I built the amp in three separate sections with the intention of stacking them or building them into some sort of case.
Heater Supply Section:
The GM-70 needs 20v 3A+ to light up so AC is out of the question because it would hum so much. I lucked out and got the power transformers on a homemade supply of some sort for 6 bucks at the Re-use-it center. They put out exactly the 18v needed and must be at least 10A each. I did not want to over tax anything so I went with a dual mono supply. I used some bridges that I had kicking around as I plan to change them with some FREDS later. The chokes I had to order from Parts Connexion, they are Hammond 195P5 rated at 30mh 5A. I was going to go with the 50mh but they were 100 bucks each. I am not sure the extra 20mh would make much of a difference. It puts out 20.2v under load with very little ripple. I have not hooked it up to the scope but it looked very good on PSUII.
Main Power supply:
GM-70 tubes can take a lot on the plate but I did not want to go over the 1kv range as the parts are deathly expensive. Just for one 2000v 100uf cap you’re looking at 200 bucks easy. If anyone can point me out some reasonably priced caps I would love to increase the voltage.
The power supply is 3 different sections. The first section is just a 12.6 volt transformer to heat the 12GN7A’s. I am changing this to a regulated DC supply soon. The second section is the plate supply for the 12GN7A’s it’s a 210-0-210 off an old console amp to a 6AX5 a big 20H freed choke and a 40uf oiler, 95R resistor to couple of 100uf oilers in parallel. I was going to use an 83 but the transformer only has a 6.3 volt tap. I could run the 83 off the main transformer but I want to control the main power separately. Puts about 170v on the plates of the 12GN7A’s. The main power section consists of a Stancor PC-8414 600-0-600 and Hammond 166F5 to two 866A’s mercury vapor rectifiers to a couple of 100UF’s in series to get the voltage rating up to about 1500v. A military swing choke that I cooked by accident and need to replace, then to a huge 100uf 1000v Russian military oil capacitor. I cooked the choke by having the heater supply unknowingly grounded. I confused the center tap with an electrostatic lead on the heater section causing a lot of amperage to go through the GM-70 plates. This severely overloaded the power supply and squirted potting compound out of the choke. It was not happy after that incident. The main supply can put 710 volts into the GM-70 plates. I would like to get it up to 850ish to get closer to 20W’s so I can try and dive my electrostats.
Amp Section:
The 12GN7A is not a very common tube. Triode strapped it has a mu of 40 and an Rp of 2K. I have miss matched tubes in there but have some NOS ones on order right now. It takes a meaty tube to drive a GM-70 and these do the job. The plate chokes are 100H and can take 40ma although I am no where close to that. The coupling caps are Jensen foils 0.47uf 1000v. I have some Hovlands that I want to try but have not soldered them in yet. I made the GM-70 tube sockets my self out of some 829 sockets by drilling out the necessary holes with a ceramic drill bit. It was quite easy, I just drilled them in a bucket of water. The cathode bypass caps are 40uf oils. The output transformers are made by Eastern Audio 10K-4/8R 150VA.
Having learned my lesson many times I built everything on boards first for testing. I do plan on putting it into a case at some point in the future. Hopefully by that time I have enough parts to make it into monoblocks. Either way it’s going to be freaking huge. The only real problem and head scratcher I had with the build is that my heater supply inadvertently got grounded through an extra lead on the heater supply transformers that I though was a electrostatic shield. I confused it with the center tap and ended up with a lot of ma going though the cathodes of the tubes. this smoked up the cathode resistors real good. I could not figure out why the voltage drop across them was wrong and they were very hot, so hot that I burned a big blister on my finger. I was helped out by a few guys on the GM-70 yahoo group and I fixed it up. I had to pick up some new 500R cathode resistors from Parts Connexion so I went with some 50W ones because overkill is cool.
How does it sound? Well I must say that it rocks. Hooked to the Calrads’s in the open baffle it pumps unbelievable amounts of bass. I still have a bit of hum that I have to fix as that cooked choke is not really doing its job very well any more. I have a Stancor 10H/300ma on order so that should clear it up. I also have to DC the 12GN7A filaments. It is something that really has to be heard to describe, I enjoy the sound of my 27/6V6 amp but it just does not have the solid punch the GM-70 has.
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