If at first you think that this is a DRD amp, look again. It is not. No slow, giant, heat inducing cathode resistor found here! The output impedance of the SMPS supply is less than 10 ohms and is voltage regulated. The same low impedance supply that is feeding the plate choke of the 841, and driving the grid of the 300B is also directly connected to its cathode. In fact, the stacked SMPS supply can easily deliver over 300mA of current to any of its voltage taps, instantaneously and continuously, while maintaining voltage to +/-1V! The ultra low impedance supply means the SMPS filtering capacitors are essentially out of the signal path. This gives incredibly good low frequency coupling… better bass than any electrolytic or oil capacitor could provide. Bias protection for over-current of the 300B is provided by the multiple voltage regulated taps that the stacked SMPS supply offers, not just the current draw of the 841 driver tube. Additionally, the thoriated filament 841 tube will begin drawing current before the oxide filament 300B tube. A large sonic improvement would be found by using a true audio rated low DCR plate choke, wound by Bud at Onetics, to load the 841 plate. A low DCR plate choke would improve dynamics and recovery of both tubes better than the inexpensive 200H chinese choke I had laying around. Even-so, listening tests show that this breadboard amp destroys my Welborne Terraplane DRD monoblocks in every audible way that matters, while putting out more wattage and dissipating less wasted heat.
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:
Have you ever been sitting at home, listening to your stereo and thought “I wonder if this is as good as it gets?” Well, I have. I guess it doesn’t matter much since the idea of what is best is different to every ear. But that hasn’t stopped me from trying! I will say that this is one of the best sounding amps in existence.
Shown below is the schematic of my finest design. Although choke loaded VT-25 driving VT-25 finals is fantastic, it sounds best using the stunning RCA 210 driving another RCA 210. For those who know, they know this is special. This is my gift to you, the DIY guy with great ears.
I have changed the amp to use filament current bias on the input tube, while allowing a choice of driver tubes. The VT-51 (841) tube is a detail monster. If you are into vinyl the 841 driving the 10 is something you’ve just gotta hear! An added bonus of using the 841 is slightly higher gain, but because of less current draw the B+ ripple is lower. However, my favorite is the schematic shown… a 10 interstage coupled to another 10.
How can I say that this is the best sounding amp in existence? Well, I can not. I haven’t heard them all yet. But compared to every amp I have heard so far… things look pretty good!
This amp would make a nice front-end for GM70 finals wouldn’t it? 😉
I wish you could hear this amp for yourself. The best I could do is make a few videos of it. They do give a sense of how well the amp images. For your entertainment… Link to Videos of the Luddite 2 10
The photos below show the amp at various stages of its development. It no longer uses the Mundorf SGO caps for coupling, instead using Hammond 126C 10K:10K interstage transformers.