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Great American Sound Head Amp
Great American Sound Head Amp
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Old 5th June 2015, 12:21 AM   #1
gasgeek is offline gasgeek  United States
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Default Great American Sound Head Amp

I have put together all of the original documentation I have on the GAS head amp and have posted it for everyone's edification. After reviewing it I can see a glaring error. The bias setup for version 2, board #334, shows the wrong PCB layout. Suffices to say that there should be 1 volt across the 51 ohm resistor R20 and also across resistor R25 in the output section.

There ya go. Hope this helps someone.
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File Type: pdf Head_Amp.pdf (793.5 KB, 254 views)
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Old 5th June 2015, 05:13 PM   #2
llwhtt is offline llwhtt  United States
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Thanx again GASGEEK, this is the first Goliath info I've ever seen.

Craig
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Old 6th October 2020, 06:04 PM   #3
Rjones5296 is offline Rjones5296  United States
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Thanks GASGEEK!
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Old 6th October 2020, 06:45 PM   #4
Rjones5296 is offline Rjones5296  United States
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Guys
I have recently gotten a
GAS Thaedra MK i preamp, function somewhat, some one had been in there,,
SO it took me a while to get it all straightened out, seems to be working well now.
I do have a few questions.
Im wondering if the MKii boards are compatible specifically the power supply and line stage,
My power supply board was pretty cooked but I was able to repair that. I do however have a set of mkii boards. also some spare mk i boards.
Any comments on this swap over?
Man this guy generates a good bit of heat, so I sit a muffin fan over the opening on the top.
But I do have to say this is one hell of a preamp, Im gonna say one of the best I have heard.
Thanks
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Old 6th October 2020, 07:06 PM   #5
llwhtt is offline llwhtt  United States
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All Thaedra boards should/do work fine in either chassis, I or II. I've run I boards in my II and vice versa. Same goes for Thoebe. Be sure to check and adjust your voltages.

Craig
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Old 25th November 2020, 10:53 PM   #6
johnfalc is offline johnfalc  United States
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Just coming upon this thread with a GAS Thoebe in hand.

The Thoebe I have has a head amp board marked 334D, for which I have not found an exact schematic. I have found a schematic called out as a Thaedra head amp and it has numerous similarities to my board ... in fact the general topology is the same.

Interestingly, my 334D board sources current to the emitters of the paralleled input bipolar transistors through three-position switches which have the effect of altering the gain of the head amp, something I haven't seen on other schematics/images but which I'm guessing was implemented similarly to the Goliath (for gain adjustment).

Additionally, it appears that the emitters of the output transistors on this board have neither the 150 Ohm resistors nor 2,200 uf caps in their circuits; rather the emitters go directly to the +/- 22 Volt lines.

Here are some photos I took of the Thoebe and the Grandson I was given.

GAS Thoebe preamp and Grandson power amp - Google Photos

The relay board has obviously been repaired previously, as has been the main board power supply. I'll figure those out but any additional info is certainly welcome.

Boards in this Thoebe are: 528B, 522B, and 334D.

Any info/suggestions/schematics are warmly welcomed. Both devices were working when received but poorly (noise/hum) and I have electrolytic caps on order for both. I'm retired and keep a bench just for fun - having left the audio field about 40 years ago (ran Central Service and private label brands for long-defunct Pacific Stereo - we sold SAE among many others).
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Old 25th November 2020, 11:29 PM   #7
llwhtt is offline llwhtt  United States
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The Head Amp with selectable gain is from Thaedra II, there are small holes in the side of the top cover to access the switches. I have the schematic but will have to get copied into a smaller size so I can scan it.

There were three different relays, that I know of, used in the Thaedra/Thoebe over the production life, all three are impossible to find nowadays. I'm curious as to what relay they used in the repair.

Craig

Last edited by llwhtt; 25th November 2020 at 11:36 PM.
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Old 26th November 2020, 12:52 AM   #8
johnfalc is offline johnfalc  United States
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Thanks Craig,. I have added a photo showing the relay to the album of photos previously shared. Any schematic or other info you find will be appreciated. John
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Old 26th November 2020, 05:26 AM   #9
johnfalc is offline johnfalc  United States
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Hi again Craig,

Success here in finding a schematic for the head amp I have.

One question, since I never dealt with GAS when it was new; in the Thoebe, was it an election made at purchase time that determined if a head amp for MC as opposed to MM was installed? I ask since, although I haven't really studied in in depth, it appears that the 334D head amp I have in this Thoebe is of low enough input impedance to suit a MC cartridge.

Thanks,

John
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Old 26th November 2020, 04:13 PM   #10
llwhtt is offline llwhtt  United States
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Not heard of an internal Head Amp option for the Thoebe, Goliath was the optional external Head Amp for the Thoebe. Thoebe provided power for the Goliath via the four-pin connector on the back. Usually you'd use a step-up device, either xfmr or pre-pre, and then on to the conventional RIAA stages. GAS made the Head Amp and RIAA all in one for MC cartridges. Then Thaedra II came out with the selectable gain Head Amp. I have not been able to find a Goliath yet so I use a Thaedra I Head Amp in my Thoebe with AT OC9 MC.

Craig
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