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The reason why
Igor exists because I was given two pairs of matched 300B triodes and because I wanted to build an amp with them. And Igor is a push-pull design because I had four tubes and don't fancy parallel single ended circuits. But there's more to that. To my knowledge, Igor is the only amp that can be–literally–switched beween direct coupled and transformer coupled. And as far as I know Igor was the first amp with differential driver and output stages. Some more details:

Stacked power supplies
Igor has two separate stacked B+ supplies per channel. Which translates into less heat generated because no voltage dividers are necessary to establish the various potentials of a direct coupled amp. And even though Igor works in deep Class A, i offered him the luxury of LCLC filtered supplies for B+ as well as for the heaters and filaments. (The picture shows the filament and heater LCLC for one channel.)

Lots of iron
Let's take a look at the signal path. There's an input transformer with 0.5 db taps on the primary. The two secondary windings take over the phase splitting duty and are coupled to the grids of a pair of curve trace-matched 6S45Pi driver tubes. The primary of an interstage transformer is their anode load. Here comes the rub: because of the stacked supplies, a single switch is enough to either couple the anodes of the driver stage directly to the 300B grids or via the secondary of said interstage transformer. Both stages, the driver and the output, employ constant current sources. I didn't need a negative supply because the 300B's filament sit on a potential of 230 Vdc. Quite tricky–I recommend to take a look at the schematic you can download at the bottom.

A never ending story
Igor's biggest problem is not the sound (although I am no longer that sure wether CSS in the output stages are really the way to go)–Igor has a major size issue. When I breadboarded the circuit, each channel took more than 3 square feet of real estate. And as you maybe know, real estate is scarce here in Switzerland. So I really really need to build a stacked chassis that's stable enough for all this iron and that looks as good as the amp sounds. The design has been finished quite some time. If only I could find the time to actually build that chassis… Stay tuned!

Download: igor_rev_4.9.pdf

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