Tweet
Just as I finish building my 5f3a my neighbour brings around his 6G7A with a problem. I asked him what the problem was and he told me that there had been smells, dripping goo and no sound.
I took the back off and pulled the valves and ran them through my little tester. One of the 5881 Tung Sols was very much in the 'bad' zone. I put in a pair of my 6P3S-E just to see if it was purely a blown valve problem. The look of joy on my neighbour's face when we got sound was a delight. However, when we went to take out the valves ( so that I could open up the chassis) i noticed that V8 (the one furthest to the left on Fender's layout diagram) was much hotter to the touch than V7. That rang alarm bells.
Open up the chassis and note that a cap job has been carried out and the bias circuit has been tinkered with (trim pot added, 100uF cap, different resistor values to the diagram). I also noted that the screen resistors had been replaced with 470R 5W ww resistors. I took some measurements with my DMM and found that the screen resistor on V7 was reading open circuit.
Question: will a blown valve (I didn't note the position) have caused the screen resistor to fail? Or would a failed resistor cause the valve to fail? If so, why would a failed resistor on V7 cause V8 to be so much hotter to the touch after only a couple of minutes? Is there something else I should be investigating?
Furthermore - I notice the ground switch has a cap to earth. Is this the dreaded 'death cap' and should I remove it? I have attached some gut shots for support.
Very many thanks for all your help.
Jeff
I took the back off and pulled the valves and ran them through my little tester. One of the 5881 Tung Sols was very much in the 'bad' zone. I put in a pair of my 6P3S-E just to see if it was purely a blown valve problem. The look of joy on my neighbour's face when we got sound was a delight. However, when we went to take out the valves ( so that I could open up the chassis) i noticed that V8 (the one furthest to the left on Fender's layout diagram) was much hotter to the touch than V7. That rang alarm bells.
Open up the chassis and note that a cap job has been carried out and the bias circuit has been tinkered with (trim pot added, 100uF cap, different resistor values to the diagram). I also noted that the screen resistors had been replaced with 470R 5W ww resistors. I took some measurements with my DMM and found that the screen resistor on V7 was reading open circuit.
Question: will a blown valve (I didn't note the position) have caused the screen resistor to fail? Or would a failed resistor cause the valve to fail? If so, why would a failed resistor on V7 cause V8 to be so much hotter to the touch after only a couple of minutes? Is there something else I should be investigating?
Furthermore - I notice the ground switch has a cap to earth. Is this the dreaded 'death cap' and should I remove it? I have attached some gut shots for support.
Very many thanks for all your help.
Jeff
Comment