Gibson Minuteman

ricach · 01-29-2009, 03:18 PM

Its a 60's GA20-RVT. Just recently its developed a loud hum on power up, but after it warms up a good 3-5 minutes the hum is only half as loud. None of the controls effects the volume of the hum, and its there even without an instrument cable plugged in. I've rechecked and resoldered the grounds. Pulled each tube seperately. Moved wires around. Even checked it in several different outlets around the house. No luck.

I remember a thread a(long)while back that had a nice checklist and troubleshooting methodology for hum problems but I can't find it anymore.

Mars Amp Repair · 01-29-2009, 03:47 PM

Most likely in an amp that old you're dealing with a filter cap in the power supply. It might look like a stick of dynamite under the chassis with 3 or 4 wires coming out of it. It could also be a metal can mounted on the chassis....OR the 3 filter caps could be stand alone single units.

You can find a similar value cap (20uf @ 450V would do) & bridge it across the caps inside by connecting the minus side to the black wire & the plus side of the test cap to the red wires one at a time...

If that resolves your hummmmm, you need to replace all 3 caps as the others are either already bad or close behind.

You can find these multi section or single caps in suppliers like Antique Electronics Supply for reasonable prices. http://www.tubesandmore.com/ Look under 'multi section capacitors'. Looks like it's a 20/20/20@450V? These caps also could be separates, too. You'll know when you get into the chassis.

Here's a link to the gibson site for the schemo; http://www.gibson.com/Files/schematics/GA-20RVT.jpg

Be very careful as there are lethal voltages in there. glen

ricach · 01-29-2009, 05:06 PM

Thanks Glen. I've been inside enough if these to where I don't bother looking at schematics initially anymore. After looking at the schematic I noticed the 20u filter cap coming off the rectifying circuitry that ties directly to the OT. No doubt a probable source of the problem. Putting a scope on it I do see quite a ripple going directly into the OT. I'll give your idea a try, unfortunately no spare caps on hand at the moment.

ricach · 02-01-2009, 02:16 PM

It appears that I have the 3 - 12AU7, 2 - 6EU7 preamp tube configuration and its considerably different than the schematics I'm finding for the GA20RVT with the 3-6EU7, 2-12AU7 configuration. Does anybody know where I can find the schematics for the former?

dkevin · 03-20-2009, 03:47 PM

Hello,
I recently worked on two 60's vintage Crestline Gibson amps and I ran into a similar problem. One of the amps (a Gibson GA-25RVT with 7591 output tubes) had a nasty hum. At first I thought it might be the electrolytic caps but it persisted after the replacement of said caps.
I gave up and went to work and chewed on it for awhile and decided to check the cathode bypass caps for the output tubes. When I quick-subbed in a new bypass cap, the hum disappeared. Subsequently, I replaced all the bypass caps as they were all original.
It's an easy test-good luck!
dkevin

TD_Madden · 03-20-2009, 07:50 PM

Quote:

Originally Posted by dkevin

Hello,
I recently worked on two 60's vintage Crestline Gibson amps and I ran into a similar problem. One of the amps (a Gibson GA-25RVT with 7591 output tubes) had a nasty hum. At first I thought it might be the electrolytic caps but it persisted after the replacement of said caps.
I gave up and went to work and chewed on it for awhile and decided to check the cathode bypass caps for the output tubes. When I quick-subbed in a new bypass cap, the hum disappeared. Subsequently, I replaced all the bypass caps as they were all original.
It's an easy test-good luck!
dkevin

that was the 20/25 cap on the outputs?

dkevin · 03-20-2009, 08:27 PM

The cathode bypass caps are located on the cathode pin (or on a connection point made to the cathode pin) of the output tube(s). They are usually low voltage (ie 25-50v) and low capacitance (ie 20 uf or so).
In my amp, I had to trace the wiring from the cathode of the output tubes off to a component board that was 4-5 inches away from the tube. My Hawk amp utilized these "cradles" that held the caps and associated wiring horizontally and allowed them to be soldered together.
Good luck finding yours!
dkevin

TD_Madden · 03-20-2009, 08:33 PM

probably better off just replacing all 'lytics in the amp.... :-)

01-29-2009, 03:18 PM	#1
ricach Senior Member Join Date: Jul 2007 Posts: 110	Gibson Minuteman Its a 60's GA20-RVT. Just recently its developed a loud hum on power up, but after it warms up a good 3-5 minutes the hum is only half as loud. None of the controls effects the volume of the hum, and its there even without an instrument cable plugged in. I've rechecked and resoldered the grounds. Pulled each tube seperately. Moved wires around. Even checked it in several different outlets around the house. No luck. I remember a thread a(long)while back that had a nice checklist and troubleshooting methodology for hum problems but I can't find it anymore.

03-20-2009, 03:47 PM	#5
dkevin Member Join Date: Mar 2009 Location: Seattle, WA. Posts: 36	Gibson Hum Hello, I recently worked on two 60's vintage Crestline Gibson amps and I ran into a similar problem. One of the amps (a Gibson GA-25RVT with 7591 output tubes) had a nasty hum. At first I thought it might be the electrolytic caps but it persisted after the replacement of said caps. I gave up and went to work and chewed on it for awhile and decided to check the cathode bypass caps for the output tubes. When I quick-subbed in a new bypass cap, the hum disappeared. Subsequently, I replaced all the bypass caps as they were all original. It's an easy test-good luck! dkevin

03-20-2009, 08:27 PM	#7
dkevin Member Join Date: Mar 2009 Location: Seattle, WA. Posts: 36	Cathode bypass caps The cathode bypass caps are located on the cathode pin (or on a connection point made to the cathode pin) of the output tube(s). They are usually low voltage (ie 25-50v) and low capacitance (ie 20 uf or so). In my amp, I had to trace the wiring from the cathode of the output tubes off to a component board that was 4-5 inches away from the tube. My Hawk amp utilized these "cradles" that held the caps and associated wiring horizontally and allowed them to be soldered together. Good luck finding yours! dkevin

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01-29-2009, 03:47 PM	#2
Mars Amp Repair Senior Member Join Date: Sep 2006 Location: Englewood, CO Posts: 673	Most likely in an amp that old you're dealing with a filter cap in the power supply. It might look like a stick of dynamite under the chassis with 3 or 4 wires coming out of it. It could also be a metal can mounted on the chassis....OR the 3 filter caps could be stand alone single units. You can find a similar value cap (20uf @ 450V would do) & bridge it across the caps inside by connecting the minus side to the black wire & the plus side of the test cap to the red wires one at a time... If that resolves your hummmmm, you need to replace all 3 caps as the others are either already bad or close behind. You can find these multi section or single caps in suppliers like Antique Electronics Supply for reasonable prices. http://www.tubesandmore.com/ Look under 'multi section capacitors'. Looks like it's a 20/20/20@450V? These caps also could be separates, too. You'll know when you get into the chassis. Here's a link to the gibson site for the schemo; http://www.gibson.com/Files/schematics/GA-20RVT.jpg Be very careful as there are lethal voltages in there. glen

01-29-2009, 05:06 PM	#3
ricach Senior Member Join Date: Jul 2007 Posts: 110	Thanks Glen. I've been inside enough if these to where I don't bother looking at schematics initially anymore. After looking at the schematic I noticed the 20u filter cap coming off the rectifying circuitry that ties directly to the OT. No doubt a probable source of the problem. Putting a scope on it I do see quite a ripple going directly into the OT. I'll give your idea a try, unfortunately no spare caps on hand at the moment.

02-01-2009, 02:16 PM	#4
ricach Senior Member Join Date: Jul 2007 Posts: 110	It appears that I have the 3 - 12AU7, 2 - 6EU7 preamp tube configuration and its considerably different than the schematics I'm finding for the GA20RVT with the 3-6EU7, 2-12AU7 configuration. Does anybody know where I can find the schematics for the former?

03-20-2009, 08:33 PM	#8
TD_Madden Senior Member Join Date: May 2006 Location: Manassas, VA Posts: 693	probably better off just replacing all 'lytics in the amp.... :-)

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