soundmasterg wrote:Many of the old Gibsons like that sound pretty cool, and often have a pentode input instead of a triode input on the amp, so they are often excellent harp amps. they are also often underpowered, use some odd tubes, and have extremely poor layouts inside the chassis as far as working on them. I HATE working on vintage gibson amps.

Some of them are pretty cool though...definitly a lot for harp, and quite a few for guitar too..
Greg
Thanks Greg! I called my friend today and he told me that the amp was made sometime between '41 and '42. He said the speaker was replaced in '51.
I found a schematic for this amp:
http://www.ampwares.com/schematics/gibson/eh-150.pdf
Here is a article about these amps from
Vintage Guitar Magazine
My friends amp is Style 4:
"STYLE 4 (Ca.1941-'42)
The final variation showed up in Catalog BB, dated 1942, with a rearranged control panel, having the tone switch replaced by a potentiometer, ranging from Treble at 0 to Bass at 9, with Normal halfway between. The picture was the once-again retouched version of the '37 catalog's shot. A major change in the circuit (that may have occurred earlier) was the tube phase inverter, with a twin-triode 6N7 replacing the transformer. Also new were the 5U4 rectifier and the three 6SQ7 high-mu triodes (amplification factor of 100), with two for the microphone channel and the third common to both channels.
An interesting placement of tubes on the amp, which was not included in the retouched catalog shot, features the power tubes on either side of the rectifier tube, not standard anywhere else in the world of amps, but somewhat logical. Removal of the tubes to satisfy our curiosity revealed marked sockets from the factory, so this apparently wasn't the result of repairman monkey business. Sadly, the Echo Speaker output was removed for the final 150, possibly to save money on the center-tapped output transformer previously used. By this time, the EH-150 and the previously supercharged EH-185 shared the same circuit design, right down to the schematic."