AO-35 to 18 Watt Lite 2b(ish) help

[ttwjmw]

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

[JMPGuitars]

Hi,

Fixing your lead dress is a good idea. Getting your power tube dissipation to 85% is also a good idea.

Besides that, it's possible there's an error you didn't find when verifying the schematic. For example, if there's a bad solder joint, it might look connected, but is not (at least not well enough).

Did you do a full highlighter test?

Post a bunch of high resolution photos so we can review your build.

Thanks,
Josh

[ttwjmw]

Thanks Josh

I've got it sorted out now. It was definitely oscillation. I moved the OT to plate wires, added shielding from volume pot to V2, shortened all my leads, and moved all the grid stoper resistors from the board to the tube sockets. It's now surprisingly silent.

I've tried a few different rectifier tubes and measured voltages and bias. With a 5Y3, its about 313V B+ and 80% dissipation. It's also has more pronounced hiss. With a 5AR4 I get 354V B+ and 103% dissipation.

I've got a 16V Zener diode I'm going to install and see what I get for dissipation.

Regards

Troy