Motorboating question - help me figure this one out!

[frequencycentral]

I just finished building another of my Murder One tube amplifiers, this one's for a friend. It powered up just fine but it's got a strange quirk that I can't figure out.

If there's no guitar lead in the input it motorboats at maximum gain/volume. Once I plug in a guitar lead, even without a guitar connected, the motorboating stops. Weird. This is my third Murder One, I haven't had this problem before. The amp is functioning normally apart from this weird quirk.

The only things different is that I'm using a 7327 tube instead of a 6111, and I'm using stereo jacks from Banzai (with the ring unused) that I've never used before. The fact that the motorboating stops when a lead is plugged in, even with no guitar connected makes me think it's the jack that's at fault. Can anyone shed any light on what's happening?

[R.G.]

Isolate your preamp section B+ from the power section B+ with a 10K and a largish filter cap on the preamp section.

[petemoore]

  How about an input jack that connects the input to ground when the plug is pulled?

[kurtlives]

No grid leak resistor?

[petemoore]

  The pot does that.

[frequencycentral]

No grid leak resistor?

  The pot does that.

Yeah, Pete's right the pots are grid leak resistors. What I can't figure is why it should stop with an unconnected guitar lead plugged in - what's different?

[slacker]

Could you have wired up the jack incorrectly so that some connection to ground is only made when the jack is inserted?

I did that recently using a stereo jack where the sleeve was supposed to ground the chassis. I accidentally connected the ground wire to the ring instead though, so the ground connection to the chassis is only made when the jack is inserted. No big deal but it confused me for a while.

[R.G.]

What I can't figure is why it should stop with an unconnected guitar lead plugged in - what's different?

A guitar cord is a -capacitor- .