GA50 aspirations

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didit
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GA50 aspirations

Post by didit »

Prototype single channel amp build from mold of mid '50s Gibson GA50. Pre-amp has cathode biased 6SJ7 approximating the microphone channel, into stock 6SN7 paraphase driving cathode biased 6L6/6V6 pair. Internal assembly is point-to-point on sockets & terminal strips.

The amp works well as is, though a few design improvements & adjustments are coming. Post power-on confirmation of correct functioning, plan now is to experiment with preamp biases exploring range of compress & gain for the spot that suits. Build objective is specifically the warm compressed sonics of late '50s & early '60s jazz guitar as exemplified by Jim Hall.

Here is a schematic along with a few photos. Topology is consistent but several values, particularly resistors, were substituted based reading, research and a realization inevitable tweaking means starting somewhere.

Big Son 50.jpg
IMG_0091.jpg
IMG_0095.jpg

Iron is all contemporary Hammond catalog. 300V power transformer into silicon diodes approximates correct rectified DC in the original. Output transformer is replacement BFDR 6.6K with multiple secondaries. Chokes selected for matching originals impedance & DC resistance. In research I'd found marked-up schematic with various measurements that was essential since access examining an example of the real thing could never be arranged. Filtering is also close, but first electrolytic is 2x original 10mF.

When experiments & adjustments are finished, I will post a follow up with a more accurate schematic annotated with voltages & so on. Also hope to post some audio recordings. In the meantime for those curious the photos and these notes are hopefully worthwhile sharing as is.

Best .. Ian
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Last edited by didit on Fri Aug 19, 2022 2:39 am, edited 1 time in total.
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pompeiisneaks
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Re: GA50 aspirations

Post by pompeiisneaks »

Looks nice! I built a Gibson EH-185 at one point and need to go back to it, and redo it like this. I used a turret board, but I screwed up the chassis drilling and reversed it... Rather than try again, I just used shielded cables for all connections across things to try and keep noise to a minimum. It didn't work super well I think as i get a lot of noise in the input. I'm going to do terminal strips and ptp type like this to fix that and retry, maybe this weekend. I look forward to hearing samples.

~Phil
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didit
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Re: GA50 aspirations

Post by didit »

pompeiisneaks wrote: Thu Apr 20, 2017 11:18 pm Looks nice! I built a Gibson EH-185 at one point and need to go back to it, and redo it like this. I used a turret board, but I screwed up the chassis drilling and reversed it... Rather than try again, I just used shielded cables for all connections across things to try and keep noise to a minimum. It didn't work super well I think as i get a lot of noise in the input. I'm going to do terminal strips and ptp type like this to fix that and retry, maybe this weekend. I look forward to hearing samples.

~Phil
Thanks Phil. I find these old pentodes to be noisy regardless of careful wiring. This amp occasionally works as a faint radio and hisses all the time. Will experiment with a grid stopper on the preamp. A socket & strips rebuild will likely improve it but expect a bit residual noise. Part of the charm perhaps. Been nearly forever since I tinkered with old school plate capped small signal tubes. Surprising if they are any quieter than their successors.

Best .. Ian
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didit
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Re: GA50 aspirations

Post by didit »

Hello -

A PM inquiry has prompted a bit of a recap on this project. It remains a raw chassis as time for hands-on projects in the past few years has been scarce. Some tweaks and a swap of the output transformer to one that is closer in spec to what I understand was in the original GA50 from Gibson are the only significant changes from the posted picture. I had hoped to get some time this week to more thoroughly capture diverging details and post those against the documented schematic, and also share a few new photos. Alas, it's busy time for many other tasks so I am only able now to quickly enumerate the current working set of transformers and chokes.

* 272DX 600V/144mA (127VA)
* 1750PA “18W” output transformer
* 159P 10H/125mA main choke
* 154E 20H/20mA preamp choke

These were selected based on an annotated schematic that had shared somewhere on the Internet that included DCR readings from chokes and the power transformer. I had initially tried to use a Deluxe Reverb 6.6K output transformer but after some experimentation had somewhat better results using the Hammond "18W" type.

My goal always had been to work up a relatively re-usable design based entirely on readily available iron and believe the results prove worthy. I and a few others have played through it few several hours using various available speaker cabinets with best sounding results unsurprisingly with lighter cone alnico.

Getting decent NOS 6SJ7 was the largest hassle. I gathered a few and best results came from a red RCA 5693. The Rogers tube in the photo was also good. Some were total duds. I do plan for DC filament supply for that socket when time does permit bring it back on the bench to finish.

Best .. Ian

(corrected error on choke spec.)
Last edited by didit on Sat Aug 20, 2022 11:50 am, edited 1 time in total.
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Phil_S
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Re: GA50 aspirations

Post by Phil_S »

It would be great to hear a sample. I built something a while back that I suspect is similar, based on the GA-20. 6SJ7 > 6SL7 > pair 6V6. (Schematic posted for reference. I hope that's OK.) It was a bone pile build with used iron and various parts in the scrap box. I recall considerable tweaking of the B+ node to raise the voltage for the 6SJ7 to get some balls on the amp, as it initially was rather anemic sounding. (I'm thinking that Va started out around 80V.) I had no particular sonic goal in mind and was just having some fun. The amp I built was rather Jimi-like in tone -- think Purple Haze or Manic Depression. I can't imagine that for Jim Hall tone, but maybe swapping the 6SN7 in place of the 6SL7 would do that? I wonder how much Jim Hall's guitar vs. his amp accounts for his tone. (BTW, I didn't know Jim Hall, but youtube came to the rescue. He was really someone to admire. I was immediately hooked. Thank you for the introduction.)

Congratulations on the build. It looks very nice and I'm sure it sounds great!
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thetragichero
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Re: GA50 aspirations

Post by thetragichero »

on mobile so i can't pull em up but check out resistance couple amplifier charts for suggested conditions for the 6sj7

edit: page 11 http://www.tubezone.net/pdf/rcachart.pdf
PRR wrote: Plotting loadlines is only for the truly desperate, or terminally bored.
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didit
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Re: GA50 aspirations

Post by didit »

Hello --

Attached is my "flotsam" annotated schematic from which I'd collected key targets for transformers, chokes, and more. Long lost a bookmark that'd be proper for referencing, but it was claimed to be correct with values for resistances and voltages taken from a working amp, and one claimed to make the right sounds.
annotated schematic_GA-50 copy.jpg
Best .. Ian
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