! Most of us can't hear below about 20 cycles, and with good hearing we might be able to hear up to 12 kilocycles.
However, there is some distortion I believe may be generated in the 27 tube detector stage."
... "Have not had to change anything to the R.F. stages, they are doin a jam up job.
This area is notoriously bad for AM reception, but these stages are bringing in stations all over the country around here with just a thirty foot antenna..
I logged about thirty stations one night."
... "In reading Atwater Kent's owners manual on this radio, there is mentioned in the manual that there is a "heavy metal cap" which is to be placed over the tube.
Those 24A's can boot out some pretty bodacious r.f. power at the grid of the 27 detector. Naturally some of this r.f. is radiated off the plate of this tube which is acting
as a transmitter antenna, enough for some of it to be picked up at the input of the r.f. stages, creating some feedback, which can cause a whistle or distortion in the sound.
The cap serves to absorb this radiation, like a shock absorber on an automobile suspension, or a capacitor in effect, to reduce the back radiation to where it can't be picked
up by the set's antenna."