- Joined
- Aug 27, 2021
- Messages
- 5
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- Guild Total
- 1
I recently acquired a sweet old Guild Thunderstar Bass amp for real cheap because of some issues - primarily some nasty, constant crackle. The filter caps have been replaced some time in the last few years with new, clean looking JJs, so I didn't touch those. The tubes are old but nice American RCA glass and seem OK.
The bias resistor was way out of spec causing an imbalanced current to the power tubes - like 75mA to one and 50mA to the other! I don't know how the tube wasn't redplating, but after replacing that resistor I got them balanced to a more reasonable level (~70% dissipation). I replaced the super old 30uF 10V cathode bypass caps with 22uF 50V caps that I had on hand - I can't find small 30uF caps anywhere. That killed off most of the crackling and cleaning the switches, pots and jacks did the rest. It sounds real cool! It's kind of between a Tweed Bassman and an Ampeg b-15 to my ears. I like it for both bass and guitar.
1: Even with all new electrolytics, there's still quite a bit of hum even with the volume at 0. Should I recap more of the amp? I'm looking at the coupling caps next, but I thought those didn't need to get changed. Should I buy a new pair of 6L6s?
My main questions are about the 3-way tone switch. The way the schematic is drawn is baffling for one. I can tell that it's providing some negative feedback to V1B but which parts are engaged when is confusing as hell. In the bright position (open circle) the amp sounds very scooped, in the dark position (closed circle) it's VERY dark - like fully-rolled-off-tone-on-a-P-bass dark. In the center position, there's about 10dB more gain in the circuit. It's way louder and gainier than the other positions. Honestly it's kinda cool to get that much tweedy overdrive, but it seems real odd to me.
2: Can anyone explain how the switch actually works? I've scoured the web for a wiring diagram for a 12-pin 3 position switch to no avail. Like, which pins are active when?
3: Is this the way the tone switch is supposed to work? Or are the components drifted way off spec? My next thought is tor replace all the resistors and ceramic caps and see if that balances the positions, or maybe just install a switch to bypass the whole thing.
The bias resistor was way out of spec causing an imbalanced current to the power tubes - like 75mA to one and 50mA to the other! I don't know how the tube wasn't redplating, but after replacing that resistor I got them balanced to a more reasonable level (~70% dissipation). I replaced the super old 30uF 10V cathode bypass caps with 22uF 50V caps that I had on hand - I can't find small 30uF caps anywhere. That killed off most of the crackling and cleaning the switches, pots and jacks did the rest. It sounds real cool! It's kind of between a Tweed Bassman and an Ampeg b-15 to my ears. I like it for both bass and guitar.
1: Even with all new electrolytics, there's still quite a bit of hum even with the volume at 0. Should I recap more of the amp? I'm looking at the coupling caps next, but I thought those didn't need to get changed. Should I buy a new pair of 6L6s?
My main questions are about the 3-way tone switch. The way the schematic is drawn is baffling for one. I can tell that it's providing some negative feedback to V1B but which parts are engaged when is confusing as hell. In the bright position (open circle) the amp sounds very scooped, in the dark position (closed circle) it's VERY dark - like fully-rolled-off-tone-on-a-P-bass dark. In the center position, there's about 10dB more gain in the circuit. It's way louder and gainier than the other positions. Honestly it's kinda cool to get that much tweedy overdrive, but it seems real odd to me.
2: Can anyone explain how the switch actually works? I've scoured the web for a wiring diagram for a 12-pin 3 position switch to no avail. Like, which pins are active when?
3: Is this the way the tone switch is supposed to work? Or are the components drifted way off spec? My next thought is tor replace all the resistors and ceramic caps and see if that balances the positions, or maybe just install a switch to bypass the whole thing.