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It has been a long time since I started, but I finally completed the first of two GM70 monoblock SET amps. The circuit is a simple two stage, interstage transformer coupled design, using an SiC diode biased E280F (7722) triode connected pentode voltage amp/driver, through a Hammond 126B 5K/5K interstage, to a cathode biased (for now) GM70. The power supplies are all-tube rectified, using oil capacitors and over-specified iron, with a B+ of 1025V for the GM70, and 190V (separate supply) for the E280F. The GM70 filament is fed 20V/3ADC through a Rod Coleman regulator. Based on the operating point, I figure I am getting around 20W in Class A, and due to using an interstage transformer, and go into A2 to the limit of the driver stage.

The B+ power transformer is dead cold when operating. It is a big UTC 1500VCT/1900VCT/300mA continuous duty unit that Grover donated, and after running a half hour or so, is still cool to the touch. Also using a big 10H/500mA choke, another Grover donation. The big oil capacitors are 88uF/1200V pulse capacitors (either paper/oil or PP/oil).

Based on initial informal listening in mono, the amp is dead quite background, and is very nice tonally. I am not using very good speakers while it is down in my shop, though the best sound so far is with a classic Dynaco A25, which I probably keep set up to amuse me while building the twin brother.

David

Outstanding layout and construction! Great job David.

Tom

Lovely work. Is the chassis a drawer? How heavy is the amp?

ray

Cool heavy-@ss amp there, Sir!! Well done!!

I am not confident that that puny wood handle can bear the strain, however!

Looks like a carbon plate GM-70. See if you can snag a comparison pair of the metal plate units which I thought were more open sounding, a somewhat different flavor, but the carbons do a nice fat "run you over" presentation in their own right.

I'd expect that this is a powerful sounding machine indeed!

Is that a fan on the Coleman reg? I believe these things get quite hot, no? Rod's instructions seem to prescribe lots of heatsink.

Joe,

I have a pair of copper plate GM70s on the way from Ukraine as we speak. I have a healthy heatsink on the Coleman regs, which I have tested and it keeps pretty cool, around 50C in operation according to my IR meter. No fan.

The sides of chassis is built out of oak stair tread (1-1/8" thick), the top is 3/4" oak plywood, poplar core. You can probably park your car on top of it. Figure the entire amp weighs around 70 lbs.

David

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Walt

I was always wonder: if one use interstage transformer, with obviously air gap, why not use it as a phase splitter so the O.T. former wont have to be air gap?

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SamB

Ceramic sockets for GM70s are available from a number of Ebay sellers, usually modified slightly (drilled out) for one of the heater pins. The fancy teflon sockets are harder to find and expensive. I just have the ceramic socket.

Regarding the air gapped (SE) vs centertapped interstage transformer, the circuit is single ended by design, including using a very fine 10K SE Tamura output transformer. A centertapped transformer for phasesplitting is applicable to a push-pull or differential amp topology. Two different approaches. One can debate the merits and disadvantages of either design. I chose SE due to having the output transformer for it (a big sunk expense), and I generally like the simplicity and sound of SE circuits. And just because. I have built compact PP EL84 amps with the same output power as this big amp.

David,

It's good to hear from you again - I hope your performing is going well!

I wouldn't expect anything else from you but a beautiful job, and you didn't disappoint. Maybe we'll be able to listen to the pair of amps in the future.

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- Guy

Guy,

I am hoping that once I get the other amp finished, I can hold a meet at my place like old times. It will be fun to reconnect to old friends as well as new.

David