I am new to the forum and recently picked up a 1967 Gibson GA5-T amp with an intermittent working tremolo. I could take the amp to a local technician to repair it, but I would like to learn more about how to repair and eventually tweak this amp. I have constructed a capacitor discharge to use. While I was checking pin 3 on V1 I inadvertently touched my DMM probe to another pin and got a pop. I noticed that the filament in the bottom of the tube in V1 was glowing, but not up top like the other three tubes. I put a new tube in its place and still get only the bottom portion of the tube to glow, so I guess I have blown something connected to V1 besides the tremolo not working. I have checked resistor values and they are ok, I don't have a way to check capacitors, unless I desolder them. The amp does work with V1 partially working.
I have included the schematic of the amp and some pictures of the components.
So, I have a couple of questions:
1. V1 pin 3 voltage should be approximately 2.5V, after the pop I am getting 0.5V. What might I have blown when I touched more than one pin on V1 and what do I need to do to repair it?
2. What might be causing the problem with the tremolo? The tremolo switch in the off position reads 0.448M ohm, in the full ON position, 14.5 ohm. Could it be a bad capacitor or resistor?
Thanks.
1967 Gibson GA5-T Amp Issues
Moderators: pompeiisneaks, Colossal
1967 Gibson GA5-T Amp Issues
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Re: 1967 Gibson GA5-T Amp Issues
It isn't clear. Does the amp work? Are you only asking about the tremolo?
I suspect multiple problems here that need to be sorted out. One thing is, the amp has been worked on and we have no idea if the work followed or diverged from the original schematic. We know the amp was worked on because there are a number of parts that are not period correct to the date of manufacture, which was probably early '60's.
For starters, I'd ask for a voltage chart for all tubes. Include readings at each B+ node -- you can see the expected as 260, 250 , and 200V near the rectifier.
Regarding the visual on the filaments, not all tubes have the filament in the same physical configuration, so they might look different. You have two 12AX7 sockets (V1, V4). Swap the two tubes and see if what you are seeing follows the tube or stays with the socket. If the latter, the problem is at the socket and suggests the filament wiring needs a bit of reworking to put it back where it should be. If the glow stays with the tube, chances are nothing is wrong.
Back to the the matter of the amp having been repaired. Since you don't know where it was, who worked on it, or whether it's bee modified somehow, it would be a good idea to print the schematic and use a highlighter as you verify each and every part and connection. You may find whoever worked on it misconnected something or used the wrong part, but be reserved in your judgment, as what may look like a misconnect to someone with an untrained eye may actually be OK. If you find something you think isn't right, post a photo and ask.
While I'm at it, there are two things I don't like seeing. One is the location of the large light blue Xicon cap -- it just feels wrong just floating there. The other is the brown paper mulitsection electrolytic cap, which is well beyond its expected service life and should be replaced. There is lots of room inside the chassis, so when you get to these things, they should not be hard to change. These are a lower priority for now but should not be ignored.
This is a fairly simple amp. It shouldn't take much to get it right.
I suspect multiple problems here that need to be sorted out. One thing is, the amp has been worked on and we have no idea if the work followed or diverged from the original schematic. We know the amp was worked on because there are a number of parts that are not period correct to the date of manufacture, which was probably early '60's.
For starters, I'd ask for a voltage chart for all tubes. Include readings at each B+ node -- you can see the expected as 260, 250 , and 200V near the rectifier.
Regarding the visual on the filaments, not all tubes have the filament in the same physical configuration, so they might look different. You have two 12AX7 sockets (V1, V4). Swap the two tubes and see if what you are seeing follows the tube or stays with the socket. If the latter, the problem is at the socket and suggests the filament wiring needs a bit of reworking to put it back where it should be. If the glow stays with the tube, chances are nothing is wrong.
Back to the the matter of the amp having been repaired. Since you don't know where it was, who worked on it, or whether it's bee modified somehow, it would be a good idea to print the schematic and use a highlighter as you verify each and every part and connection. You may find whoever worked on it misconnected something or used the wrong part, but be reserved in your judgment, as what may look like a misconnect to someone with an untrained eye may actually be OK. If you find something you think isn't right, post a photo and ask.
While I'm at it, there are two things I don't like seeing. One is the location of the large light blue Xicon cap -- it just feels wrong just floating there. The other is the brown paper mulitsection electrolytic cap, which is well beyond its expected service life and should be replaced. There is lots of room inside the chassis, so when you get to these things, they should not be hard to change. These are a lower priority for now but should not be ignored.
This is a fairly simple amp. It shouldn't take much to get it right.
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martin manning
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Re: 1967 Gibson GA5-T Amp Issues
I can see that lots of parts have been replaced, but the pictures are not too clear. Can you take better ones?
Looks like the positive end of the small black electrolytic over V1's socket is not connected to anything (should be on pin 8?), and I see an 820R resistor connected to pin 8 that is 2k7 on the schematic. Clear evidence that someone has been modifying this amp.
I second Phil's comment re checking each connection and component value against the schematic, and making a complete voltage chart like the table on the schematic. You said you wanted to learn, looks like this is a good opportunity!
Looks like the positive end of the small black electrolytic over V1's socket is not connected to anything (should be on pin 8?), and I see an 820R resistor connected to pin 8 that is 2k7 on the schematic. Clear evidence that someone has been modifying this amp.
I second Phil's comment re checking each connection and component value against the schematic, and making a complete voltage chart like the table on the schematic. You said you wanted to learn, looks like this is a good opportunity!
Last edited by martin manning on Sun Jan 09, 2011 4:17 pm, edited 1 time in total.
Re: 1967 Gibson GA5-T Amp Issues
texstrat,
Your in good hands with Phil and Martin helping you out so I would take advantage of their help.
Old Gibson amps are worth fixing right because they are great sounding amps. I hope you can get the amp fixed and learn in the process.
Taking voltage readings is very important already mentioned from the power supply forward. Do you have extra tubes on hand you may need them.
Mark
Your in good hands with Phil and Martin helping you out so I would take advantage of their help.
Old Gibson amps are worth fixing right because they are great sounding amps. I hope you can get the amp fixed and learn in the process.
Taking voltage readings is very important already mentioned from the power supply forward. Do you have extra tubes on hand you may need them.
Mark
Re: 1967 Gibson GA5-T Amp Issues
Phil_S:
The amp does work. Initially the problem was with the tremolo, working intermittently, per the PO. The PO had the amp worked on by Viking Amps in Warrenton, VA by Eric, http://www.vikingamps.com/index.html. After talking to Eric @ Viking he said it the caps probably needed to be replaced. There are three caps in the tremolo circuit, .033, .02 and .02. The cap that looks the oldest could be the .033 Cornell Dubilier after doing some research on components that are used in Gibson amps.
As I am learning my way around tube amps and the terminology, will the B+ node be pin 1 on V4, pin 7 on V2, but because V1 is split, the 200V is showing pin 1 and 6 and the voltage chart is hard to read for V1.
After I got the pop, I switched out the two 12ax7 tubes and I got the same result, the bottom filament glowed but not the top. Before I caused this issue, the top of the tube in V1 did glow. I also tested V1 with a new tube as well with the same result. So I presume the problem is at the socket?
Regarding the Xicon capacitor, it does not have a value, just 20%, and I believe Viking installed this. I agree about the multisection cap needing to be replaced and have been researching a suitable replacement.
I have drawn a schematic of the amp, splitting V1 like the original schematic, showing resistor and capacitor values.
Martin Manning:
Here are some additional pictures. If you would like to see something else I will get and post them.
The small black cap is connected to pin 8 on V1, ties in with 808R resistor going to a three tab pad between V1 and V2.
So, what is the next step I should take in this adventure?
The amp does work. Initially the problem was with the tremolo, working intermittently, per the PO. The PO had the amp worked on by Viking Amps in Warrenton, VA by Eric, http://www.vikingamps.com/index.html. After talking to Eric @ Viking he said it the caps probably needed to be replaced. There are three caps in the tremolo circuit, .033, .02 and .02. The cap that looks the oldest could be the .033 Cornell Dubilier after doing some research on components that are used in Gibson amps.
As I am learning my way around tube amps and the terminology, will the B+ node be pin 1 on V4, pin 7 on V2, but because V1 is split, the 200V is showing pin 1 and 6 and the voltage chart is hard to read for V1.
After I got the pop, I switched out the two 12ax7 tubes and I got the same result, the bottom filament glowed but not the top. Before I caused this issue, the top of the tube in V1 did glow. I also tested V1 with a new tube as well with the same result. So I presume the problem is at the socket?
Regarding the Xicon capacitor, it does not have a value, just 20%, and I believe Viking installed this. I agree about the multisection cap needing to be replaced and have been researching a suitable replacement.
I have drawn a schematic of the amp, splitting V1 like the original schematic, showing resistor and capacitor values.
Martin Manning:
Here are some additional pictures. If you would like to see something else I will get and post them.
The small black cap is connected to pin 8 on V1, ties in with 808R resistor going to a three tab pad between V1 and V2.
So, what is the next step I should take in this adventure?
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Re: 1967 Gibson GA5-T Amp Issues
And a couple more pictures
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Re: 1967 Gibson GA5-T Amp Issues
I"m on my way out, so I'll post more later.
You may need to rewire the filament supply on V1. Pins 4&5 are jumpered and the other connection goes to pin 9. You may be able to work with the wire that's there. If not, replace it. For a 6.3V circuit, you can use almost anything including lamp cord. Don't go smaller than 20 gauge. If this doesn't help, you many need to replace the socket. Those wafer sockets are cheezy and you may have damaged it when probing. It is hard to know. See if you can give us a good quality closeup of the socket from some other angles.
BTW, this amp should have wicked good tremolo. I would not be quick to replace those tremolo caps. First, get V1 working properly. One step at a time is how this is done.
You may need to rewire the filament supply on V1. Pins 4&5 are jumpered and the other connection goes to pin 9. You may be able to work with the wire that's there. If not, replace it. For a 6.3V circuit, you can use almost anything including lamp cord. Don't go smaller than 20 gauge. If this doesn't help, you many need to replace the socket. Those wafer sockets are cheezy and you may have damaged it when probing. It is hard to know. See if you can give us a good quality closeup of the socket from some other angles.
BTW, this amp should have wicked good tremolo. I would not be quick to replace those tremolo caps. First, get V1 working properly. One step at a time is how this is done.
Re: 1967 Gibson GA5-T Amp Issues
Close-ups of V1
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Re: 1967 Gibson GA5-T Amp Issues
And two more...
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martin manning
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Re: 1967 Gibson GA5-T Amp Issues
Tell me I'm not seeing things... in one of your pictures (titled V1), that black electrolytic is disconnected, right? The cathode resistor (pin 8 to ground) is an 820 ohm, 5% part, I believe (gray, red, brown, gold). Maybe you measured the value at 802 ohms with the cap disconnected? That would be the only accurate way to do it, but you can just go by the marked values on the components for now. This R-C combination is 1uF and 2.2k on the Gibson schematic.
That blue Xicon cap surely has a value printed on it, it's just on the underside. That was added later, as it is paralleling the "red wire" section of the multi-cap like a crutch for a broken leg... it's a band-aid fix. When you replace the multi-cap you can clean that up.
The B+ nodes are the 260, 250, and 200V points on the Gibson schematic, starting from pin 7 of the rectifier V3. Get a measurement at V3 pin 7, then on the other side of the 3k3 resistor connected to it (pin 9, V2, or where the blue wire from the multi-cap connects to a terminal strip), and then on the other side of the 100K resistor that follows it (where the green wire from the multi-cap connects to a terminal strip).
It would be helpful to write all the component values from your amp on the Gibson schematic so you can see what matches and what doesn't. I'm with Phil; don't be too quick to change things until you know what you have. I would mark up the schematic, and then you need to make the voltage table. Take the measurements with the trem switched off.
BTW, your sketch would be called a layout drawing, as opposed to a schematic. "Schematic" refers to a diagram like the one you posted from Gibson.
That blue Xicon cap surely has a value printed on it, it's just on the underside. That was added later, as it is paralleling the "red wire" section of the multi-cap like a crutch for a broken leg... it's a band-aid fix. When you replace the multi-cap you can clean that up.
The B+ nodes are the 260, 250, and 200V points on the Gibson schematic, starting from pin 7 of the rectifier V3. Get a measurement at V3 pin 7, then on the other side of the 3k3 resistor connected to it (pin 9, V2, or where the blue wire from the multi-cap connects to a terminal strip), and then on the other side of the 100K resistor that follows it (where the green wire from the multi-cap connects to a terminal strip).
It would be helpful to write all the component values from your amp on the Gibson schematic so you can see what matches and what doesn't. I'm with Phil; don't be too quick to change things until you know what you have. I would mark up the schematic, and then you need to make the voltage table. Take the measurements with the trem switched off.
BTW, your sketch would be called a layout drawing, as opposed to a schematic. "Schematic" refers to a diagram like the one you posted from Gibson.
Re: 1967 Gibson GA5-T Amp Issues
Martin:
The 47uf capacitor is connected to pin8 of V1. I guess that needs to be removed? The value of that resistor is correct, it is 820 ohms, 5% tolerance. I got the 808 ohms value by measuring the resistor connected like it is now. The Xicon cap is a 22 uf 450V, I needed to look a bit lower to see it.
Before I get the voltages, should I address the capacitor soldered to pin 8 V1?
Layout drawing, thanks for the correction.
The 47uf capacitor is connected to pin8 of V1. I guess that needs to be removed? The value of that resistor is correct, it is 820 ohms, 5% tolerance. I got the 808 ohms value by measuring the resistor connected like it is now. The Xicon cap is a 22 uf 450V, I needed to look a bit lower to see it.
Before I get the voltages, should I address the capacitor soldered to pin 8 V1?
Layout drawing, thanks for the correction.
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martin manning
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Re: 1967 Gibson GA5-T Amp Issues
You can't get a good measurement of component values in-circuit unless the part is isolated somehow. In this case, the cap is paralleling the resistor and throwing your reading off. You can leave everything as it is to get the voltage table filled in. If you want to take it back to the original circuit, you will want to change the non-spec parts at some point, though. I don't know what someone was thinking with the 820R and 47uF... that stage will distort pretty quickly, and that cap is huge- lots of bass response, sub-sonic even!
Re: 1967 Gibson GA5-T Amp Issues
Oddly enough the amp is not distorting.
I am getting some weird voltages. I have my negative lead grounded to the chassis. I have the proper range set on my DMM, it is an Ideal 61-322, and on VAC. At pin 7 V3 I get 116V at the lug, 60V at pin 9 V2, and 5V at the ground tab after the resistor. Along with the weird voltages when I touch the positive probe to each lug I get a scratchy sound from the speaker. The volume is all the way down as I was checking.
I am getting some weird voltages. I have my negative lead grounded to the chassis. I have the proper range set on my DMM, it is an Ideal 61-322, and on VAC. At pin 7 V3 I get 116V at the lug, 60V at pin 9 V2, and 5V at the ground tab after the resistor. Along with the weird voltages when I touch the positive probe to each lug I get a scratchy sound from the speaker. The volume is all the way down as I was checking.
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martin manning
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Re: 1967 Gibson GA5-T Amp Issues
You should be measuring DC from Pin 7 of the rectifier. There will be some ripple, and that is what you are measuring on the AC setting.
Looks like there is some error in your sketch. The two red wired from the PT should go to pins 1 and 6 of the rectifier (the anodes), but you show nothing connected to pin 1.
Looks like there is some error in your sketch. The two red wired from the PT should go to pins 1 and 6 of the rectifier (the anodes), but you show nothing connected to pin 1.
Last edited by martin manning on Sun Jan 09, 2011 10:12 pm, edited 1 time in total.
Re: 1967 Gibson GA5-T Amp Issues
Ok, so DC voltage is what I want to measure across V3 pin 7, V2 pin 9 and after the resistor at the terminal strip?
Yes, my mistake on the schematic. One of the red leads from the TF102 is connected to pin 1 on V3.
Yes, my mistake on the schematic. One of the red leads from the TF102 is connected to pin 1 on V3.