The design comes directly from a legacy of early radio and
phonograph designs dating back to the 40’s – perhaps earlier. The
Kent design you detail and another design which utilizes the 50L6
and 35Z5, are based on a table-top radio architecture known as
“The All American Five”.
"Trend" (somewhere out west ?) and "Regent" (Montreal, Quebec)
were other Canadian companies producing these small amp layouts
in the 60's. I think there were several others IINM – Belltone,
Regal, VOX, Silvertone, Harmony and Estey – all had AA5 based
offerings in the 50’s and 60’s. You know, I think VOX still has
one. There are boutique models on the market based on this
architecture as well.
I have a Trend amp which uses the 50L6/35Z5 combo.
It's a bit more powerful and I'm using a Fender 10" in mine -
sounds great. An upgraded speaker will probably sound better, but
remember to maintain impedance matching. It probably wants a 4
ohm speaker. Many of these amps were made with non polarized
plugs so a useful mod is to change out the power plug to a
polarized plug or perhaps a 3 prong job. The tubes and
substitutions are still available from web based brokers and
electronics shops – keep digging.
 
Here’s a couple of sites to investigate:
 
http://home.netcom.com/~wa2ise/radios/aa5h.html
http://www.fna.muohio.edu/dogstar/tubestuf/4x4intro.htm
 
Have fun!

I would *strongly* suggest using an isolation transformer (120v to
120v, around 50VA if you'll just be supplying one amp) -- either an
external plug-in unit or somehow wire one up inside the cab.
 
- Doug B.

One side of the supply must be chassis ground. The problem with
the old models is that they had no polarized plug. This would allow
the hot or return ac line to be grounded to the chassis depending on
how the plug was inserted in the outlet. You will also notice in
different schematics how the switch can be in either the hot or
return line.
If you have ever done house wiring you can appreciate the
importance of switching the right side of the supply. You can of
course switch either side but a hot side switch makes the thing
marginally safer.
Anyway, a polarized plug does the trick. A three prong just helps
ensure correct insertion in those cheater plugs roadies have in their
tool boxes. You don't need the central gound.
 
N

fixed bias sw

weber coppertop

isolation transformer (120v to 120v, around 50VA 

 Use a 130 volt zener diode (or string adding up to 130 volts on the screen supply

preamp stage - filtered DC filament supply.  

Note that the phase of the signal at the
speaker is inverted from that of the source signal at the grid of the preamplifier stage, and
also of the signal on the cathode. The circuit is interesting in that both the grid and the
cathode of the preamp are used as injection points for the negative feedback signal. 

Full-range feedback is applied directly to the cathode by the sum of the 2.2k and 12k
resistors. A portion of the signal is also applied to the grid, via the "bass" control circuit.
With the bass control at minimum (maximum resistance), the circuit essentially does
nothing. However, as the resistance is decreased, more and more of the high-frequency
component is fed back to the grid, causing a decrease of system gain at higher frequencies,
and by extension favouring the lower frequencies. Because of the extreme simplicity of this
circuit, there is a considerable effect on overall gain. As in the case of the "treble" control,
all tests and measurements were made with this control set at minimum (effectively out of
circuit).