Tube DIY Asylum

Posted by hifipaul (A) on January 28, 2025 at 10:33:06

I got a tap on the shoulder about a burnt out MV-52 and a conversation ensued about repair possibilities. In short it was a basket case, good for parts. I ended up in possession of it and proceeded to take it apart and see what I had. Turns out I had a good power supply, output xfmers, and chassis.

What's wrong......Too much smoke escaped from the circuit board to be reused......Also, over time the EL-34's would go into thermal runaway; 1 at a time. The remaining EL 34's looked good and tested good, new sets were installed; but, the problem persisted......My conclusion was that 525v B+ is too much for EL 34 screen grids, leading to their early demise.

On to the redesign. I chose KT-88's for the output tubes since their screens can handle the 525 volts. I strapped them in triode cuz I like the sound that way............For gain/driver stage I went with the Williamson circuit. To me, it sounds good and is straight forward to implement. The front end tubes are loctal 7n7's since the sockets fit nicely in the existing chassis holes........A 10kohm pot was installed in the feedback loop to dial it for best sound; which changes from speaker to speaker, not by a lot though.

I like the sound the way it is now. In the past I've never liked UL or EL34's or MV 52's for that matter. It strikes me odd that CJ would run the EL34's so dam hard that they burn out, but that's the way it goes.

As a side note, the circuit burned up because the owner spilled coffee on it. It seems that caffeine and electricity don't get along.