GAS Thoebe preamp problem

EspenE · 27th September 2010, 08:15 AM

I have a Great American Sound Thoebe preamp. The unit works and sounds very good, but however: it runs extremely hot. Unless I have the cover off, it will cut out after a period of use.

It seems that the power supply regulators are the prime cause of the heat. I suspect that the transformer is not original (there seems to have been done a rather clumsily conversion from 110 to 240V somewhere along the line).

Do anyone know the transformer rating (output voltage) for the original Thoebe? And the correct regulated supply voltages?

Thanks to anyone that can help.

Regards, Espen

RBorkowski · 28th September 2010, 01:13 AM

Espen,

I have a Thoebe that I purchased back in 1971. I have never experienced a single problem with it. I opened it up and measured about +28.02 and -28.05 volts regulated and filtered referenced to chassis ground. There appears to be several voltage dropping resistors in various circuits that lower the effective working voltage to around +23.10 and -23.08 volts throughout most circuits although I also measure +13.41 and -13.42 volts on an operational amplifier that is utilized near to or is within the phono section. I do not have a schematic for it so I cannot tell you what the transformer rating is. The voltage dropping resistors do get somewhat warm to the touch. Two (measure about 22 ohm) have a drop of about 5.2 volts or so across them and two others (measure about 470 ohm) have about a 9.4 volts across them.

Hope this helps.

Good Luck and God Bless.

Rich Borkowski

llwhtt · 28th September 2010, 03:18 AM

Get on the yahoo group "SAE Talk". James Bongiorno, the owner/designer of GAS back in the day hangs out there. If anyone can answer your questions he can.

Craig

audiohead · 28th September 2010, 03:22 AM

You will be really impressed by the awesome display of brain power there.

27th September 2010, 08:15 AM	#1
EspenE diyAudio Member Join Date: Jun 2003 Location: Oslo	GAS Thoebe preamp problem I have a Great American Sound Thoebe preamp. The unit works and sounds very good, but however: it runs extremely hot. Unless I have the cover off, it will cut out after a period of use. It seems that the power supply regulators are the prime cause of the heat. I suspect that the transformer is not original (there seems to have been done a rather clumsily conversion from 110 to 240V somewhere along the line). Do anyone know the transformer rating (output voltage) for the original Thoebe? And the correct regulated supply voltages? Thanks to anyone that can help. Regards, Espen

28th September 2010, 01:13 AM	#2
RBorkowski diyAudio Member Join Date: Dec 2009 Location: Bronson, Michigan 49028	GAS Thoebe Preamplifier Problem Espen, I have a Thoebe that I purchased back in 1971. I have never experienced a single problem with it. I opened it up and measured about +28.02 and -28.05 volts regulated and filtered referenced to chassis ground. There appears to be several voltage dropping resistors in various circuits that lower the effective working voltage to around +23.10 and -23.08 volts throughout most circuits although I also measure +13.41 and -13.42 volts on an operational amplifier that is utilized near to or is within the phono section. I do not have a schematic for it so I cannot tell you what the transformer rating is. The voltage dropping resistors do get somewhat warm to the touch. Two (measure about 22 ohm) have a drop of about 5.2 volts or so across them and two others (measure about 470 ohm) have about a 9.4 volts across them. Hope this helps. Good Luck and God Bless. Rich Borkowski

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28th September 2010, 03:18 AM	#3
llwhtt diyAudio Member Join Date: Jun 2008	Get on the yahoo group "SAE Talk". James Bongiorno, the owner/designer of GAS back in the day hangs out there. If anyone can answer your questions he can. Craig

28th September 2010, 03:22 AM	#4
audiohead diyAudio Member Join Date: Mar 2005 Location: Los Angeles, california	You will be really impressed by the awesome display of brain power there.

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