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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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
 
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).
 
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
 
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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
 
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
 
Interesting information, thanks.

I have found a schematic for what is titled on the engineering drawing "PHONO CIRCUIT (THOEBE, THAEDRA)" which, given its 47K ohm loading resistor at the input, is obviously for MM cartridges. This uses dual differential BJTs and is clearly quite different from what is in my Thoebe.

The Thoebe I have here does not have the DD BJT MM photo circuit; rather it has (as you point out) the MC 324D adjustable gain board from a Thaedra II with its 1.2 ohm series R1 and 1K ohm R2 loading resistor - suitable for a MC cartridge (and, of course, it includes RIAA compensation).

So ... it appears that converting a Thoebe between MM and MC is as simple as swapping boards.

Perhaps this is moot for me in that I may not use the Thoebe for vinyl at all, except to test it with an Ortofon MC20. I'm considering it and the associated Grandson likely for use in a guest bedroom or as a shop setup.

A friend gave these (and a pair of AR-2ax speakers) to me. I originally declined the gift as I have too much stuff already but, when he said he'd take it all to landfill I recanted and accepted them. Anyway, it is a fun project and takes little in expense to get it all running as new (I've already replaced the woofer surrounds and will have the tweeters repaired).

Fun for some nostalgic reflection though, since I met Jim B. several times when he was with SAE.
 
You'll have to find a MM board to use in Thoebe if you want to use a MM cartridge. I have several versions of line stages and PS/regulator boards but no spare MM or MC boards. They come up on that auction site every now and then.

I got to meet JB shortly before he passed away, he'd mellowed out quite a bit since you saw him at SAE. I have soooooo many questions for him but it's too late.:(

Craig
 
I was lucky, I think, to be so involved in audio during what I consider the best of times, basically from 1969 through 1982 or so. Travels around the country and to the Audio Fairs in Tokyo were among the highlights. I'm fortunate to still be in touch with dozens of folks who also worked for Pacific Stereo and CBS.

Good gear and plentiful material plus relatively unaltered hearing mean I can continue to enjoy the music ... that's what really counts.

Cheers and Happy Thanksgiving, John
 
Parts came in for the Grandson so that part of the duet is complete. Once all back together (except for the case) I discovered one of the meter level controls was defective. Thankfully I had some good multi-turn 2K pots to use so I installed those and calibrated the meters.

Mouser order for the Thoebe caps is due on Monday.

Grandson measures very well, distortion and noise is mostly at the limits of my HP 339. I look forward to auditioning the two pieces together once I've finished the Thoebe.

Thanks again for your input.
 
This is such a treat to see GAS chatter! I loved that stuff. I was a salesman for a GAS dealer in the late seventies and bought a Ampzilla II, a Goliath, and a Sleeping Beauty. Lost the Ampzilla having sent it in to the factory to have a channel repaired and was told they went out of business and couldn't get it back. So sad. Anyway, thanks much for the Goliath info :)
 
Interesting information, thanks.

I have found a schematic for what is titled on the engineering drawing "PHONO CIRCUIT (THOEBE, THAEDRA)" which, given its 47K ohm loading resistor at the input, is obviously for MM cartridges. This uses dual differential BJTs and is clearly quite different from what is in my Thoebe.

The Thoebe I have here does not have the DD BJT MM photo circuit; rather it has (as you point out) the MC 324D adjustable gain board from a Thaedra II with its 1.2 ohm series R1 and 1K ohm R2 loading resistor - suitable for a MC cartridge (and, of course, it includes RIAA compensation).

So ... it appears that converting a Thoebe between MM and MC is as simple as swapping boards.

Perhaps this is moot for me in that I may not use the Thoebe for vinyl at all, except to test it with an Ortofon MC20. I'm considering it and the associated Grandson likely for use in a guest bedroom or as a shop setup.

A friend gave these (and a pair of AR-2ax speakers) to me. I originally declined the gift as I have too much stuff already but, when he said he'd take it all to landfill I recanted and accepted them. Anyway, it is a fun project and takes little in expense to get it all running as new (I've already replaced the woofer surrounds and will have the tweeters repaired).

Fun for some nostalgic reflection though, since I met Jim B. several times when he was with SAE.
hello sir john, i am new here, total non techie, but i have a friend who is doing the work on a thaedra II and he needs the schematic to the phono section. can you provide, I will pay, tell me what to do, also my personal email is jrobin0909@aol.com, thank you for any assistance.