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06-26-2020, 03:28 PM
Hello,
I'm having a bit of a problem troubleshooting a Fender type reverb - the 12AT7 driver tube output has a tendency to distort the signal. Originally with a bypassed 1K cathode resistor the tube was oscillating wildly. I increased the Rk value to 2,2K bypassed which stopped the oscillation but the top of the signal was still distorted. Removing the bypass cap obviously reduces the gain in the triode and I can make up for that by lowering the Rk value to 470R. At maximum V1 output volume I'm getting 154Vpp signal swing whereas with a bypassed 2,2K Rk it was around 195Vpp. At 154Vpp reverb sounds OK. Voltage drop at Rk is now 4,7V and plate voltage measured cathode to plate is 340V, so the plate dissipation is still within spec.
The signal at the grid is clean and undistorted. There's a 430K grid stopper and a 680K grid leak. Reverb transformer sees 360V.
Is there any other way of fixing the problem with the reverb driver?
Tomi
I'm having a bit of a problem troubleshooting a Fender type reverb - the 12AT7 driver tube output has a tendency to distort the signal. Originally with a bypassed 1K cathode resistor the tube was oscillating wildly. I increased the Rk value to 2,2K bypassed which stopped the oscillation but the top of the signal was still distorted. Removing the bypass cap obviously reduces the gain in the triode and I can make up for that by lowering the Rk value to 470R. At maximum V1 output volume I'm getting 154Vpp signal swing whereas with a bypassed 2,2K Rk it was around 195Vpp. At 154Vpp reverb sounds OK. Voltage drop at Rk is now 4,7V and plate voltage measured cathode to plate is 340V, so the plate dissipation is still within spec.
The signal at the grid is clean and undistorted. There's a 430K grid stopper and a 680K grid leak. Reverb transformer sees 360V.
Is there any other way of fixing the problem with the reverb driver?
Tomi
