Having breadboarded a few GM70 amp ideas, I was still looking for an inexpensive tube to drive the GM70. The problem is that a low impedance supply is necessary to drive the grid of the GM70. Into the hole of cathode followers I fell, until Jim Dowdy mentioned that he likes the the 6EM7. Hmmm, wow this is the ticket! An inexpensive dissimilar dual triode with half the tube looking amazingly similar to a 2A3! The 6EM7 is identical to the 10EM7 except the 10V heater requirement. www.surplussales.com has a great deal on the 10EM7’s so I bought a bunch. The schematic shows voltage tap off the 30V filament supply for 10V heaters. 6EM7 or 13EM7 tubes could easily be used since the filament resistor provides enough regulation for any of these tubes. And yes, this circuit is screaming for an interstage transformer off the 2nd plate of the 10EM7, allowing it to drive the GM70 a little into A2. I am looking for good interstage transformers. DIY HiFi Supply has some Silk 3K:3K that would work well, but they are too expensive. I will update the schematic if I find something that works, and is priced in line with this ‘economy’ project 😉
UPDATE: OK! I don’t know why I didn’t think of Hammond interstage, but a Hammond 126B should work very well. It is only rated to 30ma, so I will probably bias the tube for less current.
… For your enjoyment, here is a few links to other GM70 builds…
Audio Design Guide
Audio Talk GM70 SET
Kevin Carter of K&K Audio
Fikus the Lampizator
Dr. Z Lab
Sampleaccurate’s GM70 Build
Polish GM70 Build
Thai GM70 Build
Yahoo Boat Anchor Group
A Russian response to Ongaku
A Japanese 6AS7 2A3 GM70 version
A scary system I would love to hear.
And the schematic for my 10:IT:45:IT:GM70 Amp: