Just got a JCM800 Clone, and now its blowing HT fuses...

[Low_fidelity2100]

So I bought a JCM800 clone a little less than a week ago. The amp was built by Ceriatone, and (supposedly) recently serviced by Andy Fuch, who amongst other things installed a metro zero loss effects loop. Got it from a guy on the gear page, who seems to be reputable, has lots of positive feedback from trades/sales on there.
Amp arrived on Tuesday, was packed incredibly well, like serious over kill. But thats good. So when I first turned the amp on, flicked the stand by switch off, and It fired up normally, worked fine...But had a horrendous BUZZ if the master was above 5, which is clearly incredibly loud, but still, it was a definite "not right" buzz. Messaged the guy, he was very polite, and suggested various reasons for the buzz, saying he'd never had that happen to him. But also admitted he'd never turned it up that loud. I eventually narrowed it down to the V2 tube, replaced it with a EH12AX7 (spiral filament), and the buzz disappeared. And all was fine with the amp. Until today...

Had a friend over who owns a Vintage JCM800, we wanted to compare the two and see how they measured up to each other. We played for awhile, everything sounded great. Then we decided to use each others cabs to see how they sounded (his cab is a Carvin Legacy cab with V30's, Mines a vintage sunn 412CV with Emi Swamp Thangs, and Texas heats). So I switched my amp to 16 ohms to match his cab, and he switched his amp to 4 ohms to match my cab. Turned my amp back on, let it warm up, flicked off the standby, and No sound at all (his amp worked through my cab no problem)... So clearly we were both a bit "wtf?" Switched the heads around again, his cab worked fine with his cab as it had before....But mine, still was making no noise, through a cab it worked perfectly well with minutes before. Obvious first thing to check was the fuses, Mains totally fine, HT (500ma fuse) was blown...After a trip to radio shack and back (bought two packs of fuses), popped a new fuse in, let it warm up, flicked it off stand by...No sound. Pulled the fuse, it was blown again. So I disconnected everything, check the speaker cable continuity (to make sure it wasn't shorted), it was fine. Checked the guitar cord, fine. Checked the cab to make sure nothing was weird there, and it was also fine. So I started thinking maybe the impedance selector was faulty, replaced the fuse again, switched it to 8 ohms, plugged into an 8 ohm cab, fired it up...and everything worked totally fine.
So I switched it back to the original cab, switched it back to 4 ohms, fired it up...Worked totally fine...which was clearly confusing. Figured maybe it was a fluke, maybe the impedance selector hadn't fully seated or something. Played for awhile, everything was totally fine. Switched the amp to standby to switch guitars, turned it back on...no sound. Checked the fuse, and it was blown yet again. So I basically went through the whole ordeal again, after two more fuses, it was working again fine, and I had even less of a clue what the hell the problem was.
Played for a good hour or so again, everything was totally ok, no weird noises, no strange issues of any sort. Flicked it standby, again to switch guitars...and yet again, it blew the ****ing fuse!

So by that point I was kinda starting to get pissed....went through all the same bull**** again, a couple more fuses...and finally got it running again. After which, I decided to click it to standby, just to see...and wouldn't you know it, blew the fuse again...

So by the end of this whole evening, I ended up going through 9 fuses. The one originally in the amp, and the eight I got from Radioshack...and At this point I have absolutely no idea what the possible issue could be. At this point I'm out of fuses and radio shack closed hours ago. So I can't really try anymore, and honestly I'm not sure I want to, as clearly something is wrong with this amp, and I'd rather not **** it up further.

So I've contacted the seller at this point (haven't heard back yet), and told him this same story, and Told him I'd either like to send the amp back and get my money back. Which sucks, cause Its a really great sounding amp when its not Buzzing or eating fuses. But to me this is simply unacceptable for an amp I just bought, that was supposedly serviced by a well known amp builder shortly before I bought it.

Still curious as to what could be causing this though, so any help, or thoughts would be appreciated. Thanks.

 

[Edgecrusher]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Did you check the power tubes? Thats the most common cause for HT fuses blowing. Even well packed if it got bounced in shipping it might have broken something inside a tube without being easily visible.

To check you pull the power tubes and power the amp up. If the fuse doesnt blow again you know its power tubes if it does blow again you know its another component.

This is the only thing that easy for someone to check on their own. The rest it could be a cap or a screen resistor or something those you need to take it to a tech to find. Sometimes you can look at them and see a component burned or bulging but unless you know what you are doing you still need to take it to a tech to be replaced.

If its anything but a bad power tube I would contact the seller and get him to agree to foot the bill for you to take it to a tech. That way you dont have to lose on shipping it back and he only has to give you a partial refund.


BTW if it is a bad tube I would recommend to the seller to next time pull the tubes and pack and ship those separately. Tubes seem to die more when shipped installed.

 

[Chickenwings]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

fuses are to protect you and your amp when there is something wrong inside. Just replacing the fuse and hoping the problem goes away is unlikely.
Have you eliminated the possibility that it might be one of the power tubes going awol? It is pretty common. They might have been fine when they left the vendor, but perhaps got damaged in some way during transit. They are pretty fragile after all.
I would check that first before looking for other faults. 99% of the time when something goes wrong with a tube amp, it is the tubes.

 

[TwilightOdyssey]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

HT fuse is most likely a bad power valve.

 

[DrNewcenstein]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

While I'm by no means an expert on tube amps, I'd say the Standby switch might either be the problem or contributes to it. When you put another fuse in it, power the amp down without using the Standby switch at all, then power it back up. I would think that if the tubes were bad, you'd get the same results as before. If it powers up and down fine, then I'd look at that switch. Maybe its not rated as marked, maybe it's faulty, or maybe there's a bad connection somewhere with it.

Of course that's also assuming you won't damage the amp by not using the Standby switch. Like I said, I'm no expert on them.

 

[Low_fidelity2100]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Thanks for all the suggestions/thoughts. I contacted the seller and he told me he'd be willing to pay up to $75 to have the amp checked out, but that he wouldn't take it back. So yeah.

From everything I've been reading it does seem like the output tubes are the most likely culprit, unfortunately I don't have any spare EL34's so I can't check that at the moment.
The thing that makes no sense to me is, if it's the output tubes, why would it have worked totally fine before I tried the 16ohm tap, and still after that have worked fine at a few different points for pretty decent lengths of time, only to blow the fuse again when I switched it to standby? Seems like if it was the tubes it'd be pretty consistent.

Another thing, should it be a fast blow, or slow blow fuse for the HT? I've read a bunch on the Marshall forum saying to use a slow blow cause the fast blows are notorious for blowing when it's switched on from standby because of a sudden surge of voltage from the filter caps. But then other places are saying it calls for a fast blow fuse?

I honestly don't know whether the original fuse was a slow blow or fast blow as I didn't check before I threw it away. But the fuses I'd bought from radio shack were fast blow for sure. And I can't seem to find slow blow fuses in a 0.5A rating locally.

Thanks again for all suggestions and help with this.


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[Mr. B]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

A Marshall clone that is blowing HT fuses? Thats an accurate clone if I've ever seen one...

 

[Edgecrusher]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Another thing, should it be a fast blow, or slow blow fuse for the HT? I've read a bunch on the Marshall forum saying to use a slow blow cause the fast blows are notorious for blowing when it's switched on from standby because of a sudden surge of voltage from the filter caps. But then other places are saying it calls for a fast blow fuse?

Needs to be a slo blo. Fast blow will pop in that position. That might be why you killed 8 more of them but thats not what killed the first one. Check like I said. Put a fuse in (slo blo) take the power tubes out and fire it up. See if the fuse holds.

If it does its your tubes if it doesnt then you know its something else. There are a bunch of components that could be causing it and really anyone here is just guessing at what it might be.

Take it to the tech, at least you can get 75 bucks back.

 

[DankStar]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

FWIW, the last 4 or 5 used amps/preamps I've purchased pretty much had to go right into the shop for some reason or another. I factor that cost into the purchase of the amp now.

 

[Low_fidelity2100]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Needs to be a slo blo. Fast blow will pop in that position. That might be why you killed 8 more of them but thats not what killed the first one. Check like I said. Put a fuse in (slo blo) take the power tubes out and fire it up. See if the fuse holds.

If it does its your tubes if it doesnt then you know its something else. There are a bunch of components that could be causing it and really anyone here is just guessing at what it might be.

Take it to the tech, at least you can get 75 bucks back.

Ok, so I guess the fast blow could be to blame for everything after the first fuse then. Could the first one have simply been due to the impedance selector not being fully clicked into position, or the 16 ohm tap not being hooked up properly (or at all)?
I am going to take it to the tech tomorrow though, talked to the seller, he's being very understanding (despite how pissed I was at first). So I guess I'll see what happens.

FWIW, the last 4 or 5 used amps/preamps I've purchased pretty much had to go right into the shop for some reason or another. I factor that cost into the purchase of the amp now.

I might have to start doing that myself, haha. I've just never had any issues with any decent amp's I've bought used before.

 

[Edgecrusher]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Ok, so I guess the fast blow could be to blame for everything after the first fuse then. Could the first one have simply been due to the impedance selector not being fully clicked into position, or the 16 ohm tap not being hooked up properly (or at all)?
I am going to take it to the tech tomorrow though, talked to the seller, he's being very understanding (despite how pissed I was at first). So I guess I'll see what happens.

Yes it could be. Especially if its the old style impedance selector. They have the bad habit of losing the leads internally and causing shorts which can fry output trannies. If you have one of these I would highly recommend having a tech hard wire it or installing a newer style of selector ( i prefer the hardwire method you know like on the back of a boogie where it has one 16 ohm jack 2x8ohm jacks)

That being said without seeing the amp I wouldnt say definitevely what it is or isnt. There are a bunch of things inside that can cause an amp to eat fuses.

The last time I had an amp eating fuses it was cause the power tube socket was bad and SOMETIMES was just not barely making contact on one tube pin. Was maddening that it would work for a couple weeks then all of sudden *pop*


Just have the tech give it a thorough once over, Good luck with it, sucks when new gear gives you grief

 

[Low_fidelity2100]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Yes it could be. Especially if its the old style impedance selector. They have the bad habit of losing the leads internally and causing shorts which can fry output trannies. If you have one of these I would highly recommend having a tech hard wire it or installing a newer style of selector ( i prefer the hardwire method you know like on the back of a boogie where it has one 16 ohm jack 2x8ohm jacks)

That being said without seeing the amp I wouldnt say definitevely what it is or isnt. There are a bunch of things inside that can cause an amp to eat fuses.

The last time I had an amp eating fuses it was cause the power tube socket was bad and SOMETIMES was just not barely making contact on one tube pin. Was maddening that it would work for a couple weeks then all of sudden *pop*


Just have the tech give it a thorough once over, Good luck with it, sucks when new gear gives you grief

What do you mean by "old style" impedance selector? It's one of the flat black sort of football shaped ones, with a round portion in the middle that ya have to stick a pick (or penny or whatever) in a slot to spin it. Is that the old style?
I'm not super familiar with this sorta amp, I've been using black face style fenders pretty much my entire adult life, so I'm used to only a hard wired jack with no impedance choices.



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[Edgecrusher]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

What do you mean by "old style" impedance selector? It's one of the flat black sort of football shaped ones, with a round portion in the middle that ya have to stick a pick (or penny or whatever) in a slot to spin it. Is that the old style?
I'm not super familiar with this sorta amp, I've been using black face style fenders pretty much my entire adult life, so I'm used to only a hard wired jack with no impedance choices.

The old style you pull out and then turn to select the impedance then push back in. Those blow and not in a good way. The newer style that use like a penny to turn are ok from a design stand point but still can fail, its very rare but still can happen.

Just tell the tech what you did when it started blowing fuses and he should track it down fine.

 

[the toad]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

You'rd sure he said "fuch"? With an "h"?

Sorry, couldn't resist...

 

[Low_fidelity2100]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

The old style you pull out and then turn to select the impedance then push back in. Those blow and not in a good way. The newer style that use like a penny to turn are ok from a design stand point but still can fail, its very rare but still can happen.

Just tell the tech what you did when it started blowing fuses and he should track it down fine.

Alright, definitely not that kind of switch on this one. Would upgrading to a Three position rotary pot be more reliable than the kind that is on amp currently?

Yeah, I'm gonna tell the tech exactly what happened when I take it to him. Didn't have a Chance to take it today, over slept and barely made it to work on time, and the place I normally go closes at five for whatever reason. I did pickup some 500ma Sloblow fuses at work today though, and decided to give it a try with those in there. Worked completely fine for the hour or so I was playing it before everyone else went to bed. Tried the 8ohm out put (with an 8ohm cab), worked fine though that, and continued working fine on the 4ohm cab I normally use (on the 4ohm output obviously), don't have a 16 ohm cab of my own to try it on to see if that works though. Tried flicking the standby several times just to see if it'd blow the fuse again, and nothing happened. Worked totally fine.
So I'm thinking maybe I just ****ed up and either didn't seat the impedance selector fully an blew the first fuse, or that the 16 ohm tap is simply not functional, since thats what started it. And apparently the fuses after that were the wrong kind, so that's probably what cause the other fuses to blow. Still gonna take it to a tech regardless just to be safe. Although I think I'm actually gonna take it to my Girlfriends tech, rather than my normal one. My guy is kind of a ******, and has on more than one occasion claimed he "fixed" an issue with an amp I've brought him, only to have it reappear a week later (though he has always taken it back and done whatever necessary to actually fix the problem for free)...Plus he's expensive.
Where as her guy, is an old man she knows through her dad, who actually used to be the Tech at the biggest guitar shop here in Rockford, IL in the 60's and 70's. So I'm thinking he might do a better job of actually figuring out if there is a problem or not, than the dude I have been going to. Plus he doesn't charge a bench fee just to check something out (always repairs her stuff for free plus cost of parts).

You'rd sure he said "fuch"? With an "h"?

Sorry, couldn't resist...

I had actually thought the same thing myself after the amp started blowing fuses. haha

 

[Low_fidelity2100]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

UPDATE: took the amp to my girlfriends tech. He actually let me stick around while he did his checking. Looks like the 16 ohm tap is connected, but the switch is faulty and doesn't make contact internally. Replaced the switch, all three impedance options work fine now. He also suggested replacing the output tubes and the phase inverter. The EL34's sort of 'jingle' if you shake them (pulled from the amp of course) and he said that means they're bound to become microphonic sooner or later, said it's most likely because the guy I bought it from didn't pull the tubes when he shipped it. He also said there's nothing wrong with the phase inverter, he just always recommends replacing them at the same time as the power tubes.
He did say though that the impedance switch being messed up, could not have happened from shipment. That it would have definitely had to have been like that since the amp was made. So that's interesting.

But yeah, the amp is working perfectly fine now. So that's groovy.


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[TwilightOdyssey]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Awesome!

 

[GuitarDoc]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Well, yeah. When I was first reading this (before I got to your last post) I was thinking it has to be either the output selector switch or the standby switch.

 

[Low_fidelity2100]

Re: Just got a JCM800 Clone, and now its blowing HT fuses...

Well, yeah. When I was first reading this (before I got to your last post) I was thinking it has to be either the output selector switch or the standby switch.

Yeah seemed the most obvious to me too. I'm just not super familiar with Marshall's, so I wasn't comfortable messing around with it like I am with my fenders.


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