Hello LTG.
Your Guild amp forum is unmatched on the Internet. Thanks to y'all, the web no longer has a dearth of info on Guild amps. Rather, there is just a shortage.
So here's my story and my reason for posting. About a decade ago I received a Thunderbass head. It didn't work due to some burned up resistors in the bias circuit. After my main amp was stolen ('63 Bandmaster) I had it repaired (and got a grounded cord) and have been using it since. I like it a lot. I suspect I could truly love it if it was what I imagine it could be.
I have a plan for this amp: I want it sound as good as it looks. (I think the head looks great.) I know I should overhaul this thing. I would like to do the work myself. I don't normally play in old electronics, but I know my theory, can solder well, and am very afraid of electrocution (read know I must drain the filter caps). Eventually, I might want to mod one of the channels (I'm a guitarist for the most part). And there are some problematic noises that come out from time to time. I'll start with some pron. (I have the cabinet which is 3/4 inch plywood, black tolex, and two vents on top. I just didn't include it here.)
The serial no. is 7197.
The intended tubes are (3) 12ax7, (1) 7247, (2) 6l6.
I identify this amp as a Thunderbass 1-A. Because of the cabinet styling and serial number I guess it is late in the run and from 1969. Does anyone think I'm wrong?
First question: What are the dangers and problems presented by using a 12ax7 instead of the 7247 as the inverter driver? That has been the case for the past few years and nothing noticeable has changed in the amp. I know the 7247 is half 12ax7 and half 12au7, so I suspect half of the signal is being over driven and the push-pull doesn't sum back up as it should.
Second question: What is the right schematic? I have downloaded the 'Guild Thunderbass 4 Tube Preamp' that is at prowessamplifiers.com. From everything I've traced, this is the right schematic. Specifically the filter caps. This leads me to my third questions.
Third questions: Caps… I see no damage or bulges in any caps. Should I replace all the smaller caps in the preamps or just the filter caps?
Fourth question: Sometimes, the amp will start to make noises. The noises are intermittent, loud, and sound like I am plugging in a cable without the standby on. These loud throaty clunking noises last longer than that though, more like a couple of seconds. They can accompany a slight change in tone too. They happen even if the standby is on. From my thought experiments on troubleshooting this I find it might be a bad solder joint or accidental ground just before the output stage. Any thoughts?
Fifth question: In closing this long first post, in what order should I do following?
a) troubleshoot the noise
b) replace caps
c) replace tubes
d) bring it to a professional
I'll thank anyone and everyone in advance... Thank you.
-E
Your Guild amp forum is unmatched on the Internet. Thanks to y'all, the web no longer has a dearth of info on Guild amps. Rather, there is just a shortage.
So here's my story and my reason for posting. About a decade ago I received a Thunderbass head. It didn't work due to some burned up resistors in the bias circuit. After my main amp was stolen ('63 Bandmaster) I had it repaired (and got a grounded cord) and have been using it since. I like it a lot. I suspect I could truly love it if it was what I imagine it could be.
I have a plan for this amp: I want it sound as good as it looks. (I think the head looks great.) I know I should overhaul this thing. I would like to do the work myself. I don't normally play in old electronics, but I know my theory, can solder well, and am very afraid of electrocution (read know I must drain the filter caps). Eventually, I might want to mod one of the channels (I'm a guitarist for the most part). And there are some problematic noises that come out from time to time. I'll start with some pron. (I have the cabinet which is 3/4 inch plywood, black tolex, and two vents on top. I just didn't include it here.)
The serial no. is 7197.
The intended tubes are (3) 12ax7, (1) 7247, (2) 6l6.
I identify this amp as a Thunderbass 1-A. Because of the cabinet styling and serial number I guess it is late in the run and from 1969. Does anyone think I'm wrong?
First question: What are the dangers and problems presented by using a 12ax7 instead of the 7247 as the inverter driver? That has been the case for the past few years and nothing noticeable has changed in the amp. I know the 7247 is half 12ax7 and half 12au7, so I suspect half of the signal is being over driven and the push-pull doesn't sum back up as it should.
Second question: What is the right schematic? I have downloaded the 'Guild Thunderbass 4 Tube Preamp' that is at prowessamplifiers.com. From everything I've traced, this is the right schematic. Specifically the filter caps. This leads me to my third questions.
Third questions: Caps… I see no damage or bulges in any caps. Should I replace all the smaller caps in the preamps or just the filter caps?
Fourth question: Sometimes, the amp will start to make noises. The noises are intermittent, loud, and sound like I am plugging in a cable without the standby on. These loud throaty clunking noises last longer than that though, more like a couple of seconds. They can accompany a slight change in tone too. They happen even if the standby is on. From my thought experiments on troubleshooting this I find it might be a bad solder joint or accidental ground just before the output stage. Any thoughts?
Fifth question: In closing this long first post, in what order should I do following?
a) troubleshoot the noise
b) replace caps
c) replace tubes
d) bring it to a professional
I'll thank anyone and everyone in advance... Thank you.
-E