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Old 18th April 2012, 03:01 PM   #1
MelB is online now MelB  Canada
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Talking GM-70 All done

Finally got my GM70 amp all done. Sounds just great! Could not be happier with how it turned out. Sounds even better than my Parallel 300B amp. Very good base control. It is driving into a JBL E140 in a 200 pound box tuned at 30 Hz.

The Hammond 167P18 (18v 5 amp) works perfectly for the filament supply. So perfect in fact it's 77% efficient with a linear regulator.
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File Type: jpg GM70_under.jpg (637.8 KB, 1864 views)
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Old 18th April 2012, 05:13 PM   #2
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Hi Mel,
Congratulations on a great build and nice layout. Especially the wiring underneath. I like the thick front panel with the boards mounted behind without screws showing through. And thanks for the nice big pictures. Is that a fan on the rear apron next to the input jack?
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers. That is a fairy story for people afraid of the dark." ~Stephen Hawking
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Old 18th April 2012, 05:15 PM   #3
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Congratulations! Looks really nice, and I bet it sounds superb!
How many watts, and do you have measurements? (distortion and so on)

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Old 18th April 2012, 07:42 PM   #4
MelB is online now MelB  Canada
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Thanks guys. Yes it is a fan on the back. It's running on the filament supply for the 7027 6SN7. That's the third fan I tried before I got one you could not hear. It doesn't need it. Never built a thousand volt amp before so I put it in. No measurements yet. Back to work tomorrow so it may be a few days.

You can get the cases from 8audio on ebay for $76. I actually wrecked one of the fronts and the guy sent me another one for $30 including shipping. If you use some fore thought you can slide boards in on the sides of the case.
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Old 19th April 2012, 02:00 PM   #5
Mush is offline Mush  United States
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Gosh that's one beautiful amp! It makes me want to build something grand. More descriptions and pictures would be great.
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Old 19th April 2012, 02:27 PM   #6
vuki is offline vuki  Croatia
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Do the measurements - I didn't have luck with grid choke - couldn't get rid of LF bump... although it did sound nice
Anyway - congratulations!
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Old 19th April 2012, 02:29 PM   #7
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That is a beautiful build and thanks very much for the tip on the chassis.
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Old 20th April 2012, 01:50 PM   #8
MelB is online now MelB  Canada
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Started some testing last night. Was looking at near perfect sine wave 40v peak to peak and my inrush current limiter had it's limit and went into melt down. Actually it cracked and opened up and turned into a 6.4K resistor.
I had a 6.8 ohm load attached so that is like 28 watts and looking good.
I also managed to sweep the low frequencies and it's dead flat and then at the upper end it's starting to drop at 18K. I just watched the sign wave start to fall a bit there but didn't write anything down. Humm that was with about a 14v peak to peak sine wave.
I have some 2.5 amp NTC's coming. The one that melted was a 1.5 amp. Which I thought would be fine. I must be pushing into A2???
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Old 20th April 2012, 05:24 PM   #9
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Nice job!!!
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Old 20th April 2012, 05:25 PM   #10
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Originally Posted by MelB View Post
......... I must be pushing into A2???
Then you need to add a DC-coupled cathode follower between the present driver and GM-70.
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