Trainwreck Express oddity?

[audiosalvage]

I have a trainwreck clone that I built last year. I've tweaked it to my liking but one thing remains. Under moderate to heavy overdrive, as the note or chord decays, there is a phase shifter type sound that follows. It's sort of UNIQUE and I don't mind it but I'm wondering what causes this.

[gktamps]

That's not normal.
Mind posting photos of your wiring?
Have you done any tube substituting? Tube testing:

[romberg]

Under moderate to heavy overdrive, as the note or chord decays, there is a phase shifter type sound that follows.

Do you have a "fizz" cap across the plates of your PI? I had one of my express builds generate very short (less than 1/16th a wavelength) bursts of noise right at the point the power tubes started to clip. This noise was most apparent when notes/chords decayed. My fix was to bump up the value of the fizz cap which made the noise vanish. Don't know if this was just patching another problem but that fizz on decaying notes no longer bothers me.

Mike

[stephenl]

This might be sacrilege, but it's easy enough to try and undo, add grid stoppers to the PI grids. You can go reasonably big without a noticeable effect on treble - just use higher wattage, metal films to induce the least amount of noise. The Express has plenty of treble anyway.

Merlin Blencowe discusses adding a big stopper in front of cathodynes to reduce "nipple" distortion, which can cause a swirling effect in addition to some nasty distortion. I also think I remember him mentioning that LTP's can also suffer from this effect and benefit from grid stoppers.

I've only built two Wreck type's - an Express and a Concorde, so I'm no expert. With both, I spent a lot of tweaking noise issues on the PI and was never really satisfied with either one. The clean to mean tone in these is glorious, but I had a difficult time using them because of the deafening levels at their sweet spots.