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previous: lion I'm planning my next DIY amp projec... -- 9/15/2000 3:26 PM

Re: 4xEL34 cathode bias ZNFB - done before?

9/17/2000 5:47 PM
Wild Bill	Re: 4xEL34 cathode bias ZNFB - done before? Hi Lion! Sounds like a great experiment! I'd love to hear how it turns out. Still, before you heat up the soldering iron I have a caution for you, FWIW. Watch out for oscillations, particularly ultrasonic! I've built a few parallel/PP amps and I found that lead dress, especially with the grids can be really touchy. After all, you've got more wire feeding more tube elements! Cathode bias actually seems to help - a bit of degenerative feedback. Using an existing chassis also helps by forcing you into a certain parts layout. NFB swamps out a lot of oscillation problems. Without it grid stoppers and such become even more mandatory. I used shielded wire star-gnded to feed the grids and plates and still had some problems. Ultrasonics can only be seen with a scope but will surely bite you if ignored. Tubes tend to fail early when running fullout with wasted power. To the ear there's no way to tell - the amp can sound great at guitar frequencies! Don't mean to scare you - it can be done! You may even get lucky first try. I've got an amp running 6 6V6's (3 on each side) sounding great and stable as a rock. It did take some work however and without a scope I could never have done it. Mr. Marshall had the luxury of trying layout after layout until he worked out all the bugs. We homebrewers have a harder time of it - you can't keep punching new holes for the tube sockets when chassis is so expensive! Despite my warnings - I bet it sounds awesome when you finish! ---Wild Bill

9/17/2000 5:47 PM

Wild Bill

Re: 4xEL34 cathode bias ZNFB - done before?

Hi Lion!
Sounds like a great experiment! I'd love to hear how it turns out. Still, before you heat up the soldering iron I have a caution for you, FWIW. Watch out for oscillations, particularly ultrasonic!
I've built a few parallel/PP amps and I found that lead dress, especially with the grids can be really touchy. After all, you've got more wire feeding more tube elements! Cathode bias actually seems to help - a bit of degenerative feedback. Using an existing chassis also helps by forcing you into a certain parts layout.
NFB swamps out a lot of oscillation problems. Without it grid stoppers and such become even more mandatory. I used shielded wire star-gnded to feed the grids and plates and still had some problems.
Ultrasonics can only be seen with a scope but will surely bite you if ignored. Tubes tend to fail early when running fullout with wasted power. To the ear there's no way to tell - the amp can sound great at guitar frequencies!
Don't mean to scare you - it can be done! You may even get lucky first try. I've got an amp running 6 6V6's (3 on each side) sounding great and stable as a rock. It did take some work however and without a scope I could never have done it. Mr. Marshall had the luxury of trying layout after layout until he worked out all the bugs. We homebrewers have a harder time of it - you can't keep punching new holes for the tube sockets when chassis is so expensive!
Despite my warnings - I bet it sounds awesome when you finish!

---Wild Bill