JCM 800 2203 dead...

[MitchK]

but they have blown right away. originally after the show where the amp died, I replaced the fuses turned it on and pop they blew immediately. then I put new tubes in, tried again and they did not pop right away but took a little time.

I'm not the original owner so I really don't know if the caps are original. They are obviously older, but also I've seen in a lot of pictures of these amps that the caps are blue, these ones are all black.

this is bad to admit to but the first time after the fuses had popped right away. I put some higher fuses in the HT fuses sockets. turned it on and the bias fuse began to glow red at one end and something started to smoke, looked like it was coming from pretubes or around there. I only had the amp on for a split second, and probably shouldn't have done that but I mean, wouldn't that indicate a short?

[stokes]

Bad to admit?Dont ever put a higher rated fuse anywhere.Also,it is a pretty important detail to leave out.I dont see a "bias fuse" in the schem.Where exactly in the circuit is this fuse.It doesnt make sense that there would be a fuse in the bias circuit.There is very little,if any current in a bias circuit.And if a fuse blows here you would be left with power tubes operating with no bias voltage, the tubes would "runaway" and fry.Heres a scenario,with no tubes installed you see a 12 volt drop across the 2 resistors between C22 and the cap feeding the PI (V3).It may seem small now but when you install the tubes it becomes worse.The filters are in parallel with the tube,which should be the only thing drawing current,once the cap charges up.These caps like all caps do not pass dc,so it should not pass dc to ground,which yours are.That 12 volt drop shows that.As the tubes draw current the leaky cap is drawing current also,now the total current draw in the circuit is too high causing the B+ to become lower,but your bias supply stays the same causing your tube current to increase causing a further drop in your B+,you see the vicious cycle we get going here?Eventually something gotta give.With the proper value fuse,the fuse will go first.Putting a larger fuse in could certainly have burnt something else,most likely further down the line.Not telling us about a smoking component in an amp that we are trying to trouble shoot for a short is not a good idea.Now tell us about that "bias" fuse and what smoked.

[Enzo]

Did we cover the fuses? The T in the fuse rating means "timed" so those ar slow blow fuses. Are you using slow blows? Fast blow fuses will work, but will often blow at turn on and on peaks

[stokes]

Enzo,I believe somebody went over the fuses slowblow/fast blow properties and ratings earlier,I was assuming all this time that he had the fuse thing in order,now hearing about the overrated fuse and the smoke issue,who knows."I only had the amp on for a split second, and probably shouldn't have done that but I mean, wouldn't that indicate a short"?Mitch,of course it indicates a short,but we already know there is a short someplace,thats why fuses are blowing.You describe a fuse "glowing red",that would indicate a fuse is not the proper rating,it should pop long before it glows.Tell us exactly where that fuse is in the circuit,as I said I dont see one in the bias supply on the schem,and I have never seen a fuse in a bias supply.And if there are any other glowing,smoking or heat issues you forgot to mention please tell us,these are important issues and if we dont know about them we will be going in circles forever.

[MitchK]

what I meant by wouldn't "that indicate a short" was that it was something catastrophic, or at least, just a more evident or direct short than the caps leaking.

the fuse connects right before R30, so between the white wire on the PT secondary and R30 the 27k resistor.

as for the fuse glowing to be honest it was a while ago I did this and I'm not entirely sure if it was the bias fuse that began to glow or the mains fuse, both are the correct rating for sure, I've checked and checked again. I do think reflecting back that it actually was the mains fuse that began to glow, but it did not blow? once again I had the amp on about 1 and a half seconds. it buzzed like crazy I turned it off as fast as I could.

The bias fuse, is 100ma. the thing is though is that I'm not convinced that its actually blown, the meter doesn't show continuity, but when I test resistance its a small amount of resistance (I don't have me meter here at this moment so I can't read it but I'll do that tomorrow and post it), the signal does seem to come through, also the wire inside the fuse *appears* to be intact.

the smoke looked like it was coming from one of the preamp tubes but I couldn't tell.

[stokes]

The leaky cap can act slow and take a few seconds to blow,but if the rest of the caps are charged up it can blow immediately on power up.We have been proceding here on the basis of the HT fuse blowing,right?Any fuse that doesnt show continuity is shot.Any fuse that glows red before blowing is too big for that circuit,it would blow if it were the correct amp rating.You cannot always see the broken wire inside a blown fuse,quite often the break is under the metal cap on the end and cant be seen.Make sure all your fuses are the correct rating,especially the one that was glowing.I am still certain you need to change those filter caps,there is too much current draw in that B+ rail with no tubes.If there is other damage I would think it was caused when you put the higher rated fuse and saw that smoke.I would suggest doing the caps and fixing the problem we can see and hope any other damage will be apparent when the power supply is stable, or if you got lucky something didnt completely burn out when you saw that smoke.Just be aware that whatever smoked is likely to fail sooner than later.That bias fuse makes no sense.A fuse there,if it blows will completely cut off your bias voltage and burn the tubes.Just to be sure I went and looked at a bunch of other Marshall schems and do not see a fuse where you describe on any of them,in fact I have never seen a fuse in that position in any amp.Anyway,if it were my amp or an amp I was fixing for someone,I would change those filter caps first.

[MitchK]

I am really really certain that its the right rating fuse in the mains and bias I think they are even the original fuses. I'll replace the mains fuse which is 4A just to be safe but the fuse that is in there says its a 4A fuse.

as for the tubes, I'm sure when i powered the amp up without the bias fuse in there I did just that and fried my tubes. is there any way to check them without the a tube tester? I'm hoping one of them failed before the others, or else I just blew a quad of 6550s for nothing.

any suggestion for brands of caps?

[Steve Conner]

You won't believe it, eh, but I think the Canadian electrical code requires every winding of the transformer to be fused. Even in cases like the bias fuse where it probably does more harm than good.

I'd like to think that if a quad of 6550s were fired up with no bias, they'd just blow the HT fuses pretty much straight away. If you put oversized fuses in to stop them blowing, this is just a case of straightforward overdissipation, and the only way it could hurt the tubes is by melting the plates, which takes time.

[stokes]

With absolutely no bias voltage you would have a bit more than just a case of overdisipation.The tubes would go into a runaway condition and short out very fast.I cant imagine anything that would cause the bias supply to draw more than 100ma's and blow that fuse.But turning the amp on without that fuse installed,another important fact you forgot to mention,would surely fry those tubes quickly,as well as screen resistors and possibly the OT.You know Mitch,its tough enough to try to diagnose an amp in this manner,when all we have to go on is the info you are giving,but some of the info you havent told us only makes it that much tougher.I am going to suggest you take that amp to someone who knows about amps,and has the equipment needed to test it.No offense Mitch but it seems you know just enough to get yourself into trouble,which I think you have on a couple of fronts here.It seems you have caused more damage to that amp by putting a higher rated fuse in the HT and by running the amp with no bias voltage,and I have to wonder what else you arent telling us about.Close that amp up before you hurt yourself as well,and get it to a qualified tech.

[MitchK]

you know what stokes, I want to get this amp working for sure don't get me wrong, but what I'm far more interested in is understanding how amps work and learning how to do this stuff. I'd gladly fry this amp 5 times over to do so. And honestly you sound like you've taken person offense to me leaving certain things out but you also are totally not paying attention to what I'm writing either. I told you about the bias fuse several posts ago, I'm still not even clear if its blown or not right now because like I said the resistance is only about 35ohms across that fuses so something is going through. and secondly I did put the higher value fuses in there, not a good idea, although I think now we have a better idea as to whats going on, AND whats happening with the amp, the symptoms, are from everything we've looked at exactly the same before and after me doing that.

At this point I have more money than time, I could easily take this to a tech, but I want to do it myself, thats the point, its why I bought this amp. If you don't want to help thats fine.

[Enzo]

OK, yes the Canadian version have a fuse board with fuses for everything, but that doesn't show up on our schematics here.

Tiny fuses like under half an amp do have resistance. If you measured 30 ohms with the fuse out of the clip and in your hand, then it is likely OK. If it were teh 4 amp fuse, that would not be a good reading, but that 1/10 amp will have a few ohms resistance.

WHat happens if there is a short within a tube or an arc at a socket and the B+ winds up connected to the control grid. COuld that draw 100ma through the bias supply? Maybe not.

If you want to know what is going on in that circuit, remove the fuse and connect your ammeter in its place. Now watch the current reading - it is high or not. If it is high, we need to determine why. if not, then we worry there might be an intermittant short in the system.

[stokes]

Mitch I am not taking any personal offense.We are tracking down a short,and leaving out info like the higher fuse and the smoke issue and the fact that you ran the amp without that bias fuse is counter productive.What I can see and have already said,with no tubes in the amp nothing else is pertinent except the rectifier and the filter caps,you could have 100 other shorts and they wont show without the tubes completing the circuit.Dropping 12 volts as you see between C22 and the PI (V3) cap indicates a leaky cap,which with tubes in will act like a short as I have explained.It makes no sense to track other shorts till you fix that one.I am sorry if you didnt like the suggestion about going to a tech,but when someone doesnt think that mentioning smoke and a glowing fuse is important when searching for a short,I have to wonder how capable you are at poking around a live amp,and I am not trying to insult or offend you in any way,I just dont want to see you get yourself electrocuted,or cause more damage.Trying to learn about amps is fine and I understand the desire to do it yourself,we were all there at one time,but the fact that you powered that amp up without any bias voltage by taking that fuse out is an example of knowing enough to get yourself in more trouble,not meant to offend you at all.Dont run that amp without that bias fuse,if it were me I'd replace it with a straight wire,but I am not telling you to do that,a blown or missing fuse here will fry those tubes for sure.From what Enzo says it seems that fuse is fine,so I would leave it for now.Like I said before,if the amp were on my bench my first priority would be a recap,since I can see a problem here,and look for other issues after.You may have been lucky and not cooked anything but the power tubes.But those caps are definately an issue that would cause the HT fuse to blow,which was your first problem.Since you arent sure how old they are it would be a good idea,because if they are original they are likely overdue.I dont leave caps in any amp more than 8 years and 10 years is the max some manufacturers of caps recomend for the shelf life of electrolytics.

[MitchK]

I just want to reiterate that I did not run the amp without the bias fuse, I had a suspicion that the fuse was dead, but I had actually assumed that since the fuse was such a low value that for some reason this might explain the resistance . Most of my assumptions up until this point have been wrong so I decided to ask you guys and apparently that does make sense and the fuses is not blown, I have not taken that fuse out and ran the amp at any point.

I'm going to order some new caps tomorrow and we'll see what happens. In the meantime can you suggest any way of testing the tubes so that I can figure out I'll need to order those too?

[stokes]

"as for the tubes, I'm sure when i powered the amp up without the bias fuse in there I did just that and fried my tubes. is there any way to check them without the a tube tester? I'm hoping one of them failed before the others, or else I just blew a quad of 6550s for nothing."
This is your statement,Mitch,from an earlier post.I would not recommend that you attempt to do the cap job on that amp.For one thing you will need a variac to form the new caps,and to be honest,and I dont mean to insult or offend you but from some of the things I read here,I think you dont have a full grasp on what is going on in an amp and in that power supply,in particular.The fact that you were calling that PI cap a mystery cap says volumes about your knowledge in tracing a power supply.Like I said Mitch I am not trying to offend you at all,we all started somewhere,and I understand you want to learn to do this stuff,but there are lethal voltages in there as you know,and I think you need to read a few books about basic amp circuits before you start tackling some of these things you are trying to do.Your new thread on the 5150 screen problem is a clear indication that you are delving in areas you just dont know enough about.

[MitchK]

sorry that post wasn't clear, at that time I was thinking the fuse was blown. it was still in the amp and it appears now that it was indeed fine, I didn't actually physically take the fuse out I was just running the amp and I afterwards thought it was blown and I now know thats not true.

I do have a variac, and I'll most likely do the cap job myself. with big thick rubber gloves on. I'll take it to someone in town if they'll let me watch them do the entire job and tell me in detail what they are doing, otherwise I'll be at least *trying* to do it myself.

as for the 5150 thread its an honest question, and its not like everyone jumped to give me a clear decisive answer, it could be a number of things. It does seem really indicative of a short but well no power tubes and its happening right on the pin itself I don't know if its entirely wrong to wonder how its happening, i'm going to keep asking questions no matter how stupid you think it is.

any recommendations for good books? I was really looking at the books written by kevin o'conner, but the entire volume is pretty pricey...