Thursday, May 6, 2010

Van Halen's GM70...

So earlier we discussed how Jon finally picked an output tube (link here). I need to call him out on his basement. It is full of audio and woodworking stuff. He didn't want me to see it at first, as it is a little bit incriminating. But now that he has seen my collection/pile of stuff, he is more open to letting me wander through. Apparently every schematic that we have drawn up together over more than a decade has companion parts in his basement. We started off with 2A3's.. then 45's... PX25's.. settled on GM70's now that he is doing open baffles and could use the juice. And there is also iron... rows of Mikey iron... and a growing stack of Slagle iron... I won't even begin with the raw drivers down there..

So above is the rough scheme we settled on. He provided the output iron (big nickel) and the tubes. I made him buy a pair of nickel 1:1's. But we hit a snag. The driver was to be the Emission Labs 20. It can swing big volts and he has had a pair on his shelf for years (see above about his basement). I built for that tube, but when I hit the B+ the measurements weren't working out. The EML20's were weak and super weak.. ouch! So I noticed that I could drop an 801A in without changing a single part (probably should have caught that from the outset - blush)... much better.. well within a dB from 20 to 20k...

So here she is up and running. Too early to really tell, but man, not a bad combo... BIG sound.... use triodes in their linear range and you just about can't go wrong. Jon is building a full coverage chassis for these. They need it. Too much iron and oil cap showing. I like that stuff, but I know I am in the short line. It also has too much glow. We need to tame it down with a screen....

I am not sure how much tweaking I will do before Lone Star Audio Fest, but so far, just spinning vinyl is making it sound better by the hour... (Coleman Hawkins just morphed into REM "Life's Rich Pageant" - must be about noon... )



  1. Sweet!
    'cept i'm trying to figure out "Too much iron and oil cap showing..."
    i thought the 304tl was just about right, the way it was displayed @ vsac...

  2. agreed!

    Nice pics and I'm looking forward to hearing it in dallas.

  3. Great thoughts you got there, believe I may possibly try just some of it throughout my daily life.
    van for sales


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