J. Dan Posted August 20, 2008 Posted August 20, 2008 Hello all - visiting from the Keyboard forum. I'm working on a Fender 75 combo amp for a friend and figured there may be a tech on this forum that could help. I've got everything checked out and seems to be working properly except the Lead pot. I already figured out the switching method on this amp and that it's working properly: Lead Master Pot switches in/out properly (Red Ring from Footswitch Grounded/ungrounded) and Lead pot switches in/out properly (Plain Tip from footswitch grounded/ungrounded). However, in the lead position, there is not much gain, and the lead pot, when turned up reaches max volume at about 50% then goes back down. You cannot overdrive it with the lead channel. The next two tube stages must be fine because the clean channel also runs through these. Before I dig much further into it, I was reviewing the schematic to see if I could determine what it might be. Half of the 7025 tube labelled V3B looks like a buffer stage feeding the lead channel from the clean channel - is this correct? Any reason why the lead pot would affect it's operation or should I move beyond that since I AM getting signal? I noticed that the clean signal is present at the stage with V2B when the lead channel is activated - any chance some weird phasing thing could be going on, or a voltage bleed-through that could be screwing with the V3B stage (bad capacitor)? FYI - schematic is at: http://www.geocities.com/twoseriesfenders/fender_75_schem.pdf FYI - Background: I am an electrical engineer and have done much of my own service, but do not have a great deal of experience with tube amps, though I understand how they work and am capable of basic troubleshooting. Dan Acoustic/Electric stringed instruments ranging from 4 to 230 strings, hammered, picked, fingered, slapped, and plucked. Analog and Digital Electronic instruments, reeds, and throat/mouth.
The Geoff Posted August 20, 2008 Posted August 20, 2008 Probably best to post this in the 'Feel Free to Ask Myles' sticky near the top of the page. Geoff "When the power of love overcomes the love of power the World will know Peace": Jimi Hendrix http://www.soundclick.com/bands/default.cfm?bandID=738517&content=music The Geoff - blame Caevan!!!
J. Dan Posted August 20, 2008 Author Posted August 20, 2008 Thanks - I've now posted there with a little more detail. I overlooked it as I don't frequent the guitar side of the forum that much. Dan Acoustic/Electric stringed instruments ranging from 4 to 230 strings, hammered, picked, fingered, slapped, and plucked. Analog and Digital Electronic instruments, reeds, and throat/mouth.
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