Help identify head

[jackryan]

Trying to replace the tubes in my head, couldn't find the exact model, serial number matches the mark 3 era
The old tubes are GT6L6B and recently bought some JJ6L6C tubes. Noticed they have a different amount of pins and don't know what the exact tube to buy would be, any help would be appreciated

 

[Guitarfreak]

I had an '86 MK III looks accurate. It had Mesa 6L6GC (STR420?) Not sure why there are different amount of pins on the tubes you got but JJ is quality. Look up the tube numbers. Not sure if the manual on Mesa site has that spec on there. Did you try them yet? Judging from the Ser# looks like an '85. Great condition.

 

[jackryan]

I had an '86 MK III looks accurate. It had Mesa 6L6GC (STR420?) Not sure why there are different amount of pins on the tubes you got but JJ is quality. Look up the tube numbers. Not sure if the manual on Mesa site has that spec on there. Did you try them yet? Judging from the Ser# looks like an '85. Great condition.

I didnt put the new tubes in cause I'm new to replacing tubes and don't wanna kill the head haha. I replaced the preamp tubes which were the "original" mesa tubes, still had the Mesa stamp on them. From what I found in other forums/posts that the 'B' and 'C' mark are just from iteration of the tubes and they should work find, but I also found a chart that said the groovetube 6L6B are the same as the Sovtek 5881/6L6WGC, but from the images of those they also don't have 8 pins. Another user on this forum mention that old manufacturers just left the the pins in and 2 of them didn't connect to anything.
Will there be any harm in putting in the wrong tubes to try them?
Here's some pics of the new and old tubes from the pin layout

 

[Guitarfreak]

You wont kill the head. If you want to go with original spec then buy them from Mesa or find the same P/N. IF the old tubes are still working put them back in and play the amp. I would call them and they will sell you the same thing.

 

[Guitarfreak]

Wish I would not have sold my MK III head and 480W 4x12 EV/BS speaker cab. Really a great amp. I never turned the master over 3.5-4 it would drown out the heaviest hitting drummers.

 

[electric mayhem]

I didnt put the new tubes in cause I'm new to replacing tubes and don't wanna kill the head haha. I replaced the preamp tubes which were the "original" mesa tubes, still had the Mesa stamp on them. From what I found in other forums/posts that the 'B' and 'C' mark are just from iteration of the tubes and they should work find, but I also found a chart that said the groovetube 6L6B are the same as the Sovtek 5881/6L6WGC, but from the images of those they also don't have 8 pins. Another user on this forum mention that old manufacturers just left the the pins in and 2 of them didn't connect to anything.
Will there be any harm in putting in the wrong tubes to try them?
Here's some pics of the new and old tubes from the pin layout

Pins 1 and 6 are not used in a 6L6GC which is what that amp came with in all 4 sockets if the amp is non-simul-class. The only practical way to tell if it is simul-class is looking at the output transformer which will say simul-class if it is, in which case the outer tubes were designed for EL34's (same pins).

The tubes you are pulling out with dummy pins is more the concern with old Marks since the factory railed up unused pins for stability.
My guess on the date is early 1987 putting it in purple/red stripe time frame.

 

[electric mayhem]

To follow up with just a little bit more and since this is your 1st boogie, when you have 4 power tubes in a Mark IIB through the Mark III blue stripe (which includes your amp) it is either 60/100W or simul-class Class A 15W/Class AB 75W. For the normal 17 inch chassis the switch on the back right can typically be trusted.

You have what is called a long chassis or 15 inch or Big Brother if it specifically is in a 15 inch combo. You have a 1/2 power switch on the front which works as a hi/lo power switch but tells you nothing about your power section.

Intuitively you would think 60/100W is closer to half power but intuition will fail you here.

To see the output transformer stamp, the only sure way to know what you have, you need to pull the chassis. If you pull the chassis you will also see in marker on 1 or both sides of the chassis the intended options for that specific amp and the 1st letter is the power section option- S, H, D, K and in your case either H for hundred or D for simul-class. You'll have an R after that for reverb and a 2 number code for the week the amp was started.

On the bottom of the chassis there is usually a finish date in sharpie. If there is any doubt good clear pics of the preamp board should be enough for some of the trained eyes on here to affirm for certain if you have a purple stripe or red stripe. I don't have many 18xxx serial numbers in my reference archive so I am curious if pulling the chassis doesn't bother you.

To pull the chassis I would keep the power tubes out, amp unplugged, again amp unplugged, disconnect the fan and reverb RCA connections noting the white one to the back, unscrew the top 4 screws and grabbing the transformers slowly pull back lifting up ever so slightly, watching for snags on the tolex or front grill.

Keep fingers away from the big capacitors but do look at the ends of them for bulging. Look at the date code for the big capacitors if you can see them. The code is YYWW, e.g. 8642 for 1986 42nd week, usually seen towards the bottom of the print. This date will tell you if they are the original caps or if they have been replaced and if so when they were replaced.

Enjoy!

 

[allbutromeo]

It has a black dot over the power outlet, and the first pot is labeled Volume 1 instead of just Volume means its a 1985 "Black Stripe." The serial#s are weird, I've seen 1986 Purple Stripe combos in the 17xxx range. If it went all these years with 4×6l6s in it then it's a 60/100w, the new tubes should work, but if they not in the right spec range for Mesa they won't last as long in high volume playing.

 

[electric mayhem]

It has a black dot over the power outlet, and the first pot is labeled Volume 1 instead of just Volume means its a 1985 "Black Stripe." The serial#s are weird, I've seen 1986 Purple Stripe combos in the 17xxx range. If it went all these years with 4×6l6s in it then it's a 60/100w, the new tubes should work, but if they not in the right spec range for Mesa they won't last as long in high volume playing.

Of course you could be right but I would put zero faith in a long chassis face plate for dating anything.

I would also put zero faith in it being 60/100W based on tubes someone else had in an amp.

As for a no stripe- 18xxx? Really? Sure anything can happen but I see no black dot, line or X. I do see the faint markings of what to my bad eyes looks like sc written over the power cord- like simul-class.

I'll put 2% faith in it being simul-class and 'sc' was the only outward marking of it that someone added.

But why guess when the answer is pull the chassis and look?

 

[vrdyer]

Get the numbers off of the transformers. That should give you the year of production. Then contact Mesa support services.