AO-35 to 18 Watt Lite 2b(ish) help
Posted: Wed 04/15/26 8:11 am
Hello All
I converted an AO-35 to a CG and it worked well, but I broke a tube socket tab while trying to make some modifications so I decided to strip the whole thing and start over. I'm only reusing the chassis and transformers, everything else is new including tube sockets. I also changed to a horizontal turret board. It's wired to the Lite 2b(ish) schematic and I've quadruple checked everything. I used shielded wire from the input to V1 and from the 0.0047 uF cap to the volume pot. I left the same rectifier socket and I am using a 5AR4 for a tube.
This is were things get weird. With the tone knob fully off, everything seems to work great. Volume works perfect, guitar sounds great, although with no treble. The A, B, C voltages are about 10V higher than posted on the schematic, but everything else is within 5V and I check bias and I am at 95% for one EL84 and 90% for the other.
However, when I turn the tone knob anywhere but fully off, I get terrible distortion when playing guitar, the volume knob doesn't seem to actually increase volume, and the B+ voltage climbs about 30V when the volume is turned up to about 80%, then reduces to about 20V up at full volume. I tried changing the rectifier tube to a 5Y3 to reduce voltage but have the same results. Tried swapping all the tubes, same results. I pulled the tone pot out of the circuit and still same results. I've tried chop sticking things around and can change the hum a little bit by moving the preamp and PI wires but voltage stays 30V high when volume is increased. I did leave the original wire dress from the OT and it comes out under the pots and runs directly under the board parallel to pretty much everything. If I move the OT to plate wires around a little (only have about 1/4" I can actually move them) I did get the voltage to drop about 10V from the 30V increase.
I think I'm going to try adding wire to the OT to plate wires and try rerouting them around everything as a next step. Any other thoughts to try?
Regards
Troy
I converted an AO-35 to a CG and it worked well, but I broke a tube socket tab while trying to make some modifications so I decided to strip the whole thing and start over. I'm only reusing the chassis and transformers, everything else is new including tube sockets. I also changed to a horizontal turret board. It's wired to the Lite 2b(ish) schematic and I've quadruple checked everything. I used shielded wire from the input to V1 and from the 0.0047 uF cap to the volume pot. I left the same rectifier socket and I am using a 5AR4 for a tube.
This is were things get weird. With the tone knob fully off, everything seems to work great. Volume works perfect, guitar sounds great, although with no treble. The A, B, C voltages are about 10V higher than posted on the schematic, but everything else is within 5V and I check bias and I am at 95% for one EL84 and 90% for the other.
However, when I turn the tone knob anywhere but fully off, I get terrible distortion when playing guitar, the volume knob doesn't seem to actually increase volume, and the B+ voltage climbs about 30V when the volume is turned up to about 80%, then reduces to about 20V up at full volume. I tried changing the rectifier tube to a 5Y3 to reduce voltage but have the same results. Tried swapping all the tubes, same results. I pulled the tone pot out of the circuit and still same results. I've tried chop sticking things around and can change the hum a little bit by moving the preamp and PI wires but voltage stays 30V high when volume is increased. I did leave the original wire dress from the OT and it comes out under the pots and runs directly under the board parallel to pretty much everything. If I move the OT to plate wires around a little (only have about 1/4" I can actually move them) I did get the voltage to drop about 10V from the 30V increase.
I think I'm going to try adding wire to the OT to plate wires and try rerouting them around everything as a next step. Any other thoughts to try?
Regards
Troy