Reverb motorboating

[drewl]

Got an Epiphone EA28-RVT that the reverb will "motorboat" or feedback with the volume all the way up and the reverb all the way up.
It uses half a 6EU7 into a 6C4 to drive the reverb tank.
I tried lowering the gain of both stages to no avail.
Will a 6C4 drive a tank on it's own?
That way I could parallel the 6EU7 for a thicker sound.

The other way around it is to use a voltage divider to lower the reverb after the tank.
Any other ideas?

[Rob Mercure]

Hey Drewl,

I'm not sure what the symptoms you describe actually are. "Motorboating" is caused by insufficient decoupling between amplifier stages that are in phase with each other - a triode's plate voltages are 180 degrees out of phase with it's control grid so any connection from plate to grid would be negative feedback. But add another stage and the ouput of that stage is back in phase with the signal hitting the first control grid so any signal coupling from the second stage plate to the first stage grid produces an oscillator (yeah, I know you know this but I'm just laying a thought foundation for my slow brain this morning) - this RC oscillator usually sounds like a motorboat as it starts cycling and any controls within the loop often allow you to speed up or slow down the "motor boat." If this seems like the symptoms in your reverb circuit the classic cause of motorboating is bad filter capacitors which are easy to diagnose by clipping a known good filter of sufficient capacity and voltage across the suspected bad filter - the problem should immediately cease.

But if you've got a reverberated signal feeding back and howling - not what I'd describe as "motorboating," then your problem may involve other factors besides bad filters (still not a bad place to start looking). Since this circuit controls the reverb level by dividing the signal before it hits the 6EU7 preamp stage and then mixes the output back right into the volume control there are several places where you can easily drop the signal level if needed (hmmm, any chance that your problem is mechanical feedback? Does the tank have a bag over it and a piece of cardboard under it? Is it original? Is the speaker original?). Since you mentioned a desire to free up a triode stage you could change the 6C4 socket to a 9 pin miniature and use a 7247 for the reverb preamp and driver leaving you with the original 1/2 6EU7 reverb preamp available.

Just some Saturday morning ideas,

Rob

[Enzo]

The first and second things into my head were filter caps and filter caps. SOunds like a lask of decoupling to me.

[drewl]

hmm, sounds logical...even to my cloudy sat. morn. brain.
Tried a different tank just to let you know..it's a deep type of feedback which is more akin to motorboating. The frequency of which varies with either vol or reverb controls...like I said with both cranked it makes the noise, back either off and it goes away.

The cap can looks like it was replaced sometime in it's life, but who knows when! Ooh, but it also has one of the original orangish dual caps with the wires sticking out....they usually go bad.

These are great sounding amps, too bad they could be a bit louder with a bigger OT highr B+ they could be screamers!
Thanks guys, have a good weekend

[drewl]

It was a filter cap.
Thanks again.