Great American Sound Head Amp - Page 2

johnfalc · 26th November 2020, 05:28 PM

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.

llwhtt · 26th November 2020, 06:17 PM

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

johnfalc · 26th November 2020, 08:08 PM

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

johnfalc · 29th November 2020, 12:36 AM

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.

pcb121055 · 29th November 2020, 01:43 AM

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

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26th November 2020, 05:28 PM	#11
johnfalc diyAudio Member Join Date: Mar 2009	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.

26th November 2020, 06:17 PM	#12
llwhtt diyAudio Member Join Date: Jun 2008 Location: SoCal	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

26th November 2020, 08:08 PM	#13
johnfalc diyAudio Member Join Date: Mar 2009	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

29th November 2020, 12:36 AM	#14
johnfalc diyAudio Member Join Date: Mar 2009	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.

29th November 2020, 01:43 AM	#15
pcb121055 diyAudio Member Join Date: Sep 2008	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