What's the most common cause of parasitic oscillation?

[cgoodson]

I would like to get rid of the extra caps on my power tubes(snubbing caps?). If I disconnect them it really seems like I get parasitic oscillation(a major drop in volume). When I built this amp(Vibrolux 864AA in what I think is a '73 Super Reverb Chassis) I read some info on lead dress and such but apparently I still didn't do something right.

So I was wondering where did I most likely go wrong? It seems to me that there are probably a certain area/s that are the typical source of trouble.

BTW:Is this even worth the effort? How much difference do those caps make in loss of tone?

Also, I've noticed many Fenders have a certain group of wires wrapped with a wire that is grounded on one end I think, I didn't do this, should I have?

[dai h.]

you have a wire interacting with another wire, one radiating signal, then reinforcing the signal on the other wire too much where it leads to oscillation. So you want them to interact less. So I would try moving away, plus putting one or both flat against the chassis (capacitively couples more with chassis which is ground which means less coupling to the other wire. Putting two parallel wires down flat on a ground plane makes them couple less to ea. other.), protecting a sensitive wire with shielding (use shielded wire--this is like putting the ground all around the wire--the copper wire provides electrostatic shielding--shielded wire has a capacitance between hot and shield and this capacitance reduces the electrostatic, or capactive coupling betwee noise source - shield - hot, since the shield makes for a series capacitance and caps in series is less capacitance. Some wires such as the output tube plate leads can have especially strong radiation (it has the output signal on it), so make sure it's flat to chassis, possibly route along corner if able (two sides of ground against wire bottom and side instead of just bottom). Same thing happens as mentioned above with alu sheeting stapled or glued on combo or head innards, so put the "lid" on before assessing. Conversely, if two wires do not reinforce but cancel out signal (such as a grid and plate lead), you could try moving closer together away from chassis (giant ground plane) so they couple together more. Some Marshalls for example have the plate wire wrapped around the grid wire, or use a shielded wire with the shield connected to the plate using the shielded wire as a "capacitor" which couples the output back to input a little (you can also just use an actual cap which is probably safer). Putting a grid stopper close to the tube pin can help, such as for inputs (might see this on high gain amp mods to help contribute to stability) and output tube grids. Just some ideas off the top of my head.