Analogue revived !

A low voltage GM70 / гм70 amp

Having breadboarded a few GM70 amp ideas, I was still looking for an inexpensive tube to drive the GM70. The problem is that a low impedance supply is necessary to drive the grid of the GM70. Into the hole of cathode followers I fell, until Jim Dowdy mentioned that he likes the the 6EM7. Hmmm, wow this is the ticket! An inexpensive dissimilar dual triode with half the tube looking amazingly similar to a 2A3! The 6EM7 is identical to the 10EM7 except the 10V heater requirement. www.surplussales.com has a great deal on the 10EM7’s so I bought a bunch. The schematic shows voltage tap off the 30V filament supply for 10V heaters. 6EM7 or 13EM7 tubes could easily be used since the filament resistor provides enough regulation for any of these tubes. And yes, this circuit is screaming for an interstage transformer off the 2nd plate of the 10EM7, allowing it to drive the GM70 a little into A2. I am looking for good interstage transformers. DIY HiFi Supply has some Silk 3K:3K that would work well, but they are too expensive. I will update the schematic if I find something that works, and is priced in line with this ‘economy’ project 😉
UPDATE: OK! I don’t know why I didn’t think of Hammond interstage, but a Hammond 126B should work very well. It is only rated to 30ma, so I will probably bias the tube for less current.

… For your enjoyment, here is a few links to other GM70 builds…
Audio Design Guide
Audio Talk GM70 SET
Nicholas’ baby/monster
Kevin Carter of K&K Audio
Fikus the Lampizator
Dr. Z Lab
Sampleaccurate’s GM70 Build
Polish GM70 Build
Thai GM70 Build
Yahoo Boat Anchor Group
A Russian response to Ongaku
A Japanese 6AS7 2A3 GM70 version
scary system I would love to hear.

And the schematic for my 10:IT:45:IT:GM70 Amp:

 

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4 responses

  1. Winston James

    Hi Erick,

    An acquaintance of mine has recently purchased an Audio-Research SP-8 pre-amp. However, while he welcomes its outstanding performance in most of the frequency-range, he laments the fact that it is not as good as his passive pre at high-frequency clarity and transparency (while the passive, of course lags in the active’s areas of strength).

    I’ve suggested to him that your Optocoupler Volume-control kit may be the solution. My question to you is this: Would your V-C kit (with balance-control) elevate the SP-8’s h-f performance to the level of the passive pre? If so, what would be the cost of this mod?

    Separately, perhaps, you could also quote the cost of mods to the coupling caps and resistors in the signal-path? The latter are mods which may also be considered – or not. The LDR V-C is positively on the agenda, however.

    Thanks for your time.

    Sincerely,

    Winston.

    June 21, 2012 at 2:43 pm

    • Thanks for the note Winston,

      High frequency performance of a volume control is a capacitive issue. Make sure his interconnects are short and read the capacitance per foot specs of his interconnects. If the manufacturer does not provide that information, do not use those interconnects. The LDR volume control is the best passive volume control I have heard, but if he has long interconnects, insists upon using high capacitance interconnects, or the input impedance of his amp is low, an active preamp may be the only solution.

      June 21, 2012 at 2:58 pm

  2. Winston James

    Hi again,

    At the Transcendent forum I’d seen where you’d installed LDRs as volume/balance-controls into some of the members’ G-G active pre-amps. This is the service I was actually seeking to inquire about.

    I wondered whether you still offer such a service, and what the cost would be, for such a mod. My acquaintance (the owner of the active ARC pre-amp in question) may also be interested in mods to the coupling-caps and resistors in the signal-path. If you still offer such services, could you quote both costs separately so that my acquaintance may be able to decide on one or both options?
    Perhaps you’d prefer to contact me by e-mail. If so, then my e-mail address was provided at the time of my first post.

    Thanks for responding.

    Winston.

    June 21, 2012 at 10:57 pm

  3. Lc

    Do you sell your amps?

    March 5, 2015 at 7:06 am

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