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Hello All.
I've just completed a new amp build - a Mojotone 5E8-A low power tweed twin. The amp sounds great - no hum or hiss, but there are two problems i can't track down. I've searched through the forum, but haven't found a similar issue.
The problems are:
1. The treble pot acts as a volume control (affects both channels)
2. Overall output is lower than expected.
Another thing I've noticed is that as soon as power is switched off the filter caps discharge themselves. With the amp on, the filter caps have 455 V. Within approx 10 seconds of turning the power off the Filter caps read 8-11 VDC. (I've tested this both with the meter on the cap as it discharges, and without the meter - results are the same.). This seems to indicate that there is a component grounding out somewhere.
With the treble pot on 0 the volume is very low and can barely be heard. start turning the treble pot and the treble increases as does the volume. With the volume and treble on 12 (full) the amp is loud, but not as loud as i would expect from a 40W amp. The sweet spot for the amp is Vol 7, Treble 7 and bass 3. At these setting the 5E8-A is a little louder than my Deluxe Reverb clone with the vol on 3.5. I was expecting the volume to be closer to my JTM45 clone.
I've tried isolating the problem and believe its in the preamp section. I checked and there is no voltage between the wiper of the treble pot and ground. I then inserted an Audio signal (tape deck) onto the wiper of the treble pot and controlled the volume using the tape deck controls. This output signal was loud.
The amp is wired using the original Fender schematic and layout. I've used the 8ohm tap on the OT and 240V (Australia) input on the PT.
Any suggestions on isolating and fixing the issue would be apprecated.
MT-FENDER-MOJO752EX.pdf
MT-FENDER-MOJO767.pdf
Here's voltages on the valve pins. Although they are higher than on the fender schematic most are within 20%. Those marked with * are outside +/- 20%. The heaters are 6.8v.
V1
1: 144.6v
2: .0004v
3: 2.41v
6: 141.1
7: 0
8: 2.41
V2
1: 141.2v
2: .0001v
3: 2.42v
6: 139.5
7: 0
8: 2.40
V3
1: 129.9v
2: .0002v
3: 2.068v
6: 266.8
7: 130.3v
8: 132.6
V4
1: 223.4v
2: .0003v
3: 1.78v
6: 278.1
7: 24.18v
8: 66.4 * (50v on schematic)
V5
3: 445.6v
4: 447.6v
5: -55.9v * (-42v on schematic)
6: -55.85 * (-42v on schematic)
V6
3: 444.6v
4: 445.7v
5: -55.8v * (-42v on schematic)
6: -55.9v * (-42v on schematic)
Rectifier Heater voltage across pins 2 and 8 is 5.5VAC
V7
2: 7
4: 358
6: 358
8: 5.7
V8
2: 6.9
4: 358
6: 358
8: 5.5
I've just completed a new amp build - a Mojotone 5E8-A low power tweed twin. The amp sounds great - no hum or hiss, but there are two problems i can't track down. I've searched through the forum, but haven't found a similar issue.
The problems are:
1. The treble pot acts as a volume control (affects both channels)
2. Overall output is lower than expected.
Another thing I've noticed is that as soon as power is switched off the filter caps discharge themselves. With the amp on, the filter caps have 455 V. Within approx 10 seconds of turning the power off the Filter caps read 8-11 VDC. (I've tested this both with the meter on the cap as it discharges, and without the meter - results are the same.). This seems to indicate that there is a component grounding out somewhere.
With the treble pot on 0 the volume is very low and can barely be heard. start turning the treble pot and the treble increases as does the volume. With the volume and treble on 12 (full) the amp is loud, but not as loud as i would expect from a 40W amp. The sweet spot for the amp is Vol 7, Treble 7 and bass 3. At these setting the 5E8-A is a little louder than my Deluxe Reverb clone with the vol on 3.5. I was expecting the volume to be closer to my JTM45 clone.
I've tried isolating the problem and believe its in the preamp section. I checked and there is no voltage between the wiper of the treble pot and ground. I then inserted an Audio signal (tape deck) onto the wiper of the treble pot and controlled the volume using the tape deck controls. This output signal was loud.
The amp is wired using the original Fender schematic and layout. I've used the 8ohm tap on the OT and 240V (Australia) input on the PT.
Any suggestions on isolating and fixing the issue would be apprecated.
MT-FENDER-MOJO752EX.pdf
MT-FENDER-MOJO767.pdf
Here's voltages on the valve pins. Although they are higher than on the fender schematic most are within 20%. Those marked with * are outside +/- 20%. The heaters are 6.8v.
V1
1: 144.6v
2: .0004v
3: 2.41v
6: 141.1
7: 0
8: 2.41
V2
1: 141.2v
2: .0001v
3: 2.42v
6: 139.5
7: 0
8: 2.40
V3
1: 129.9v
2: .0002v
3: 2.068v
6: 266.8
7: 130.3v
8: 132.6
V4
1: 223.4v
2: .0003v
3: 1.78v
6: 278.1
7: 24.18v
8: 66.4 * (50v on schematic)
V5
3: 445.6v
4: 447.6v
5: -55.9v * (-42v on schematic)
6: -55.85 * (-42v on schematic)
V6
3: 444.6v
4: 445.7v
5: -55.8v * (-42v on schematic)
6: -55.9v * (-42v on schematic)
Rectifier Heater voltage across pins 2 and 8 is 5.5VAC
V7
2: 7
4: 358
6: 358
8: 5.7
V8
2: 6.9
4: 358
6: 358
8: 5.5