just got a '78 MKIIa need some help, kinda long

[jdandry]

I have a new to me, MKIIa 100/60; EQ and reverb . It needs some work on it. Their are a couple of problems . first, when the pull lead is engaged and drive up and the vol 1 is up to get some overdrive it is sounding like a gate is on. Noise/hum after the envelope of the note. The sound is a mushy, farty overdrive , kinda harsh also. Could this be old CAPS...speaking of CAPS, looks to have the original Mesa caps. I should change those out for fresh ones? Should I do all the electro's, bias, cathode caps (12ax7's), filters, ect? BIAS, on the bias/cap board it looks to have a trim pot. Small blue/white thing with small slit in the top. I know Boogie usally have a fixed bias resistor, could this be a mod or what do you think? TUBES.. the amp has Sovetk 5881 and all Ruby 12AX7A CZ select. I have loads of mullard, RCA black plate, Brimar, Amperex 12AX7s. Any one have any suggestions of what would work the best in this amp? Is V3a in the gain / non reverb part of the amp and V3b the reverb driver? or V3 a all signal and just V4 for reverb. One more thing.. I am only giong to use the amp in the 60watt mode. I am thinking of trying some NOS RCA Black Plate 6L6CG but i only have a pair ot used. I will replace only the two OUTSIDE 6L6s right and would this be a good choice,(sound killer in a BF Fender!). I hope the blue/white thing is a adj. bias! I may have more questions if you guys don't mind helping me. My reverb is shitty, speaker suggestions (G12H100 now) and other stuff. Thanks in advance!- John

 

[RussB]

If it were MY amp...

I'd replace all the electrolytics, clean and tighten the tube sockets, replace the grid resistors (470 ohm), clean all the jacks and noisy/scratchy pots.

Sounds like you have a bias pot in there. I'd chuck them Sovteks and put in a good set of tubes. Sounds like you have some nice 12AX7's too...try 'em out and hear what it is 8)

Don't count out that Celestion! Them are nice sounding, underrated speakers. Big Bass, nice crunch.

welcome to the forums!

 

[jdandry]

if I use the amp in the 60watt only with just the two outside 6l6 working will having the unmatched sovteks in the middle effect the bias on the outside tubes being used if the bias is set on the outside pair? and I know boogies like -52v or so, is this any different when biasing for 60watt use instead of 100w?

 

[Boogiebabies]

I think the cathode caps should be fine. They are 25V caps and only see between 2 and 3.5V in the whole preamp. If they are tants, I would say leave them, but electrolytics may need a change. You would probably do more damage trying to resolder them or desolder them. You should be able to test them to ground with a capacitance meter. If a 22uf is putting out 19-24uf I would bet they are fine.