Biasing trick for a Rivera R-55 ???

[H Chandler]

This amp has an interesting "failsafe" feature where the first speaker jack is switched so that when the plug is pulled, the cathodes of the power tubes are lifted from ground.

Well, has anyone else thought of inserting a 1 ohm resister somewhere in the circuit for measuring the bias current?

Is this a well known mod or just a brain fart?

Thanks

[Enzo]

Inserting the 1 ohm resistor in the cathode lead is hardly a new trick. The cutout on the jack I think is a good idea. One does not need you to remove the other. Add your resistor between the cathodes and the jack cutout. That assumes the jack was the ground ultimately. If the jack is not the ground, then insert the resistor ANYWHERE between cathode and ground.

[Alex/Tubewonder]

This amp has an interesting "failsafe" feature where the first speaker jack is switched so that when the plug is pulled, the cathodes of the power tubes are lifted from ground.

Well, has anyone else thought of inserting a 1 ohm resister somewhere in the circuit for measuring the bias current?

Is this a well known mod or just a brain fart?

Thanks

It's "fail safe" only half way. What if you pull the plug out of the speaker cab?

[H Chandler]

Sorry for not being clear

I guess I ought to do a bit more thinking before I dash off a message like that.

Sure the 1 ohm R from cathode to ground is not new. Finding a way to do it neatly on a PCB is a bit of a trick though. But that's not what I was thinking.

What I was thinking - and not saying - was what if there was a way to put the 1 ohm in a box with a stereo plug, and a couple of terminals for measuring the bias current through the speaker jack.

I need to take a closer look at the jack though.

Get back with you later.

Howard