[John L Stewart]

John Stewart Nov 2015

This circuit is based on an amplifier I built for a friend to be used with a guitar & mike. The date on the schematic is Dec 7, 1958, almost 57 years ago. The entire project was built on a sub chassis & installed inside an 8x12x3 Hammond chassis. Only the power amp is covered here.

The original amp used PP 25L6s driven by the same cct as shewn here. There are no pentodes in Electronic Workbench software so here we have used the low mu power section of 6EM7s. That still manages to demonstrate the effectiveness of +ve FB on the performance of the cct.

The power supply used a 170-0-170 HT transformer, something that was plentiful at a good price at one of the surplus shops adjacent to ElectroSonic when they were downtown at 543 Yonge St. The 25L6s & 6SL7 heaters ran directly off the line in series with a 50Y6GT rectifier.

Oddly, this transformer had no 6.3V winding but rather a very useful 25V at 150 mA. That ran the preamp, a 6BH6 for the mike & a 12AX7 for the guitar inputs & tone cct driver.

In the original cct there was not enough gain. To get around that problem I decided to try a simple +ve FB cct. The full loop NFB would control the overall gain & get distortion down to a reasonable number. The extra gain here is possible by returning the –ve lead of the split load phase inverter to the previous cathode rather than the amps common buss. An added bonus is the much improved Damping Factor.

The OPT used was a Hammond 125D, the 125E had yet to be developed. My notes show the amp was down one db at one watt, 20 Hz to 28 KHz. And down one db at 6 watts, 30 Hz to 15 KHz. D = 0.5 % at 6 watts, 7 watts clipping.

The no FB version still has a very respectable DF, typical of power triodes. For the 6EM7 mu is 5.5 while rp is 750R, very good for driving a loudspeaker. The OPT loading could be increased resulting in more audio power.

On the schematic & spreadsheet switches labeled Space, z & x control the feedback & loading conditions.