trinityamps.com

Your suggestion led me to the solution! The filter caps were grounded at the PT, not on the preamp buss. However, the screen resistor and cap were jumpered to the filter caps. I removed the jumper, and grounded them to the input star ground, and then powered up the amp. After waiting for the filaments to heat up (no standby switch), I turned the master all the way up, and no hum. I then turned the preamp volume all the way up, and no hum. Thinking something must be wrong, fear set in for a moment, until I plugged in my guitar. Perfectly silent. Ah bliss!!!

Henry

My sentiments exactly after I figured why my Spitfire had a hum from hell when I first built it. It's amazing what moving one wire 1/8" away from another parallel wire can do!

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PJM

http://www.diyguitarist.com

Getting back to the original topic...

The 68K resistors are used as mixing resistors in this amp just as they are in most Fenders. So the idea isn't to have them act as grid resistors at all. If you move them to the tube socket, you can enjoy both benefits.

I almost always use grid resistors on the inputs. I learned the hard way when I took an amp to a gig and picked up some religious radio station. Never had that problem before, but it showed up there. That amp got some grid resistors and every amp I have built since get them too. Always soldered right to the tube socket with the shield grounded at the input.

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--s2
www.s2amps.com
www.category5amps.com