Chmuzz Posted June 2, 2012 Share Posted June 2, 2012 Hi all, I'm new here, coming from Reunion island. I've a GA 40 Les Paul on the Bench ; I've found only one schematic (see attachment) The component values on this one seems to be good, I've compared with the circuitry Unfortunately, without no voltage chart, it's hard to verify the value of the power tube's cathode resistor. This one is broken on my amp ; according to the schematics, it's 150 Ohms. I've tried this value, but the power tubes dissipate too much ( 360V on the plates, 18,5V on the cathodes, 22W per tube !!) I don't know if the amp is in class A or AB. In pure A class, the best cathode resistor's value seems to be 450 Ohms (400V on the plates, 29,3V on the cathodes). But with such a plate voltage, I doubt that the amp is in class A. In AB class, 750 Ohms looks safe : I've 425V on the plates and 35V on the cathodes. I've put this value, but as I'm not sure, I'm looking for more accurate informations on this amp. Kind regards, Chmuzz gibsonga40lespaul.pdf Link to comment Share on other sites More sharing options...
59Panhead Posted December 19, 2014 Share Posted December 19, 2014 Hi all, I'm new here, coming from Reunion island. I've a GA 40 Les Paul on the Bench ; I've found only one schematic (see attachment) The component values on this one seems to be good, I've compared with the circuitry Unfortunately, without no voltage chart, it's hard to verify the value of the power tube's cathode resistor. This one is broken on my amp ; according to the schematics, it's 150 Ohms. I've tried this value, but the power tubes dissipate too much ( 360V on the plates, 18,5V on the cathodes, 22W per tube !!) I don't know if the amp is in class A or AB. In pure A class, the best cathode resistor's value seems to be 450 Ohms (400V on the plates, 29,3V on the cathodes). But with such a plate voltage, I doubt that the amp is in class A. In AB class, 750 Ohms looks safe : I've 425V on the plates and 35V on the cathodes. I've put this value, but as I'm not sure, I'm looking for more accurate informations on this amp. Kind regards, Chmuzz I know that this is an old topic, but a couple years ago when I was trying to clean up some very nasty previous repairs on one of my early GA-40's, I just could not read the only schematic that I could find. So, I put it in CAD and proceeded to determine the component values. I had a great deal of help from friends and from another old GA-40 of mine that has never been touched for repairs. It is non-working, but I now have two working models of the same. So, I leave it as-is, for future reference. I've attached that drawing, just in case someone else is having the same trouble. I haven't looked to see if someone else has redrawn the same. Anyway, have a look and comment. Have a good one. Jack Gibson GA-40 schematic - Early - rev D.pdf Link to comment Share on other sites More sharing options...
mggearjunkie Posted February 5, 2015 Share Posted February 5, 2015 I know that this is an old topic, but a couple years ago when I was trying to clean up some very nasty previous repairs on one of my early GA-40's, I just could not read the only schematic that I could find. So, I put it in CAD and proceeded to determine the component values. I had a great deal of help from friends and from another old GA-40 of mine that has never been touched for repairs. It is non-working, but I now have two working models of the same. So, I leave it as-is, for future reference. I've attached that drawing, just in case someone else is having the same trouble. I haven't looked to see if someone else has redrawn the same. Anyway, have a look and comment. Have a good one. Jack Hi, I'm new here. I have come across an old gibson tube amp that I can't get any info on. The previous owner (now deceased) wrote EH 180 from the 40's on it and sounds great except for some background noise. I would like to change the caps but the values have faded on a lot of them and I can't find the schematic. The tube compliment is 5U4, 6V6,6F6,6V6,6SN7,6SJ7x3. Very similar to the GA-40 except for a 6F6 instead of 6V6 ( same pinout) hence why I am inquiring here. The tubes stick out the back of the chassis rather then the bottom and the black facing of the chassis has strictly a Gibson label above the jewel light. Some of the cap values are similar but I found 1 Screen cap rated at 600VDC and one off the pilot light at 750 VDC. They all look original. My photo files are too big to upload, even after compression. BTW Jack, your schematic is excellent ! Thanks in advance for any info. MG Link to comment Share on other sites More sharing options...
59Panhead Posted February 15, 2015 Share Posted February 15, 2015 Do you have a lamp limiter? These are a must for bringing up all old amps, and whenever you replace the caps in any amp. Done right, you can save the old electrolytics, 90% of the time. Also, I like to install a thermistor on the line input to the primary of the power transformer. Limits the inrush current, and voltage, until the thermistor warms up. Really good soft start. Without it, B+ voltages go sky high, until the tubes begin to conduct and start loading the power supply. Jack Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.