My bass player just scored a free jmp mk2 master volume head!!! I am going to help him make sure it is workng correctly and fix it up if needed. Heres what I found odd (Possibly Awesome odd that is), when I asked what power tubes are in it he took a look and said KT66. I didnt think that amps from the mk2 master volume came equipped with kt66s, so I am hoping someone didnt just put them in there in place of the typical el34 or 6550 or whatever came stock on this specific model.  Anyone familiar with this era of Marshall? (I am willing to bet a testicle that someone on this forum is)

Post pics!

EL34's would have been stock but the amp could have been modded to accept KT66s. IMO if it is the amp I think it is KT66s would not be the best option for that amp the EL34s would be. Not saying it is but I have seen amps built into old chassis before therefore it may not be a JMP mk2 at all.

I've never heard of stock KT66s in the MkII Master Volumes either. I've had a couple over the years and they came with EL34s.
In fact I still have one now.
It may be a good idea to get the amp fully checked out as other components may have been "tinkered with" over the years. I'm sure you friend has already considered this.

BTW which model is it?  2203, 2204 etc...
How does it sound? It may be great with the 66s!
A good score for free, anyway you look at it.  

I actually prefer these amps to the later JCM800s as they don't seem to have the awful ice-pick treble that the 800s have.

This amp was never fitted with KT66s stock. EL34 (also 6550 in the States) only.

Thanks for the replies all! I was pretty sure that those era of amps were el43 or 6550s. Did anything differ between the amps that were shipped with el34 vs 6550s? My guess would be different versions of transformers.  I would think that using the wrong tubes than the amp was made for could cause posssible damage. Anything specific I should look out for. I am pretty familiar with the inside of a 2203 and I dont think this amp should differ too much. I have just seen a few pics of the amp and we are going to open it up later in the week

I'm pretty sure el34s and 6550s are direct replacements.

On the subject of 6550s, just a question.  Aren't they an American version of the KT88?  I haven't ever used them but I remember reading that.  Sorry to digress.

got to take alook at the amp today. The tubes are groove tubes and say 6l6 then beneath that say aka kt66 aka 5881   What kind of lie is that!  Anyways, probably going to throw some el342 in there and add some caps here and there and a few other tweaks!   Nothing that isnt 100% reversible of course.

As far as I know, Kt88s are the "big brother" of the 6550s. More headroom

05702M is the s/n of the amp btw. trannies are original and state el34 or 6550!
And It is a 2203 100 watt master volume

The transformers were the same regardless of EL34s or 6550s. The only circuit difference I can remember was that one of the resisitors in the biasing chain was a different value for the export models with 6550s.

So I opened the amp yesterday and cleaned out all the cobwebs,rust/corrosion and dirt   Then retubed and fired it up. Alls I got was a weaksignal/quiet volume.

THere had been minimal work done to the component board that included a shielded wire from v1 pins7(that the ground was connected to pin 6) to the v2 cathode resistor. So I replaced the shielded wire with a new better quality one and upgraded the cathode resistor. The gain pot was intermittent and when turned up past 7 was squeeling/motorboating.
Replaced the gain pot. Now the amp is alive and well!!!  I also changed the filter caps due to them being around 30 years old. Next is changing the bias resistors to be more compatible with the el34s I put in there. Then playing with the cathode resistors to add some gain and bite. Everything seems to be going well so far! I am glad I was able to get it up and running! SWeet!!

Awesome man!  Post some clips if you can.  

Actually, Ive got some kind of digital recording ummmm device at my house right now. Ill see if I can figure it out. Ill also try to post some of my metro 2203 clone. Trying to nail the GNR AFD tone with that one. Gettin close!

No, bias settings are different hence the resistor value change on the 6550 export models.

I changed the resistors to acommodate el34s. I believe the resistors are 1 27k(from 15k) and 2 220k (from 82k'S) for el34s. I piggybacked 2 56k's in place of the 27k..I was out of 27k, hope this is ok.  I am having trouble setting the bias. My multi meter is on millivolts and I measure pin 1 and get a reading of like 2.1. Im not sure if I am doing something wrong..  The amp sounds great though.