I just rewired the PI and output section of my V18 to Vox AC30 values and was checking for solder bridges and open solder joints when I found that they miswired the 470k grid loads on the EL84 output tubes (these resistors mislabeled as R71 and R98 on the schematic would be the bias splitters in a fixed bias amp).
The grid loads are normally wired at the junction of the coupling caps from the phase inverter and the grid stoppers going to the output tubes (as is shown on the schematic). However on the circuit board itself the grid loads are wired directly to the grids. So that the signal from the coupling caps goes first through the 10k grid stoppers (mislabeled on the schematic as R73 and R96) and then to the grids, with the grid loads wired to the grids.
Obviously in a fixed bias amp a mistake like this could be a total disaster but it kinda works in a cathode biased amp so evidently no one else has noticed it... The fix is simple- pull out the old grid loads and rewire the new ones so that lead nearest the grid stopper is connected to the *other* terminal of the grid stopper. There is plenty of room for that "tack joint" on the component side of the board.
Steve Ahola
P.S. If anyone here can enlighten me as to the effects of the miswiring the power section, it would be appreciated! I'm going to correct it right now because I'm trying to make this amp more like what we call "a real amp"...
Here's a link to the V18 schematics (I still need to add 12 corrections to the PI and power amp section):
http://www.blueguitar.org/new/schem/...d_drawings.pdf
P.P.S. I've attached a photo of the "fix" for this problem- note the blue heat shrink tubing on the extended leads from R45 and R46. BTW the identifying marks on the audio board are "OASIS 03V-0 E99661 99V-0 0726" (If your numbers are different your board might not have this problem.)
The grid loads are normally wired at the junction of the coupling caps from the phase inverter and the grid stoppers going to the output tubes (as is shown on the schematic). However on the circuit board itself the grid loads are wired directly to the grids. So that the signal from the coupling caps goes first through the 10k grid stoppers (mislabeled on the schematic as R73 and R96) and then to the grids, with the grid loads wired to the grids.
Obviously in a fixed bias amp a mistake like this could be a total disaster but it kinda works in a cathode biased amp so evidently no one else has noticed it... The fix is simple- pull out the old grid loads and rewire the new ones so that lead nearest the grid stopper is connected to the *other* terminal of the grid stopper. There is plenty of room for that "tack joint" on the component side of the board.
Steve Ahola
P.S. If anyone here can enlighten me as to the effects of the miswiring the power section, it would be appreciated! I'm going to correct it right now because I'm trying to make this amp more like what we call "a real amp"...
Here's a link to the V18 schematics (I still need to add 12 corrections to the PI and power amp section):
http://www.blueguitar.org/new/schem/...d_drawings.pdf
P.P.S. I've attached a photo of the "fix" for this problem- note the blue heat shrink tubing on the extended leads from R45 and R46. BTW the identifying marks on the audio board are "OASIS 03V-0 E99661 99V-0 0726" (If your numbers are different your board might not have this problem.)
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