fly-back diodes causing distortion

[Randy Magee]

I'd been running my Express clone without any fly-back diodes on the power tubes and decided that I should add them for protection. After wiring them in with the striped end towards pin 3 and the other end on pin 1, my amp started to distort on just notes in the higher register. It sounds almost like blocking distortion, but in the higher notes as opposed to the lower notes you'd expect the amp to "fart out" on. The only thing I have wired differently than the original schematic is that I have added external bias points. I clipped out the fly-backs and the problem went away. Does anyone have an explanation for this odd phenomenon?

[VacuumVoodoo]

If you used only one diode per side then probably you used diodes with too low reverse voltage rating. You can try using two in series per side.
IMO using general purpose rectifier diodes for this job is way outside boundaries of "general purpose". It's a specialized job that requires specialized component like a Transorb --just google it.

[Randy Magee]

I had three 1N4007s in a chain like the schematic and layout calls for...

[Tillydog]

...wiring them in with the striped end towards pin 3 and the other end on pin 1...

I'm pretty sure they should be between 3 and 8 (anode / plate and ground) - you might like someone else to confirm this.

Edit2: ^DOH! if you're using EL34s, pins 1 & 8 are connected anyway! Sorry - ignore me.

Andy

Edit - See page 29 of the Trainwreck Pages

If you have bias monitoring resistors, it may be better that the diodes go direct to ground rather than pin 8, but I don't think it's a big deal.