Fender reverb of AB763 on 10. ??

[Raybob]

I never run Fender reverb over about 2 on the knob. Never realized until a customer complained about it, but turning the control to 10 not only adds much hum but seems to cause power tubes to glow blue at base. I checked one, seems to do just that, also pulls both screen and plate voltages down on power tubes. Is this normal or did I get a bad batch of tanks last time I ordered?

[potatofarmer]

Sounds like oscillation. Have you scoped the output or measured AC voltage at the speaker while you do this?

[Tom Phillips]

I concur with potatofarmer. That behavior is definitely not normal.

[Raybob]

Not yet, moving it to bench with scope/gen now (later) will see...

[tubeswell]

What is happening at the PI when the power tube voltages take a nosedive? (And is it the power tube(s) on one side only (hint, hint)?)

[Raybob]

What is happening at the PI when the power tube voltages take a nosedive? (And is it the power tube(s) on one side only (hint, hint)?)

Need to clear crap on bench with scope still. Meantime, what's going on with PI as I bring up reverb is plate on left, v6b, takes at least 10v dive at point of this happening (around 4 on control). Plate on right gradually lowers, not as sharply as other. Both tubes glow blue near base but left glows more.

[Tom Phillips]

It would also be useful to monitor the overall B+ supply voltage when the problem occurs. If there is a oscillation causing the power tubes to draw more current then the B+ will be sag and all the other plate voltages in the amp will follow. Therefore, when you measure a drop in one spot, such as the plate of the PI, it is only meaningful if it is localized to that circuit and not just proportional to a drop in the B+ rail.

[Raybob]

Found it. Using a multi-tap tranny with impedance switch. Had secondaries too close to v4 reverb driver. Re-routed them my old way and oscillations are gone. Was trying to do an esier, shorter path, but didn't work. Need to correct my BMR I just put same tranny into not too long ago. Has same problem but never notice, never turning reverb past 2. Yea. Man, that was baffling.