Clbraddock
Well-known member
Recently built a point to point LA-2A using the attached 1967 schematic. It works great and everything tests correctly, except the plate voltage to V3 -12AX7 - pin 1 and screen voltage to V4 - 6AQ5W - pin 6.
The plate voltage to V3 should be about 105, but I am measuring around 85 VCD.
The screen voltage to V4 should be 100, but I am measuring about 80 VCD.
Both tubes are fed from a 275 VDC B+, each across a separate 220K resistor to the relevant pin. The B+ measures correct at 275, but I am getting too much voltage drop across the 220K resistors. The resistors measure within 5% of spec.
These are both sidechain tubes - i.e. not in the signal path.
Edit: The first time I fired the unit up the cathodes for V3 and V4 were not connected to ground (wired wrong). I guess this could have damaged both tubes? Still seems like some other cause though since the unit works overall.
1) Should I worry about this/does it make any difference
2) What is causing this? Im assuming to the tubes are drawing more current than expected, thereby causing a larger voltage drop with ohms law. If it was just one tube I would think it was a bad tube, but since two different tubes show too low VDC I would guess its something else.
3) Should I swap the 220K resistors for a lower value? Or should I not worry about it? (Sorry kind of a repeat of question 1)
The plate voltage to V3 should be about 105, but I am measuring around 85 VCD.
The screen voltage to V4 should be 100, but I am measuring about 80 VCD.
Both tubes are fed from a 275 VDC B+, each across a separate 220K resistor to the relevant pin. The B+ measures correct at 275, but I am getting too much voltage drop across the 220K resistors. The resistors measure within 5% of spec.
These are both sidechain tubes - i.e. not in the signal path.
Edit: The first time I fired the unit up the cathodes for V3 and V4 were not connected to ground (wired wrong). I guess this could have damaged both tubes? Still seems like some other cause though since the unit works overall.
1) Should I worry about this/does it make any difference
2) What is causing this? Im assuming to the tubes are drawing more current than expected, thereby causing a larger voltage drop with ohms law. If it was just one tube I would think it was a bad tube, but since two different tubes show too low VDC I would guess its something else.
3) Should I swap the 220K resistors for a lower value? Or should I not worry about it? (Sorry kind of a repeat of question 1)
