It was listed as an "unknown amp" I did some sleuthing and found that it was either a Radio shack, National Radio, or a Merrell kit amp.
We'll call it a Merrell SA-30 for now.
Gratuitous glowing artsy tube picture.
In the picture above of the back of the amp, there are 4 connections for each speaker. I determined that the far right connection is common, the other connections are from left to right. 4ohm, 8ohm, 16ohm. I input a sign wave in to the unit and noted that the amplitude of the wave increased as I went from right to left. The 2 RCA connections below the left speaker connectors. They are not Left and Right Tape out.
The unit supposedly does NARTB equalizing on the phono inputs (I cannot tell).
After about 2 hours listening the Power transformer gets to be about 140°f.
I need to:
get matched tubes for it, though it plays pretty good with the hodge podge that I have in it.
make a cage/cover for it, it had one at some point and there are screw holes for one.
Make a wood base for it.
Properly label everything on the back and remove all the marking pen marks on it.
The filament voltage for the 5U4 on this this amp was over 7 volts A/C, it should be 5. I did some research and calculations that to drop 7.16 volts to 5 volts at 3 amps would take about .76 ohms 6.5 watts. I knew from past experience that it would probably not be that exact a resistor. Most calculators I found said it would take a 1 ohm 6.5 watt resistor.
I picked up 2 - 1 ohm 10 watt resistors. I found that 1 - 1 ohm resistor dropped the voltage down to about 3.5 volts. I tested the 2 resistors in parallel for .5 ohms. That brought the voltage down to 5.07 volts They do get pretty hot.
For now they are mounted sub chassis on a standoff in free air.
For anyone that is interested here is the Schematic.
Here are the original catalog ads for the amp.
Here is the schematic after I changed it to match my amp and making the mods that improve bass.