Is my Mark IIB in good shape and has it been modded?

[JCZxyZ]

I feel weird asking for help without being a contributor, but I haven't been able to find much in comparative pics and I don't know much about tube amps anyway. I would be very much grateful if somebody here could help me.

It is the 60/100 simul class version and the date on it is 1980.

It appears that the filter caps (?) have been replaced, although I'm not sure with what except that one says "J/J" and "500v"

The tracers are brown, but I'm not sure if it's dirt or rust.

The tubes in it are electro harmonix and I have no idea if they were biased and matched or not, but they don't seem to be running too hot or cold just from sticking my hand back there. I plan on replacing them anyway in the future.

I didn't buy this amp for the sake of having or looking at, I really plan to "punish" this thing in the years to come, assuming that it has the health to withstand it. My mini recto and DC-5 each get at least a good 20 hours a week at high volume, and I'd like to think that this thing will survive in the mix with them.

I was rather worried when I first got it and looked inside, but it fired right up and works fine. Guess I'm wondering if you should judge a book by it's cover when it comes to old amps. I've put countless hours of punishment into my DC-5 and it still looks brand new inside. Maybe someone left the Mark IIB in a hot garage for a long time?

On an unrelated note, I had assumed from all the pics on the internet that old marks were full size heads, but this thing is small and blocky, not even twice the size of my mini recto. How did they fit that much power into something so small :shock:

And just for the heck of it, the cleanness of my dc-5 :mrgreen:

 

[b00g13mk2B]

Looks stock except for the caps, which is normal to have replaced after that many years. J/J are the brand of electrolytic caps. I changed all of mine, including the thin silver ones on the rectifier board and the smaller electrolytics on the preamp board, while I was changing the rectifier caps.

If it still plays well, let her rip. If you are worried, have a good tech check it out. Can only image some cleaning, lubricating of the pots might be needed. They were made to be played hard and simple to repair.

What speaker do you have? Depending on the storage conditions, the cone could dry rot or degrade, but not necessarily.

 

[JCZxyZ]

Thanks for the reply. It's in head form so it doesn't have a speaker. I guess I'm just worrying because the amp is old. Every time I crank it up I'm like "Woah, wait! This amp is 34 years old! It's going to EXPLOODDE!"

I guess I'm just too used to modern electronics? I'd hate to blow it up one way or another when it might have otherwise gone on for another decade or two in a limited role... but then again if the spirit of old boogies amps is to die in battle, then maybe I should just go for it.

 

[b00g13mk2B]

Go for it. Caps are easy to change if needed again in a few years. If an output tube goes, screen resistor change. Should be able to find transformers, IF they would ever go.

 

[McBarry]

Looks totally stock in there except for 2 things.
The JJ caps as mentioned.
And possibly the treble cap at the far end of the preamp board behind the treble pot. It's not silastic-ed to the adjacent cap - either the silastic has given way, or it's replaced. Not a biggie though.
Wiring all looks completely original.
Bias caps are original and I personally replace them. And I'd do the two on the GEQ board too.
Personally, in no particular order, I'd check B+ resistors for value - all of 'em - and as mentioned, clean/lube pots/jack and go for it.
I'd also check for DC on pots to check coupling caps are OK (though you'd hear the crackle if they weren't), clean/re-tension sockets (especially the 6L6 sockets as they get bloody hot and the metal fatigues as the decades roll by...), and chase a signal to see what's going on.

Looks a nice amp though - congrats !
Dave

 

[JCZxyZ]

Thanks for all of that information and advice, McBarry. I put a demo up in case anyone wants to hear this amp in action:

http://youtu.be/gxt-IpqYnAc

 

[MrMarkIII]

Just FYI, your amp can be a 60/100, or it can be a Simul-Class, but it can't be both.

 

[JCZxyZ]

That's good to know, MrMarkIII. Do you know which tubes it switches off when you set it to 60 watts? I re-tubed it just yesterday but of the four mesa tubes I bought one of them just couldn't go in any of the tube sockets at all but went in my DC-5 no problem while the tubes from the DC-5 fit into the MKII no problem as well. So I put the new tubes in the middle and the dc-5's several months old tubes on the outside. Not sure if that's really a good thing to do.

 

[MrMarkIII]

Inside pair and outside pair, as you have done, should be correct. I think the outside pair is switched out when running at 60 watts, with a 50% chance of being right.

On a Simul-Class, the outside pair is 15 watts, and the inside pair is 60 watts, so a Simul-Class IIb is 75 watts total. I had a 60/100 Mark III for a short while, but just can't remember which tubes did what.