GAS Thoebe Preamp Needing Repair

I have a GAS Thoebe preamp that works but needs repair to eliminate random noise and cutting out. A local repair shop was not able to fix it so I'm looking elsewhere. Does anyone know of someone in the USA (I'm in New Mexico) who would be interested and able to repair it?

I also have a Thaedra with similar and other problems that the same repair shop is now looking at but not having much luck.

They both are normally hooked up to Son of Ampzilla amps that work perfectly. I am currently using Dynaco PAT-4 preamps in the meantime and have no problems. I'd just like to have the GAS preamps back up to snuff.

Thanks,
Dan
 
Thanks!

The local repair shop told me they had or would be able to find schematics. Since they never could figure out what the problem is it makes me wonder. If I don't find somewhere else to send it for repair I will take this schematic back to them to see if it will help though I'm not sure I really want to go back to them.

I appreciate the help.

Dan
 
civildan said:
I have a GAS Thoebe preamp that works but needs repair to eliminate random noise and cutting out. A local repair shop was not able to fix it so I'm looking elsewhere. Does anyone know of someone in the USA (I'm in New Mexico) who would be interested and able to repair it?

I also have a Thaedra with similar and other problems that the same repair shop is now looking at but not having much luck.

They both are normally hooked up to Son of Ampzilla amps that work perfectly. I am currently using Dynaco PAT-4 preamps in the meantime and have no problems. I'd just like to have the GAS preamps back up to snuff.

Thanks,
Dan

Hi Dan,

I can take a look at them, and I'm sure fix them, if you run out of options. I had a business that specialized in the GAS products for many years, until life dictated otherwise. I would not mind helping to get it working again. I'm in Virginia. Send me an e-mail and we can discuss it.

Mike.
 
civildan said:
Mike,

Since I am still under moderation I am not able to send you an email. However, I am interested. Please let me know how I can get in touch with you.

Thanks,
Dan

Hi Dan,

You can e-mail me at MikeBettinger @ comcast. net.

Thanks, for the kind words Bill. Seems like quite a long time. I've raised my kids and had an almost 20 year alternate career in the mean time.

Mike.
 
Thanks!

MikeBettinger said:


Hi Dan,

You can e-mail me at MikeBettinger @ comcast. net.

Thanks, for the kind words Bill. Seems like quite a long time. I've raised my kids and had an almost 20 year alternate career in the mean time.

Mike.

The Thoebe is back and sounding better than ever.

A huge thanks to Mike for the terrific job he did getting the Thoebe back into working order. I could not be more pleased with the results. Mike, your efforts are greatly appreciated.

Dan
 
Re: thoebe

snowdog51 said:
mike
i was audio tech in the 70's but not enough money to get by when i had kids about 80 's cars were getting harder to fix there was some good pay to fix the fuel injection so i started doing that
now i'm old and getting back in to audio this is a great site

bill

Hi Bill,
I left audio in the late 80's for the semiconductor industry. I find it's a lot more fun to build stuff just to test an idea and see what it sounds like rather than whether or not it will pay the bills.

This is a great site with a lot of good ideas to fuel the fire. Compared to the information readily availible in the 70's this is quite an improvement!

Have fun, Mike.
 
Re: thoebe

DIYspaceW said:
Hi!

I just want to replace the stock mc1458cp1 whit lme49720na op amps!!!
Please help me!! Is this a good one or should I look after something else?

Thanks!!!

Hi,

They will work great for the servo amps on the lineamp and phono cards but I would not use them for the two mc1458's in the power supply on the mother board. The supply to the chips is 33 vdc (equal to +/- 16.5v), which is within the spec but from experience it's rolling the dice. The'll work a few times and then die on turn on. The first time it happened it threw me a curve since I didn't really suspect the chip. The second time I decided it wasn't a good match.

The servos are working well.

Mike.
 
Hello All.

I just replaced all of the capacitors on my thoebe and luckily is still works. My thaedra wasn't so lucky, which I believe to be a open 2n6316 transistor. I was able to adjust the line card, thanks to Mike B. I would like to properly adjust the phono stage card on both. Any guidance would be greatly appreciated.

Craig
 
just bought a VERY RARE UNHEARD OF UK VERSION Phoebe on a whim.

quite an astonishingly good preamp
shame about the cut outs and buzzes that come and go. ??faultering caps
God only knows how it will sound if i remove all tone and balance controls out of the signal path, rewire recap etc etc???

Is it true that when the tone controls are at 12 o clock they are out of the signal path??
 
just bought a VERY RARE UNHEARD OF UK VERSION Phoebe on a whim.

quite an astonishingly good preamp
shame about the cut outs and buzzes that come and go. ??faultering caps
God only knows how it will sound if i remove all tone and balance controls out of the signal path, rewire recap etc etc???

Is it true that when the tone controls are at 12 o clock they are out of the signal path??

Sonically both the Theobe and Thaedra are very good designs. The design of the line amp stage sports a cascoded dual JFET differential input stage coupled to a class-A power amp stage which would be right at home with today's high-end designs.

Both of them share the same Line-amp and MM phono circuits so the family resemblance is there. The Thaedra has a separate power supply for the phono stages where the Theobe has a common supply for both the line amp and the phono. The Thaedra has a better transformer design but the supply design is identical. The controls in the Thaedra are better quality with the controls elements being fired on to ceramic substrates.

The tone controls are considered by design to be out of the circuit when in the twelve o-clock position but in reality they are located in the feedback loop and running the feedback loop to the front panel controls and back is worth pondering if this is trully the best approach, although it works well and the turnover points for the controls are placed well from a music perspective.

I get a kick out of your comment about bypassing the controls as I had many people ask me to do this in the 80's; it actually makes a noticable mprovement. To do it is quite easy. Just cut the tone traces coming from the tone controls on the line-amp card and install a feedback loop onto the board consisting of a 15K resistor from the output to the differentials and a 1K to ground. This mimics the tone control network as far as gain. Simple to reverse as well. I can post a picture of a board with the tone control mod installed if you respond with the line amp board number.

I read a article of GAS guru and he says the best pre-amp ever made in his opinion is Theodra, so I think the Theobe should be also very good!)

I don't know about best preamp ever but the Thaedra is surprisingly very good. As the source recordings, hardware and speakers have improved it's qualities are even more apparent.

Mike
 
HI MIke

thanks for your input. I was thinking of totally by-passing the tone controls but are you suggesting that there is a feedback loop contributing to the db gain which needs to be addressed??

I am also assuming that the vol pot might not be up to todays standards and was thinking of removing the line board and phono boards then putting them into a seperate chasis. I will add a new power supply regulation board silver plated phono's and hard wiring. Although I fear I may mess up that wonderfully warm and vibrant sound I seem to get from the original unit as it is????? I may have no choice as i seem to have wrecked the rather sophisticated (for me that is)power supply.

Luke
 
Hi Luke,
The tone controls are included in the feedback loop of the lineamp circuit, there is not a separate stage to bypass. To remove them from the circuit you need to remove the return from the controls and replace the feedback from the controls with a voltage divider. I'll post a picture later in the week when I have a few minutes.

Mike

HI MIke

thanks for your input. I was thinking of totally by-passing the tone controls but are you suggesting that there is a feedback loop contributing to the db gain which needs to be addressed??

I am also assuming that the vol pot might not be up to todays standards and was thinking of removing the line board and phono boards then putting them into a seperate chasis. I will add a new power supply regulation board silver plated phono's and hard wiring. Although I fear I may mess up that wonderfully warm and vibrant sound I seem to get from the original unit as it is????? I may have no choice as i seem to have wrecked the rather sophisticated (for me that is)power supply.

Luke