GM-70 Build Issues - diyAudio
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Old 24th June 2009, 01:43 PM   #1
Pyre is offline Pyre  Canada
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Default GM-70 Build Issues

After much collecting of parts from every part of the world I am in the building/testing stage of my GM-70 amp based on Nicholas' GM-70SE Amp found at :

Nicholas GM-70

Schematic: Click the image to open in full size.

I have pretty much everything put together and was sad when one of my GM-70s failed to light up but they are easy to get.

My problem is with the driver tube. I have never used a pentode as a driver and seem to think the schematic over simplifies it. I am running about 250v on the plate through a 100H plate choke but am getting very low unclear sound. My 22R and 560R cathode resistors on the GM-70s are getting very hot. I am going into the grid on the 12GN7A at pin 2 but what do I do with the other grids on pin 3/9 and 8? The schematic does not show anything.

The schematic is pretty simple and I have checked it many times to be correct but the sound, she is just not there.
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Old 24th June 2009, 01:59 PM   #2
316a is offline 316a  England
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Default Re: GM-70 Build Issues

Quote:
Originally posted by Pyre

My problem is with the driver tube. I have never used a pentode as a driver and seem to think the schematic over simplifies it. I am running about 250v on the plate through a 100H plate choke but am getting very low unclear sound. My 22R and 560R cathode resistors on the GM-70s are getting very hot. I am going into the grid on the 12GN7A at pin 2 but what do I do with the other grids on pin 3/9 and 8? The schematic does not show anything.

The schematic is pretty simple and I have checked it many times to be correct but the sound, she is just not there.
Did you connect the 12GN7 screen grid to the anode via a 100 ohm resistor ? The schematic does not show that

Try reflowing the filament pins of the GM70 , it's probably an open filament though .

316A
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Old 24th June 2009, 02:10 PM   #3
Pyre is offline Pyre  Canada
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Hello,

I have not tried triode strapping the 12GN7A. I will give it a shot when I get home from work. I figured that would lower the gain further.

The GM-70 filament is definitely open. Hard to say what abuse they go through on the trip from Russia to here. Good thing they are nice and cheap.
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Old 24th June 2009, 02:15 PM   #4
Pyre is offline Pyre  Canada
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Should pin 3/9 (g3) be connected to the cathode as well?
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Old 24th June 2009, 03:13 PM   #5
316a is offline 316a  England
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Quote:
Originally posted by Pyre
Should pin 3/9 (g3) be connected to the cathode as well?
If it's not connected internally to the cathode then yes . You can also tie G3 to the anode via G2 .

316A
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Old 24th June 2009, 06:45 PM   #6
JoshK is offline JoshK  Canada
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The author intended the driver pentode to be triode strapped, it is implied in the circuit but not explicit in how that is done (assumed reader knows).

"After some searching on the net I found a lovely pentode called 12GN7A. It has a Transconductance of 36mV. When triode strapped it has a mu of 40 and an Rp of 2K. "
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Old 24th June 2009, 10:28 PM   #7
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If you're about to get into triode strapped 12GN7/12HG7A's or other pentodes,you might like to have a look at this page
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Old 25th June 2009, 12:36 AM   #8
Pyre is offline Pyre  Canada
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HI Guys,

Thanks for all your help. I would imagine that the triode strap would have caused a problem but it was not THE problem. I feel kinda dumb but I have to come clean. I built my own sockets by drilling out some 829 sockets and when I did I wrote the pins on the bottom..... well I mixed up the grid and the plate. Ooops, I am lucky the whole thing did not go Chernobyl on me. No wonder the cathode resistor was heating up. I should post a picture of the blister I got on my finger after accidentally touching it 4 min after it had been turned off.

Well it works fine now and sounds very good considering the abuse that poor tube was subjected too. I should probably order new cathode resistors as well as they took a beating. I will post some pictures of the creation when it has a little less alligator clips for wiring.
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Old 12th July 2009, 01:54 AM   #9
Mush is offline Mush  United States
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Juts curious. What voltages, transformers, speakers and etc. did you use?
What do you think of the amp compared to other thinsg you've owned/heard?
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Old 12th July 2009, 06:14 PM   #10
Pyre is offline Pyre  Canada
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Funny you should ask. I got the replacement tubes in the mail on friday so I spent all weekend getting it finished. The transformers are Eastern Audio, you can get them on eBay for about 60 bucks each. I am not sure of the real specs but they say they can take 150ma each. They are 10K into 4/8 ohm. Been running them for a couple of hours and they are only warm. I am using a 600-0-600 stancor transformer into some 866A's, a swing choke then a couple oil caps then into a huge Russian 100uf oil cap. Puts out about 600V after its all done. I may try a different transformer to get the B+ up to about 900. I am only using 20W Calrad full ranges in some open baffles and its seriously loud. I had it hooked to some KEF 104a/b's downstairs and it made enough bass to shake things off the wall.

I may be a little biased because I spent the last year getting the parts together and building this beast but it sounds amazing. I like it more then my 27/6V6 right now which is what I generally compare everything to. I really notice a difference in the low range, which I would expect going from 1 watt to 15. If any one wants a listen I am in the Hamilton/Burlington area.



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