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GM-70 Amplifier
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Author:  David McGown [ May 1st, 2016, 1:31 pm ]
Post subject:  Re: GM-70 Amplifier
Dave,

I gave this more thought over the past few days, and came to the same conclusion that it would be easier to just add a negative supply. I do not need much, perhaps around -150V or so, which I can easily do by using a 120VAC:6.3VAC filament transformer in reverse fed by my (AC) heater supply for the driver and driver rectifier. I have alot of current reserve, and filament voltage is running high, so a bit more load would do it good.

I just added 2-470uF/250V capacitors to each GM70 cathode bypass, in addition to the film caps I have there already. My initial impression is that it tightened up the bottom, with better bass definition, and perhaps a greater spaciousness. It did not seem to negatively impact the midrange or highs.

Finally, my interstage is gapped, it is designed for 30mA DC, and I am putting around 15mA through it. One thing I am considering is transformer coupling the 7722 to the MOSFET in lieu of a coupling cap. I am sure that driving the gate of a MOSFET will be a much easier task for the interstage transformer.

Thanks.

David
Author:  David McGown [ May 3rd, 2016, 4:12 pm ]
Post subject:  Re: GM-70 Amplifier - Fireworks
Well, I came home early today from work not feeling too well :crazy: and decided to sit down and listen to some music to recuperate. Well, my right channel amp, exhibiting some sympathy to my condition, decided blow a fuse a few seconds after turn on. Of course the first thing one does is to try another fuse. :sad-roulette: That one similarily blows. :doh: Having one fuse left, and not wanting to take a trip to my local Radio Shack(...wait, there ARE no Radio Shacks anymore! :angry-cussingblack: ), applied some systematic debugging. Nice thing about having tube rectified B+ for everything but the heater supplies, means you can isolate a problem more easily. So took out tube rectifiers and started the amp up. The GM-70 lit up nicely, OK then. Next put in the driver stage rectifier. That fired up A-OK as well. Well time to put back in those 6AX4 diodes. Fired up the amp and saw some nice arcing out one or both 6AX4s, at the bottom of the cathode sleeve, which promptly blew my remaining fuse. :idea: Then I remembered the 6AX4 has a 900V heater to cathode maximum rating, and perhaps I was lucky I got this far all this while. Anyway, I can either bias the heaters to some nominal value above ground, perhaps 400V or so, or just go the safe route and put in a solid state rectifier and a 6AX4 on centertap ground to give me a turnon delay and a way of disabiling B+. The easiest route is the first option. Need to get more fuses.

I just hope the output transformer is okay.

Also, glad I haven't sold my 300B amps, they really sound so much better than a dead GM-70 amp. ;)
Author:  Stuart Polansky [ May 3rd, 2016, 4:22 pm ]
Post subject:  Re: GM-70 Amplifier
David,

Sorry to hear about this. Really sucks.

I do love the internet and all the access to information and purchasing that it brings. But it's not without a price. That being the loss of convenience stores such as RS.

We are fortunate to still have stores like Mark Electronics left, but as usual, limited store hours make getting there a PIA.

Looks like we need to become stocking supply houses for our own needs on a short-term basis.

Good luck with GM70 1.1a

Stuart
Author:  Roscoe Primrose [ May 3rd, 2016, 5:07 pm ]
Post subject:  Re: GM-70 Amplifier
If the 6AX4s are on their own winding, I'd just raise it up to 400V or so and call it done. I didn't have to do that in my stereo 845 amp which uses 6AX4s since my B+ is only about 660v....

Roscoe
Author:  SoundMods [ May 3rd, 2016, 5:37 pm ]
Post subject:  Re: GM-70 Amplifier
Then there is Baynesville Electronics for those that either live in the Baltimore area, or nearby, or that don't mind a reasonable commute to get immediate gratification.

Here is the link to their WEBsite:

http://baynesvilleelectronics.com/home/3583680
Author:  David McGown [ May 7th, 2016, 7:30 pm ]
Post subject:  Re: GM-70 Amplifier
Roscoe,

All fixed for now. I had around 200V or so available from my driver B+ supply, that will at least get me below the 900VDC heater/cathode limit, though more voltage would be nice. I was also thinking about a voltage divider off the GM70 B+ supply, I am using a pair of series connected 660VAC oil caps as my input caps, so I could always take 550V off the lower of the two caps, and divide further. But at least the amp is back up and running, though one of the 6AX4s is discolored at the lower mica supporting the cathode so I used a fresh pair.

David
Author:  ratbagp [ June 2nd, 2016, 9:24 am ]
Post subject:  Re: GM-70 Amplifier
Since you have had a few weeks to listen with the new topology, did the sound change as you hoped?

ray
Author:  David McGown [ June 2nd, 2016, 9:31 am ]
Post subject:  Re: GM-70 Amplifier
I continued to have problems with arcing on the 6AX4 until I put in a TDR on the primary of the HV transformer, as well as putting in a CL-80 thermistor, also on the primary. Now the amp comes up cleanly with no issues. At this point, I have only added capacitance to the cathode bypass capacitor per Dave Berning's suggestion, which did improve the bass and openness with no apparent impact on the top end. I have not changed my driver and coupling yet, and probably will not have the time to do so before the summer, due in great part because of my musical commitments.

David
Author:  David McGown [ December 29th, 2020, 10:40 pm ]
Post subject:  Re: GM-70 Amplifier
A lot of water under the bridge since I posted anything on my GM70 SET monoblocks. Over the past weekend, I finally (after 4 years) converted the biasing from cathode bias to fixed bias. Picked up another 100V across the tube (now 1100V), and got rid of pesky capacitors in the signal path. A very nice stepup in clarity and control. Nice with the colder weather to get the boat anchors warming up the listening room.

I have also concluded that, at least in my amps, the storied copper plate GM70s just do not sound as good as the much cheaper and more robust graphite plates. The copper plates seem to have a resolution floor, when the music gets quiet, they seem to stop playing. No problem with low level resolution on the graphite plates, and they have more meat to the sound besides. "Everyone" says the copper plates are the bee's knees, but I don't get it, after trying several times to like them, I now realize it is not worth my time giving them another chance. Anyone playing with GM70s are welcome to try them.

David
Author:  SoundMods [ December 29th, 2020, 11:42 pm ]
Post subject:  Re: GM-70 Amplifier
David McGown wrote:
A lot of water under the bridge since I posted anything on my GM70 SET monoblocks. Over the past weekend, I finally (after 4 years) converted the biasing from cathode bias to fixed bias. Picked up another 100V across the tube (now 1100V), and got rid of pesky capacitors in the signal path. A very nice stepup in clarity and control. Nice with the colder weather to get the boat anchors warming up the listening room.

I have also concluded that, at least in my amps, the storied copper plate GM70s just do not sound as good as the much cheaper and more robust graphite plates. The copper plates seem to have a resolution floor, when the music gets quiet, they seem to stop playing. No problem with low level resolution on the graphite plates, and they have more meat to the sound besides. "Everyone" says the copper plates are the bee's knees, but I don't get it, after trying several times to like them, I now realize it is not worth my time giving them another chance. Anyone playing with GM70s are welcome to try them.

David

I have had the exact opposite experience with 845s. The metal-plate version has better clarity and is more musical than the graphite-plate versions.
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