Hi! First post here. First I want to thank this site and members for posting info about Guild stuff - its been great to find some info for this amp I would otherwise be lost without.
So, I'm a hobbyist with a bit of experience building and fixing amps and I agreed to fix up a Thunderbass head for a friend. I am posting the progress here and hope to contribute some info and learn more and seek help etc. etc.
When I first researched a little about this amp I found mixed reviews about "Oh - its a good amp but the chassis is flimsy, they are hard to work on, they take weird tubes, and so on. Well after spending some time with it I find most everything about it cool and different and I can't wait to get it working good.
I brought it up on the Variac and found the amp actually worked (not well) and had really dirty pots and switches etc. I played it for a few minutes and decided to take it out of the cabinet to see which version it was (I found other good posts here about what version is what - Thanks capnjuan and other posters!). Based on what I saw I did not have a clue what I was looking at. Someone had converted to EL34's, replaced the 7247 driver tube with a 12AU7, and added some parts to the bias supply. After some more checking I found the 6GF7 voltage regulator and that led me to find the correct Quantum schematic on the Prowess Amplifiers website.
Some measurements (630 Volts on the plates!) and the Transformer part number on the schematic confirm this amp is supposed to have the long discontinued 8417 power tubes. I dont know how the EL34's survived at that voltage especially because when I checked the pin 4 of the output tubes I find the regulator is not working. Pin 4 is supposed to be 300V and I'm getting the same 630V on that pin as the plates. Hmmm...... Also the extra parts added turned out to be a voltage doubler circuit obviously added because the EL34's needed more negative voltage to stay alive! The schematic below is what I traced for the voltage doubler. It also ties to the regulator adjustment pot through a 22k resistor. I'm not sure if the alternative wiring for the doubler will cause the regulator to not work. Any suggestions?
My intention is to put it back stock with the 8417's which should be fairly straightforward. Parts are on the way - more to come.
So, I'm a hobbyist with a bit of experience building and fixing amps and I agreed to fix up a Thunderbass head for a friend. I am posting the progress here and hope to contribute some info and learn more and seek help etc. etc.
When I first researched a little about this amp I found mixed reviews about "Oh - its a good amp but the chassis is flimsy, they are hard to work on, they take weird tubes, and so on. Well after spending some time with it I find most everything about it cool and different and I can't wait to get it working good.
I brought it up on the Variac and found the amp actually worked (not well) and had really dirty pots and switches etc. I played it for a few minutes and decided to take it out of the cabinet to see which version it was (I found other good posts here about what version is what - Thanks capnjuan and other posters!). Based on what I saw I did not have a clue what I was looking at. Someone had converted to EL34's, replaced the 7247 driver tube with a 12AU7, and added some parts to the bias supply. After some more checking I found the 6GF7 voltage regulator and that led me to find the correct Quantum schematic on the Prowess Amplifiers website.
Some measurements (630 Volts on the plates!) and the Transformer part number on the schematic confirm this amp is supposed to have the long discontinued 8417 power tubes. I dont know how the EL34's survived at that voltage especially because when I checked the pin 4 of the output tubes I find the regulator is not working. Pin 4 is supposed to be 300V and I'm getting the same 630V on that pin as the plates. Hmmm...... Also the extra parts added turned out to be a voltage doubler circuit obviously added because the EL34's needed more negative voltage to stay alive! The schematic below is what I traced for the voltage doubler. It also ties to the regulator adjustment pot through a 22k resistor. I'm not sure if the alternative wiring for the doubler will cause the regulator to not work. Any suggestions?
My intention is to put it back stock with the 8417's which should be fairly straightforward. Parts are on the way - more to come.