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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Peavey VK 112 of 2005
Valve amp, little used then stored for 3 years now very low level
wanted, undistorted output and constant buzz, minimum wiht middle setting of A/AB pot on rear. Good signal out of SEND and bad output from test signal in RET, so PA problem. Nothing seen weeping nor dodgey solder. I was hoping S/R bypass switch corrossion but not that. |
#2
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Peavey VK 112 of 2005
I've not fathomed the schematic, how the relay functions. Looks as
though 1 of 3 relays switch over from external to internal and the footswitch socket seems to be involved in that, so perhaps corrossion there or a stuck relay. |
#3
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Peavey VK 112 of 2005
Looks as though corrossion of a contact on ECC83 driver valve and the
Return tip contact |
#4
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Peavey VK 112 of 2005
"N_Cook" wrote in message ... Valve amp, little used then stored for 3 years now very low level wanted, undistorted output and constant buzz, minimum wiht middle setting of A/AB pot on rear. Good signal out of SEND and bad output from test signal in RET, so PA problem. Nothing seen weeping nor dodgey solder. I was hoping S/R bypass switch corrossion but not that. That A/AB pot on the back is a bit weird from memory. Does it kind of turn one output tube off? Cheers, Gareth. |
#5
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Peavey VK 112 of 2005
On 25/03/2015 18:49, Gareth Magennis wrote:
"N_Cook" wrote in message ... Valve amp, little used then stored for 3 years now very low level wanted, undistorted output and constant buzz, minimum wiht middle setting of A/AB pot on rear. Good signal out of SEND and bad output from test signal in RET, so PA problem. Nothing seen weeping nor dodgey solder. I was hoping S/R bypass switch corrossion but not that. That A/AB pot on the back is a bit weird from memory. Does it kind of turn one output tube off? Cheers, Gareth. From the explanation of controls page "As the TEXTURE control is rotated counterclockwise, the effect of one half of the 6L6GC power tubes is progressively subtracted from the circuit, while the gain of the driver tube is slowly increased. The driver's low-frequency response is also altered along with the gain, resulting in more even-ordered harmonic distortion from your power amp, even at lower-than-stage-volume settings. Finally, with the TEXTURE knob in the fully counterclockwise position, the result is a real single-ended power amp section that operates and responds exactly like a true Class A power amp, driven by a real single-ended high-gain tube stage. This setting still allows the unused power tube(s) to draw idle current, thus retaining the efficiency of the standard Class A/B topology. In this mode, power output is also reduced by as much as 60% versus maximum rated power." |
#6
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Peavey VK 112 of 2005
"N_Cook" wrote in message ... On 25/03/2015 18:49, Gareth Magennis wrote: "N_Cook" wrote in message ... Valve amp, little used then stored for 3 years now very low level wanted, undistorted output and constant buzz, minimum wiht middle setting of A/AB pot on rear. Good signal out of SEND and bad output from test signal in RET, so PA problem. Nothing seen weeping nor dodgey solder. I was hoping S/R bypass switch corrossion but not that. That A/AB pot on the back is a bit weird from memory. Does it kind of turn one output tube off? Cheers, Gareth. From the explanation of controls page "As the TEXTURE control is rotated counterclockwise, the effect of one half of the 6L6GC power tubes is progressively subtracted from the circuit, while the gain of the driver tube is slowly increased. The driver's low-frequency response is also altered along with the gain, resulting in more even-ordered harmonic distortion from your power amp, even at lower-than-stage-volume settings. Finally, with the TEXTURE knob in the fully counterclockwise position, the result is a real single-ended power amp section that operates and responds exactly like a true Class A power amp, driven by a real single-ended high-gain tube stage. This setting still allows the unused power tube(s) to draw idle current, thus retaining the efficiency of the standard Class A/B topology. In this mode, power output is also reduced by as much as 60% versus maximum rated power." OK, so it seems to turn one output tube off then, as I thought. It looks rather drastic on a scope, very top heavy IIRC. Cheers, Gareth. |
#7
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Peavey VK 112 of 2005
"Gareth Magennis" wrote in message ... "N_Cook" wrote in message ... On 25/03/2015 18:49, Gareth Magennis wrote: "N_Cook" wrote in message ... Valve amp, little used then stored for 3 years now very low level wanted, undistorted output and constant buzz, minimum wiht middle setting of A/AB pot on rear. Good signal out of SEND and bad output from test signal in RET, so PA problem. Nothing seen weeping nor dodgey solder. I was hoping S/R bypass switch corrossion but not that. That A/AB pot on the back is a bit weird from memory. Does it kind of turn one output tube off? Cheers, Gareth. From the explanation of controls page "As the TEXTURE control is rotated counterclockwise, the effect of one half of the 6L6GC power tubes is progressively subtracted from the circuit, while the gain of the driver tube is slowly increased. The driver's low-frequency response is also altered along with the gain, resulting in more even-ordered harmonic distortion from your power amp, even at lower-than-stage-volume settings. Finally, with the TEXTURE knob in the fully counterclockwise position, the result is a real single-ended power amp section that operates and responds exactly like a true Class A power amp, driven by a real single-ended high-gain tube stage. This setting still allows the unused power tube(s) to draw idle current, thus retaining the efficiency of the standard Class A/B topology. In this mode, power output is also reduced by as much as 60% versus maximum rated power." OK, so it seems to turn one output tube off then, as I thought. It looks rather drastic on a scope, very top heavy IIRC. Cheers, Gareth. i.e. you will not get a sine wave at any output power, it is purely distortion. I think. Gareth. |
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