crackling sound - Marshall Major

[fperron_kt88]

Thanks to all for the pointers! I am back at the Major now with better tools to troubleshoot and all your ideas to shake things up until clean!

(I got diverted to an Expression with VVR + building a wooden cab for it... and found myself completely drenched in swirly, nicely overdriven min7th chords of all sorts - plus my pedal board now fits within a single 500K pot in my guitar )



Ok, now back to work... Ah, yes: now it comes back tome. Must drain those capacitors first!

[fperron_kt88]

Ah! Good repeatable crackling sounds!

The Major has a split load phase inverter feeding a diff. amp that then drives the grids. See http://www.drtube.com/schematics/marshall/1967u.gif even though mine is sort of a mixture between a Bass and a Lead... Go figure...

Anyway, the inverting output of that split-load PI generates a very nasty "blip" that radiates all over the place when it starts missing headroom. I just posted a video of the setup and the scope output (http://ca.youtube.com/watch?v=TRfByCL74rM). The trace on the scope happens to be one of the grids.

So it is P.O. !!!

Now, I managed to find a tube (probably with less gain?) that tames the oscillation. Now only one big blip remains when that stage is driven too far. Still sounds like bullcrap with a guitar though.

I could swap the preamp for a 1959SLP easy, but then I might need some cathode followers to feed the KT88... ???

BTW, there is a guy called riffguy on youtube that posted some amazing stuff using a modified Major. Any idea who this guy is ??? Or what he did to his major ???

[fperron_kt88]

First, this is a beast: 180W CLEAN SINE (53.8Vrms, true RMS into a 16Ohms wirewound, sorry I don't have a distortion meter, used my eyes and ears compared to the input...).

Then, get this: 287W at full bore (67.8Vrms @16Ohms wirewound res, true RMS meter, continuous for over 10 seconds...).

Happy camper. This is a very clean amp though.

Two things needed attention.

First: the phase relationship between splitter, diff amp, grids and plate-screen was backwards. Probably since the OT replacement 10 years or so ago. There was much more clean headroom available by reversing any two of these connection pairs, the beast was riding the wrong side of the PI. That got rid of the nasty clipping, oscillation and weirdness in the sound.

Then the second stage (the one that receives the mixer wipers on the grid AND the feedback signal on the cathode) was biased so hot with the default 470+470 Ohms that the signal was clipped very early-on into saturation. I used a combo of 1.5K+1.5K instead. One side effect of this is I got a working presence pot now