Marshall JCM 800 4211 Combo Blowing HT Fuses

[sohosteve]

Bit of a strange one.
The amp worked fine for a whole tour and then died mid set.
When I got it home I discovered the HT fuse to have blown.
Replaced it and it worked fine.
Then when I was messing with various sounds, while chugging the low strings, I turned up the volume (this is a JCM 800 4211 so it has TWO Volume controls - one on the channel after the Gain and one in the Master section to control the overall output - the one causing the crunch is the channel Volume), there was a crunch (as in a dirty pot) and the volume dropped considerably.
So I turned the pot back down to zero and turned it up again.
At this point there was a huge crunch followed by silence.
Having seen my dad 'fix' the tv on numerous occasions by thumping the top I did the same.
Bingo!
The amp worked fine again.
Messed with the turned up the channel volume - CRUNCH!! - total silence.

And this time it didn't come back on.

Checked the HT fuse and sure enough it had blown!

So I replaced it again.

And as I had noticed during part of my questing that one of the output valves was making a fluttery, crunching sound if I tapped it, I replaced that as well.

It seemed to be working fine and the valve noise was no longer there when I tapped it.

But as soon as I started chugging and gently turning up the channel Volume when it got to around 8 - CLUNK! - it cut out again.

I suspect that POSSIBLY when the amp gets to past a certain volume the sheer vibration from the speakers (it's a combo) may be causing a dry joint to rattle and break contact.
Or crap in the pot once it gets to about 8...

Does that make any sense or does anyone have any other ideas of what may be that is causing this?

Would it be possible that a dirty pot could blow the output fuse?
Or a dry solder joint?

At no point have the Mains or T1A fuses blown.

Any ideas or suggestions will be gratefully received and tried out.

[52 Bill]

I suspect that POSSIBLY when the amp gets to past a certain volume the sheer vibration from the speakers (it's a combo) may be causing a dry joint to rattle and break contact.
Or crap in the pot once it gets to about 8...

Does that make any sense or does anyone have any other ideas of what may be that is causing this?

Would it be possible that a dirty pot could blow the output fuse?
Or a dry solder joint?

While all of your thoughts are good ideas, my first thought would be a failing power tube. If you have a light bulb limiter, it's time to use it and save your high tension fuses.

Gently tap each power tube with a pencil or dowel and see if any of them react to the physical stimulation.

[sohosteve]

While all of your thoughts are good ideas, my first thought would be a failing power tube. If you have a light bulb limiter, it's time to use it and save your high tension fuses.

Gently tap each power tube with a pencil or dowel and see if any of them react to the physical stimulation.

I don't have a 'light bulb limiter' - I have to confess that I am not an electronics engineer though I have successfully modded a few amps and effects pedals without torching myself...

One of the power tubes was (as already mentioned) reacting to the 'pencil test' and I replaced that tube.
Though I guess there could be another going the same way.
I'm off on tour tomorrow (hired amps fortunately in this instance) but I'll check this out on my return and see what I find.
Thanks for your suggestion.

[g1]

Since you replaced that power tube, it's still blowing the fuse or just cutting out?

[sohosteve]

Both - though not every time.

It seems to be an issue when I crank the channel volume up past 8
The Master Volume wasn't past 3 (still very loud though!)

I'm not sure whether it's the sheer volume that is causing a vibration or the sudden CRUNCH that happens that makes it cut to silence.
Also I'm not sure whether the crunch is coming from the pot (as in a dirty pot) or the crunch is the sound of a loose connection breaking free due to vibration..

Sometimes a thump on the top brings it back to life though not when the CRUNCH takes out the HT fuse...

[52 Bill]

Both - though not every time.

Sometimes a thump on the top brings it back to life though not when the CRUNCH takes out the HT fuse...

You may have or have had more than one problem here. Possibly a bad tube and a bad connection.

It is probably time for a 10,000 mile check up.

[TimmyP1955]

A little tube reaction from a good whack is likely no big deal. A loud noise and a spark inside is trouble. Could also be a loose socket connection, a bad solder joint, or a failing filter cap. Maybe a bad screen resistor.

[Tom Phillips]

You may have or have had more than one problem here. Possibly a bad tube and a bad connection...

That's what it seems like to me too. (Potentially two problems) Regarding the potential tube problem, any one of your 4 power tubes could have an intermittent short that causes the HT fuse blowing. When the tube doesn't stay dead shorted then it can be very difficult to isolate the problem. One approach is to replace the whole set of tubes, set the old ones aside and then determine if the fuse blowing problem returns. If the problem does not return then you can be pretty sure that there is a bad tube in the old lot. Then you need to decide if it's worth your time to try to isolate the problem to a specific tube or scrap the old set and move on. It's also prudent to inspect the tube sockets and verify that they are clean, the individual sockets are tight, the wiring is good and there is no evidence of arcing (Telltail carbon tracks).

[sohosteve]

Hi there

I thought I'd sorted this out but the problem has now returned in a slightly different form!

As I noticed that problem tended to mainly happen while turning up the Master Volume on the Lead Channel - as in when it got to about 6 there was a 'CRUNCH' and the HT fuse went, I got to thinking that maybe there was dirt on the track of the pot causing the Master to jump from 6 to flat out with the 'CRUNCH' being simultaneously amplified to full volume. Causing an instant overpowering surge to the fuse.

So I stripped it down and gave all the pots a good sluicing out with Caig Deoxit D5 and Bingo, the amp sounded great, all the pots worked cleanly and I could set it at any volume without the fuse blowing.

So I took it out on the next tour.
Accompanied by a spare head in case it popped mid gig.
Did most of the shows at bone crushing volume with various boost pedals (I use a couple of very high output pre-amp pedals for solos) and it working beautifully until about seven gigs in it went down again.

Same deal - blown Output fuse - Mains light still illuminated but totally silent.

So at soundcheck the next day I gingerly replaced the fuse and gently turned up the volume.
Sounded absolutely fine until I noticed it was not as loud as when I first switched it on.

At this point I realised that the Output volume level was fading and becoming ever more distorted.

So I switched it off, unplugged it and used the spare for the rest of the tour.

I would assume, from this, that this that there is possibly a failing power valve though I am NOT an electronics engineer - I can handle a soldering iron and have performed quite involved mods on valve amps to instructions - but I don't actually know exactly what I'm doing.
My diagnosis is partly based on forty years of plugging into amps and experiencing them fail for numerous reasons - not technical expertise!

So what are the thoughts of the real experts out there?

Does it sound like a dead/dying Output valve (tube for you American amp experts!) and if so, how might I best work out which one?
I know the pencil test for pre-amp valves but I tried this and it doesn't seem to offer the same clues in the Output stage.

I do have a good (spare) Mullard EL34 that I guess may possibly cure the problem if I can identify the culprit.

Or does my description sound like something far more serious/costly? (dying Output transformer or something...?)

As I don't have any transport (or much money) to get it to a tech I'd rather try to fix this at home if possible.

[Jazz P Bass]

I have a feeling that the output tubes went bad.

[sohosteve]

All of them?
Isn't it more likely to be just one, or is it common for the whole set to go down sometimes?

I have a feeling that the output tubes went bad.

[Jazz P Bass]

That is what tech's do.
Ascertain which one(s) went bad.

I am not at all aware of your test setup or abilities.

[sohosteve]

As I mentioned in my post, I'm not a trained electronic engineer though I have performed some fairly complex mods on my valve amps (swapping out big caps, resistors, diodes etc) though I can't read a circuit diagram!
I wondered (as the problem no longer appears an intermittent one - with the volume of the amp fading quite quickly once switched on) if a sensible route would be to swap each power valve (one at a time) with the good spare EL34 I have.
Does that make sense or is there more chance in just damaging the good spare (vintage NOS Mullard) that I have?

Sorry if my questions are a little 'dumb' but as I say, I'm not actually a tech, though I've picked up a few helpful troubleshooting clues over forty years of playing through valve amps...

That is what tech's do.
Ascertain which one(s) went bad.

I am not at all aware of your test setup or abilities.

[Tom Phillips]

I have a feeling that the output tubes went bad.

I agree. They symptoms are typical of a power tube that has an intermittent short. It can be very difficult to sort out which power tube is at fault AND it can be more than one of the tubes at fault.

My personal approach in this situation is to replace the whole set of power tubes after giving the amp a good internal inspection to make sure there isn't something amiss with the wiring etc. I pack up the old set of power tubes and save them until we find out if the amp blows the fuse again or not. If it does blow the fuse again then you have an extra set of power tubes and you continue the investigation. If the fuse blowing stops, then you pretty much know that one or more of the old power tubes has an intermittent short. It's then time to decide to dispose of them and move on or try to sort out which tube or tubes are faulty. That's usually not worth the effort unless they are new or valuable NOS tubes. One could do something like put one at a time in an amp and see what happens but that's a real hassle and certainly not cost effective to pay a repair tech to do for you.

[nsubulysses]

don't use an NOS mullard to test an amp that doesn't work right and you don't know why it doesn't work right

Bill and Tom are right in posts 6 and 8. You need a light bulb limiter and you need to know why the amp is failing. It could be a loose connection in the power supply or output section that's going in and out, shorting the tube or blowing the fuse. If you had a current limiter you could poke around and try to make it short, then know you're getting close to the problem when the lamp lights.

And as Tom said you could put a power tube in one at a time and tap it around to see if you can draw out an intermittent short. When the lamp lights it is shorting. If you've done some fairly complex mods you should have no problem building a light bulb limiter.