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Old 07-18-2010, 01:48 AM   #1
moejama
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Oringinal JCM 800 MK2 how do I tell what tubes

I have a JCM 800 in my attic I thought was burned out. Put a new preamp fuse in it and it works. It has the red/black switches.

I want to get new tubes for it

However it has Sovtek 6v66T power tubes in it. As far as I can tell these aren't the right tubes. So from reading I see it's supposed to take EL34s or 6550s. Are these Sovtek 6V66T's some version of 6550s ? It has 12zx7wxt Sovtek pre-amp tubes.

Also the tubes seem too short for the spring tube holder things so I'm wondering if this was an EL34 amp converted to 6550?

Are the tube sockets interchangeable ? What, in general happens when you have the wrong power tube in the amp if so? I'm not getting as much gain as I want and the amp seems to sort of stop getting louder past 5 ish. I however had the OHM set wrong, just took the cab apart to determine that and it has some old g12-30 Celestions make in England 4 ohm wired in series. Now I see it has the wrong tubes I'm not running it again.

How can I tell for sure what tubes this thing takes?

Also is the S/A number relevant to anything it says X00975 ?

I'm electronically inclined but I haven't messed with tube amps for about 10 years I noticed these JCM 800's have shot up in price so I don't want to screw anything up. Should I just take it to an amp tech. I have no idea what mods if any were done before I owned it.

Tube brand/supplier suggestions would be great too but I can research that on other threads... just throwing it out there since it's a tube replacement question. I'm looking for a smooth higher gain sound, but I have nothing to compare it to since this thing isn't setup right. I do plan to get an OD pedal hot mods seem cool but I worry they could effect the amp negatively.

Thanks for the help guys
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Old 07-18-2010, 02:42 PM   #2
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Maybe you could post some pictures? It could help us ID things and make the best suggestion.
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Old 07-18-2010, 03:49 PM   #3
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Yeah those tubes don't sound right. I would not run the amp until you get it sorted out first as if you mismatch the ohms from amp to speaker cab you can destroy the transformer and possibly other components.

Preamps should be 12AX7 and new ones can be had for pretty decent price. Here's a review of good tubes and you can purchase them there. You mentioned they are a little short but proper 12AX7's should be 2 1/4 inch long from tip to the end of the pins. Tung-Sol's seem to be a good all around tube for the 800.

12AX7 Tube Type Review

Otherwise as Pinto said post some pics of the internals if you can as that will determine if it was set up for 6550's or EL34's for sure. There are a few good brands depending on what you need there.

The sockets should be 8 the pin type. EL34 and 6550 tubes are not interchangeable but can be switched with a simple mod by a tech who knows what he's doing.

Amp is from 1989 as indicated by the X in the serial.

Great amp you got there you lucky bastard!
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Old 07-18-2010, 03:58 PM   #4
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Your best bet would be to take it to a qualified amp tech, your not gonna make that 800 better by the internet.
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Old 07-18-2010, 08:29 PM   #5
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Hey guys.. to clarify the preamp tubes are fine they are 12ax7wxt I just had a type there.

It's the power tubes that are in question they are Sovtek 6v66t and I can't find any info on them.

When I asked if el34's or 6550 are interchangeable I meant physically. Can you plug an El34 into a 6550 socket or vice versa. If so what would happen ?

I thought I had the OHM thing worked out though it's kinda of odd because the speakers appear to all be the same Rola g12-30s, but 3 of them have an orange sticker that says 4 ohm on them and one has no orange sticker and 15 ohm is printed on the speaker from the manufacturer. On the other speakers the orange sticker covers up where the same manufacturer ohm wold be. I will peel one of those orange stickers off but I assume for whatever reason they all say 15 ohm underneath but are actually 4 ohm. I can't imagine why they'd have 4 15 (yea I know it's not 16) ohm Celestian 30s in a 4x12.
I bought this head and cab for 400 bucks about 10 years. The speaker looks like this


Anyone have any idea what up with these orange stickers saying 4 ohm is that normal for there to be a sticker on the speaker? It's definitely wired in series. I'll take it apart and post more pics once I get the camera. This post is getting way to long. I assume that one speaker just didn't have an orange sticker and they were all 4 ohm but now reading more about it I'm wonder if it's just a 16 ohm speaker.


This makes it more confusion.. because why the hell does this thing have 3 of these speakers labeled 4 ohm with orange stickers and one labeled 15 ohms. Did somebody put the wrong damn speaker in as replacement not knowing what they were doing.. Man the plot thickens.

Even though I probably should just take it to a tech I've never had too much luck with them for one and for two the amps works and sounds ok. I has great bottom end just lacks some distortion. To be honest I haven't used it much in 10 years and I didn't play it much even then because I had other amps. I seems I should have worked on just getting this amp to sound good instead of my constantly defective Crate Stealth amp which was a pretty good sounding amp but constantly tore itself apart. I bought the Marshall thinking I'd get it and it would last forever but being an 800 it just didn't have the over the top metal distortion I needed to keep up with a more modern amp. Again.. now there is this whole internet thing I realize that I just needed an OD pedal or hot mod or you know.. the right tubes in it.

Ok.. so here are some pics. If I can figure out the right tubes I'll just go with that solution and if I still feel it's not right or it blow a fuse I'll take it to the amp tech. I'm just working part time now and with an OD pedal and new tubes I'm already pressing 200 bucks. Adding an amp tech charge would start to be like the cost of a nice new modern practice amp, which is ultimately something I need as well along with a new Ibanez since BOTH of mine were water damaged sitting near a ****ty old window (in their cases) in some ****ty rental. I will forever hate that house. I have a Epiphone Les Paul and a new Fender Strat standard I got for 275 with a hard shell case off craigslist. In fact I almost sold this JCM 800 thinking it didn't work and not knowing they had jumped up in price which would have been a huge mistake.

Anyway enough about me...I'm just saying I know the amp tech would be a good idea even if just to give the amp a look over and if I have no other choice I'll do that but I'd rather resolve it myself if I can with any certainty. Here are pics of the tubes, socket and back of the amp. Oh yea but all I have is a webcam right now I can post better pics when the camera comes back. Fact is this amp worked with these tubes, if I remember right, for years though it was never heavily used. I did always think it could be louder but it is only 50 watts.

I guess I might be able to take the amp in to the store and just ask the guy without actually paying him and that might work too but the closest place is 40 min away. I live in the country... sucks.


Thanks again guys
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Old 07-18-2010, 08:35 PM   #6
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Make sure you know what you're doing - if you put the wrong tubes in without bias setup, etc, you can destroy the amp (essentially). You'll probably want to have a tech replace the tubes with the correct ones, bias it, and get a cap job.

It's not cheap (not horribly expensive either), but if you want the amp to last and sound good, it needs done.

(By the way, it's 6V6GT, not 6V66T)

Or you could just give it to me, I can take care of it juuuuust fine, I'd even pay shipping :thumbsx:
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Old 07-18-2010, 09:47 PM   #7
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Ok.. well I looked up cap jobs and I guess since it's been sitting unplayed in the attic for 10 years it probably does need a cap job and if it seems to have weak output that's yet another symptom.

Ok.. well any suggestions on where to get this done? I'm not sure about my local tech. I bought an amp from them and they worked on it half a dozen times at least. It was a a bad model amp but sounded great. Still the guy could never fix the thing to stop it from tearing itself up.

Anyway... is guitar center any good? I live in the country so local options are limited. Should I send it to some amp tech specialist place? Alternatively I can ask around a couple local bands have some kick ass guitar players.

Also should I get any special caps for this thing. I'm looking for smoother higher gain distortion than it has now though I'm guessing most of that will be achieved via an OD pedal.

Anything else I should have done when I take it apart or any special way I should review amp techs? Seems like some of the guys online have really nice services and cheaper but of course I'll have to ship it. Anyone use a service like that?

I'm sure I could do the caps myself but you know since I can't even tell what tubes it needs I guess I should just pay the 300 bucks and get it all done professionally.

So when I get this thing biased I can get any tubes I want in it or what? I read people raving about some KT88 tubes I've never heard of. I'm looking for a strong bottom end with lots of gain basically. I don't want the high pitched whining distortion so much. I'll be playing rock, blues and metal. Just asking though I can research that on my own.

Anybody know a site for different Marshall JCM 800 sounds .. like with different tubes and pedal setups for instance?

Thanks again guy you've been tons of help.
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Old 07-19-2010, 02:25 AM   #8
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

We have a few great techs on this forum... depending on where you are one of them might be close enough to ship it to.
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Old 07-19-2010, 03:16 AM   #9
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Ok there's a lot of info in your posts to sift through so hopefully I can help as much as I understand.

Looks like the tubes you have are related to 6L6 beam tetrode type so they are not at all interchangeable with EL34 type tubes. Both of those types of tubes ARE 8 pin sockets but that's where the similarities end so NO they are not interchangeable. Now whether they should be in this amp only a qualified tech will know for sure once they take a look at the amp. It may have been modified for this tube.

The speakers again are pretty odd, they may have been repaired and made to be 4 ohm or modified, not entirely sure but why is beyond me. MartyStrat54 is a well known speaker guy around here and might shed some light on that. You can take an ohm meter to each speaker and get a reading to tell you what ohm rating each is. If it were me I would replace them with 16 ohm speakers and wire them up correctly. There are threads on this here as well.

I sent my 2204 (similar to your amp only older) off to Jon Wilder, one of the great techs here and he's been awesome! Very nice guy, fast and responsive and very helpful! But that's me. I would get a quote for a cap job and tube replacement and depending on where you live shipping may not be that much but you are assured of a good job being where you live and your limited options. You can take the chassis out of the case and save money on shipping as well. That's what I did with my 2204.

Honestly after being around this forum for about a year now there are some things I've learned not to attempt on amps due to lack of knowledge. There are LETHAL charges left in amps even long after being unplugged so caution is much needed in repairing them.

And since most all 800's came with either EL34's or 6550's from the factory, the fact that it has 6V6's should tell us here that you should send it off to get a thorough check up.

As far as all the different tube types (KT66, KT88, 6V6, ect) I personally don't get caught up in the hype of this tube or that tube does such and such. I would just go with the tried and trued EL34 or 6550 and be done with it. After you start playing the amp again you're gonna forget all about the nuances of what that other tube may have done because it's an 800 and they sound AWESOME!
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Old 07-19-2010, 03:27 AM   #10
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

BTW you're on the right track about putting an OD pedal in front of it as it was the norm for metal guys in the 80's paired with their 800's to get that metal sound. Unless of course they had a tech hot rod it for them.

800's sound awesome in their own right but for metal usually do need a pedal.
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Old 07-19-2010, 03:44 AM   #11
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Is it just me or does the positioning of the components look odd?
Probably just me, and I'm guessing the images are mirrored, but it just looks weird.
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Old 07-19-2010, 06:45 AM   #12
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

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Originally Posted by Jesstaa View Post
Is it just me or does the positioning of the components look odd?
Probably just me, and I'm guessing the images are mirrored, but it just looks weird.
My thoughts exactly when I saw those images. I've attached pics of my '83 2204 which is all stock. Does anyone else have a component layout like the OP's amp?

There's an old thread HERE about changing the tube type in a 2204, maybe it would be of help.

It would be good to see more detailed images of the wiring, moejama
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Old 07-19-2010, 11:44 AM   #13
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Yea the actual layout is reverse of that. It's pretty much just like your amp but the blue capacitor is in on the other side of the preamps... closer to the back of the amp instead of the front. Also I have a DI jack. For some reason my webcam mirrored the image.

I live in Maryland where do I find these amp techs to get a quote ?

They said my amp is an 89 so I guess it's a bit different.

The wiring... like take it apart. I'm not sure that's worth it if I ultimately have to send it to an amp tech anyway. Not even sure how the thing comes apart.

Sooo... amp tech suggestions for Maryland ?
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Old 07-19-2010, 11:57 AM   #14
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

I have no idea who would even think to put 6V6s into a Marshall. On the load they run and the voltage they run at 6V6s would melt the plates in a heart beat unless you mismatch the load up. Sounds to me like a bad/unlogical attempt at dropping the amp's output power.

That being said, I'm located in northern California. My website is down at the moment due to a technical difficulty...waiting on a new hard drive for the server. But I can be reached at (707) 584-8827 if you'd like a quote.
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Old 07-19-2010, 12:06 PM   #15
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

website down because of a hdd... shouldn't your host have a redundant raid setup and all that. My expertise is in computers so I'm just throwing my 2 cents out there. Not particularly web hosting, but really websites being down for maintenance in the days of cloud and cluster computing is no longer common unless it a very small outfit. In my experience that's a recipe for getting overcharged and downtime.

I'll give you a call in a bit have a phone interview coming up first to ready myself for. Thanks !
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Old 07-19-2010, 12:08 PM   #16
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

Quote:
Originally Posted by moejama View Post
website down because of a hdd... shouldn't your host have a redundant raid setup and all that. My expertise is in computers so I'm just throwing my 2 cents out there. Not particularly web hosting, but really websites being down for maintenance in the days of cloud and cluster computing is no longer common unless it a very small outfit. In my experience that's a recipe for getting overcharged and downtime.

I'll give you a call in a bit have a phone interview coming up first to ready myself for. Thanks !
Not a problem. Actually my website is normally hosted by a common home PC...kind of a project I took on just to do it which turned into my business site. Didn't have a RAID array set up on it unfortunately.

Hopefully in the next month or two I won't be self hosting to prevent things of this nature from happening.
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Old 07-19-2010, 02:01 PM   #17
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

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I live in Maryland where do I find these amp techs to get a quote ?...The wiring... like take it apart. I'm not sure that's worth it if I ultimately have to send it to an amp tech anyway. Not even sure how the thing comes apart.
I live in Northern Ireland, So I wouldn't be of much help in finding a tech

However, regarding taking your head apart: it's not as drastic as it might seem. The whole chassis is held in with just 4 (four) scews at the base, the ones next to the feet (if the amp has them). To take those pics I posted earlier, I only removed the rear grille, which is also held on with 4 screws.

The safest way to do this is to make sure the caps are discharged before you start, or discharge them when you open her up. LOOK HERE to find out how to do that safely.
I usually put the amp upside down on a protective surface, remove those eight screws, and the chassis slides out horizontally quite easily. Beware of the fact that it's upside down; you'll see the wiring immediately, but remember there's no support for those valves (tubes) which are now on the underside! I have a protective 'brick' of the right size ready to support the amp on that side.

You may want to do this anyway, if you're thinking of sending it away for service, as it will save on the shipping costs.

Hope that helps
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Old 07-19-2010, 03:47 PM   #18
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Re: Oringinal JCM 800 MK2 how do I tell what tubes

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Originally Posted by core View Post
BTW you're on the right track about putting an OD pedal in front of it as it was the norm for metal guys in the 80's paired with their 800's to get that metal sound. Unless of course they had a tech hot rod it for them.

800's sound awesome in their own right but for metal usually do need a pedal.
Kerry King made it fine with his 2203 and nothing more than a noise gate and a 10-band EQ. Pedals aren't necessary unless you can't drive the amp to its full potential in a residence.
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