Tweet
I've built an amp with a 6dj8 cascode out front, a simple tone stack and a fender style LTPI. It's built in a steel marshall valvestate chassis (the original amp blew up). Looking at the amp from the front the power transformer (an antek pt) is at the right, power tubes next to the left, vibroverb OT next, then preamp tubes at the far left. All tube sockets are bolted to L shaped metal brackets.
Here's the problem- dc leakage somewhere and ground hum and noise. I haven't tried to measure for leaky caps or bad gorunds with an o-scope but using voltmeters I can't seem to find a bad ground or leaky cap anywhere.
The pots are scratchy and just touching the ground lug at the preamp end changes the hum and scratching. The hum goes away completely when I touch the preamp end ground lug. Connecting an alligator clip to that same lug and grounding it anywhere (literally anywhere) on the amp quites things quite a bit. Measuring that same lug to ground without the alligator lead shows exactly 0 ohms. What the heck? All the grounds are solid. I've rebuilt the thing with star grounds, preamp grounds at the left and power all to one lug at the right, input ground at the left, pre in the middle and power at the right and every combo outside that I could think of. I soldered a 16 gauge wire to the back of the pots and grounded it just in case the pots weren't grounded correctly.
I recycled a tagboard from an old oscilloscope and I suspect DC leakage from the ancient board. I'm going to build a new turret board tomorrow to eliminate the recycled board as the problem if it is...but I doubt it.
I don't understand it. I've rebuilt the amp and rearranged the grounds many times. I'm wondering if it's because of the cascode or something. It's not like I've never built a dead-quite amp in a steel chassis before. It's extremely frustrating!
On the other hand the 6dj8 cascode sounds better than any previous 12ax7 cascodes I've built. I chalk it up to having tons of gain with a much lower plate impedance. Route 66 style tone controls, LTPI into cathode biased 6L6's- really makes for a neat sounding amp.
OK, done venting now.
jamie
Here's the problem- dc leakage somewhere and ground hum and noise. I haven't tried to measure for leaky caps or bad gorunds with an o-scope but using voltmeters I can't seem to find a bad ground or leaky cap anywhere.
The pots are scratchy and just touching the ground lug at the preamp end changes the hum and scratching. The hum goes away completely when I touch the preamp end ground lug. Connecting an alligator clip to that same lug and grounding it anywhere (literally anywhere) on the amp quites things quite a bit. Measuring that same lug to ground without the alligator lead shows exactly 0 ohms. What the heck? All the grounds are solid. I've rebuilt the thing with star grounds, preamp grounds at the left and power all to one lug at the right, input ground at the left, pre in the middle and power at the right and every combo outside that I could think of. I soldered a 16 gauge wire to the back of the pots and grounded it just in case the pots weren't grounded correctly.
I recycled a tagboard from an old oscilloscope and I suspect DC leakage from the ancient board. I'm going to build a new turret board tomorrow to eliminate the recycled board as the problem if it is...but I doubt it.
I don't understand it. I've rebuilt the amp and rearranged the grounds many times. I'm wondering if it's because of the cascode or something. It's not like I've never built a dead-quite amp in a steel chassis before. It's extremely frustrating!
On the other hand the 6dj8 cascode sounds better than any previous 12ax7 cascodes I've built. I chalk it up to having tons of gain with a much lower plate impedance. Route 66 style tone controls, LTPI into cathode biased 6L6's- really makes for a neat sounding amp.
OK, done venting now.
jamie
Comment