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Had a MB MKIII in here ....unfortunately, off and on for a couple weeks.
With new Svet EL34 tubes and biased correctly (just trust me on that).. the stupid thing would blow the primary fuse after about thirty to forty five mins of gig use... but it would do it two or three times in one night and not sound bad until right at that point.
It would do it regardless if it was in that bogus triode "class A" mode or simul-class.
Drove me crazy until I decided to tear it apart one more time and feed it another fuse or two or three...
I've never done this before, but using my good signal gen at about 250Hz and 100mvac, driving into a hard distortion stomp box gassed up... right at ++ full power output into a dummy load and a boat load of stomp box distortion... the fuse would pop. Every time.
Yet there was nothing wrong as soon as I'd replace the fuse.
What I found was that there was a 600v O'drop 100nF cap, (and not found in the schems I have), that was right at the center tap of the OT and shunting the tap to ground.... which under those hard hitting conditions... yup it would momentarily dead short the entire B+ rail right to ground.
What's weird is that until that moment I never suspected it and right before it would short, the darn thing would buzz like a tiny chainsaw!!
You know "eeyanngGGGG.... click-pfft".
I finally got it to stay shorted with a 6a fuse! ha ha... I know, don't say it....
Anyhow.. never ran into that before and the fix was a new 630v Polystyrene .1uF cap and using a 2w, 100 ohm resistor on each plate to screen node of the fake triode mode, "class A", El34s.
Those 100 ohm resistors are not on the original amp but after that mod, the owner thought that amp never sounded this good in "Class A"... sheesh!
It's wonder I still have any hair.
OK anyone seen that in these MB amps before?
With new Svet EL34 tubes and biased correctly (just trust me on that).. the stupid thing would blow the primary fuse after about thirty to forty five mins of gig use... but it would do it two or three times in one night and not sound bad until right at that point.
It would do it regardless if it was in that bogus triode "class A" mode or simul-class.
Drove me crazy until I decided to tear it apart one more time and feed it another fuse or two or three...
I've never done this before, but using my good signal gen at about 250Hz and 100mvac, driving into a hard distortion stomp box gassed up... right at ++ full power output into a dummy load and a boat load of stomp box distortion... the fuse would pop. Every time.
Yet there was nothing wrong as soon as I'd replace the fuse.
What I found was that there was a 600v O'drop 100nF cap, (and not found in the schems I have), that was right at the center tap of the OT and shunting the tap to ground.... which under those hard hitting conditions... yup it would momentarily dead short the entire B+ rail right to ground.
What's weird is that until that moment I never suspected it and right before it would short, the darn thing would buzz like a tiny chainsaw!!
You know "eeyanngGGGG.... click-pfft".
I finally got it to stay shorted with a 6a fuse! ha ha... I know, don't say it....
Anyhow.. never ran into that before and the fix was a new 630v Polystyrene .1uF cap and using a 2w, 100 ohm resistor on each plate to screen node of the fake triode mode, "class A", El34s.
Those 100 ohm resistors are not on the original amp but after that mod, the owner thought that amp never sounded this good in "Class A"... sheesh!
It's wonder I still have any hair.
OK anyone seen that in these MB amps before?
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