Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SirBlackmore on November 09, 2024, 03:55:26 am
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Hello everyone,
For a while now, I’ve been building the Blackface 22 kit from Tube Amp Doctor, which corresponds to a Fender Deluxe Reverb (AB763).
When it came time to adjust the BIAS and plug in the guitar to test, I noticed that the sound was unpleasant and somewhat distorted. So, I started taking measurements to locate the issue, tracing through the Vibrato channel.
I input a 1 kHz sine wave signal at the vibrato input. In the output of the preamp tubes, the measurements with the oscilloscope appear correct, yielding a sine wave at their output. But it’s at the power tubes where I see the problem, and I hope someone can guide me on where the issue might be.
The image attached is the measurement at the output (pin 3) of V7 and V8 (6V6GT):
(https://i.ibb.co/cJgRfCS/osciloscopio-pin-3-V7-V8.jpg) (https://ibb.co/dBWXCLw)
The yellow signal corresponds to the signal at the input of the amplifier's vibrato channel. The blue signal is at pin 3 of V7 (left image) and V8 (right image). In the output signal, I notice the wave is distorted and also has noise.
I’ve measured these voltages at V7 and V8:
V7
Pin 3: 442 V DC
Pin 4: 441 V DC
Pin 5: -60.70 V DC
Pin 2 to 7: 6.22 V AC
V8
Pin 3: 442 V DC
Pin 4: 441 V DC
Pin 5: -60.70 V DC
Pin 2 to 7: 6.22 V AC
I’d like to ask:
- Where could the problem be coming from?
- I also notice that in V7 (left), the output signal has (almost) the opposite phase of the input signal. In V8 (right), the
phase is almost the same as the input. Are these phase differences significant? Could they be caused by the same issue?
- Is there any other measurement or test you would recommend? Right now, I don’t have other 6V6s to test, so I’d like
to try other possible causes first.
I hope someone can help me with this. Thank you very much.I hope someone can help me with this. Thank you very much.
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-60V is way too much voltage for pin 5 of a 6V6. Set the bias pot for -36V at both pin 5.
The signals on the plates of the output tubes are supposed to be 180° out of phase. Connecting a scope probe to the plates is putting your scope at risk. A better/safer point to monitor output signal is the speaker output jack. Adjust the amp volume control for maximum clean sine wave.