Gibson Super Thor Bass Amp

[CJ]

here is an example of some early transistor work, the Gibson Super Thor amp, about 50 watts on an optimistic day into a couple of 15 inch speakers, looks like this>

 

[CJ]

we have a couple of schematics floating around the web, this one is right in between revs which means confusion city,

one version has a fet after the opamp, and the early version does not, a few different resistor values and what appears to be a missing 10K bias resistor makes life a gas, 

early schemo.

 

[CJ]

here is the version we have, notice the primitive dual opamp, external compensation on a 14 pin dip, draws major current as did the early chips of 1970, we used to test these dips for current draw with much fallout, American Micro Systems was the place, Homestead and Lawrence Expy, one of the first chip makers, however this one came from the Motorola plant somewhere,


notice the 10K bias resistor now showing up on the base of Q5,

our amp had no holes in the pc board for this resistor,

and a different resistor feeding Q2 of the diff pair,

here is a site for easier to read pdf files on this amp and other gibson amps>
http://www.gibson.com/Support/Schematics.aspx

 

[CJ]

here is the chassis, notice the 10K resistor splice onto the transistor, without it we only get about 10 watts, with it, about 45,

don't know how this amp worked without it,

one cap can was open so we updated with some modern axials,

that horseshoe loop holds the 2 diodes, ???

 

[CJ]

there is a total of 3 listings for the motorola chip on evilbay , we lucked out and scored a pair that worked, 

why not replace with a modern dual opamp?

weird pinout, would have to chop and splice, better to keep the antique stock just for kicks,

 

[Gus]

CJ

The IC should not matter much unless it causes excess noise it is a bass amp.  You could mount two opamps on a plug in header
Make sure to check the bootstrap cap C17
Noticed they added some extra cap to the - supply with C18 to the - supply I am guessing it is close to the output amp, check that.  I would guess it is used to control a oscillation in the power amp

+- 35VDC at the supply and +-22VDC shown at the amp?  Are they showing unloaded and loaded supply voltage?

Looks like it could sound cool with the 15" speaker
Does it thump at turn on and turn off? I am guessing it does

fet looks like it is for some extra gain after the tone sections, note the 2nd opamp is now a new channel and not used after the tone section.  I did not work the gain out but it has to be under 4700/330 I will guess about X10, should be  good for the tone stacks losses

Look like a fun circuit to build with new parts in the preamp or build just the preamp

 

[CJ]

yes Gus the schemo is wrong about the rail voltages,

finally got it workin ok,

R14 is very sensitive, it changes the balance of the output transistors, 680 ohms and 2.2 K ohms have been used, we used 1 k ohms to get the DC offset reasonable at the speak jack,

R21, 22 ohms, is also very sensitive, we could not get the 5 mv voltage drop across the .47 ohm resistors with the pot, so we increased it from 22 ohms to 47 ohms and now we have a lot more range on the adj pot,


R22, 10K, did not seem to do much, so we left it out as is done on one of the revs,

this thing cranks out about 40 watts into 8 ohms, not too bad for a 1970  budget amp, sound is not that bad,

here is the final output schemo with resistor changes and voltage readings>

 

[Gus]

amp section reminded me of this article
https://www.passdiy.com/project/amplifiers/build-a-mosfet-citation-12

 

[CJ]

Thanks Gus!  been thinkin about converting a few amps to MOSFET's so that article is just what i was lookin for, 

that schemo in the article looks like the same thing as the Super Thor, just flipped on the vertical plane,

updated the schemo in the above post,

cj