HT fuse just won't stop blowing.

[MitchK]

This has been an epic journey so far. This is a little long but I'd appreciate it if you all could give me a hand here.

I have a marshall jcm 800 2203, canadian market model. I have another thread about it but I'm just going to give a quick rundown:

1. Amp cuts out for me, no sound, I dime it and it will cut in and out. I do this for about a minute or two trying to figure out what it happening and a tube blows. Turns out the speaker cable had actually died I figured out later on.

2. I pull the dead tube and replace the fuse it took with it. I now run the amp with just 2 tubes in it. I play two shows that night, the amp works for part of the night and then dies on me mid set. I open the amp up the HT fuses (which are 2x 0.5A slo-blo btw) are blown.

3. I replace the HT fuses turn it on and pop, they blow immediately. So I figure I have a bad tube. I replace the el34s in the amp and try out a quad of 6550s.

4. Power the amp up with the 6550s and some new fuses it works for about 10 minutes and then pop. the fuses blow again.

5. I recap the amp, all new power filter caps. While doing this i also happen to come across the fact that the bias pot reads at max 16Kohms when it should be reading 22Kohms. So I also replace this with a new pot.

6. I test the 6550 tubes and they all test good, I go back and test the el34s, 3 of the 4 work but one upon plugging it into my tube tester cause the tube tester to smoke. I throw this tube away.

7. I then power the amp up with a quad of KT88 tubes I have from another amp that I know work fine. The amp turns on and seems ok while idling but at one point makes some not so loud scratchy buzzy sounds. I turn the standby off, wait a second, turn it back on and it sounds fine again. I finish biasing the amp and have it sitting at roughly 20watts static dissipation.

8. I plug in and start to play, the amp sounds good it gives me a feeling better than any woman has ever made me feel (hahahaha) and as soon as I hit a nice chunky chord.... you guest it, the HT fuses blow. I recheck the tubes and they still test the same as before I ran them in this amp.

I'm just lost at this point. I want to check the driver tube plate to power tube grid coupling capacitor based on what I'm reading at the tube amp debugging page (http://www.geofex.com/ampdbug/ampdebug.htm), I've read pin 5 on all the sockets and they are sitting at -37v. When I read one of the sockets it seemed to oscillate between 0 and -37 for a second but I think I just wasn't making good contact with my probe because it seemed to go away, but I'm just so stuck at this point I'll look at anything.

What can you suggest??? I'm just out of ideas! I'm going to go over all the resistors in the amp to make sure they are still to spec but I don't really know how I can go about testing all the capacitors though? I have a replacement choke OT and PT that I was going to use to build a clone of the amp that I can try if necessary but that seems like a huge without looking at everything else first. and above all, I'm sicking of buying 0.5A fuses and watching them constantly blow! Those things aren't cheap at radio shack.

[Mark Black]

Probably a long shot that it could be the same exact problem, but I did once work on a Canadian model JCM800 and it turned out a bias filter cap was leaking in such a way that it gradually dragged the bias toward 0V over time causing excessive current draw. Something of that nature could be verified by monitoring the bias voltage at one of the tubes while leaving the amp on for a while to see if it changes.

Like I said - probably a long-shot...

Good luck!

[ken]

Bum speaker cable? Check your output transformer - you probably have an intermittent winding short from overheating it. It happened to me the same way...

Ken

[Amp Kat]

http://www.antiqueradio.org/dimbulb.htm
This should help with the fuses and I know RG has another lightbulb current tester on his site but I couldn't find it. Almost a must for testing fuse blowing problems. I have had some of these amps that I go up to 1 amp fuses and it never blows again but not any higher than that. Marshalls are pretty common for blowing those 1/2 watt fuses and I agree that the speaker cable would be a concern but I've seen them blow for no apparent reason at all just sitting there and poof. Monitoring the bias voltage also sounds like a good idea and even monitoring the current draw of the tube would be good to find out what's tugging on it.

[stokes]

Like KB says the current limiter/dim bulb tester is an absolute must.It seems to me,from what I am reading you are going between 4 tubes to 2 tubes,El34 to 6550 back and forth and so on.When trouble shooting stick with one tube type,either 6550 or EL34 and use all 4 tubes.Pulling two tubes will require rebiasing as will changing from 6550 to EL34.Switching back and forth is just complicating things.

[MitchK]

Originally Posted by ken

Bum speaker cable? Check your output transformer - you probably have an intermittent winding short from overheating it. It happened to me the same way...

Ken

But this is the problem if its intermittent I don't really know how to check it? Aside from just replacing it that is, but thats really a last resort.

I'm going to monitor the bias voltage and see what happens when the fuse blows, but how does this work? the negative voltage goes closer to 0 so therefore the plate voltage increases which means more current being drawn on the B+ and this blows the fuse. correct?

as for the dimbulb test I have a variac and when I power up with that its fine, the lamp slowly gets brighter while I turn up the variac and I've replaced the caps so I know they aren't the problem, its not a dead short (well if it is its intermittent) it takes a while before the fuse is blown.

would it make any sense to hook up an ammeter in place of the fuses and see how high the spike is? I'll put like a 1A fuse in series with the meter just to make sure the spikes don't get too high? I mean I know I may as well just put in 1a fuses and see what happens at that rate but I'd like to know whats going on because if it won't run stock them you know how long is it going to last as it is now if there is some problem that could get worse.

so far on the list of things to test is:

1. check the OT but I don't really know how to test for an intermittent short

2. monitor bias voltage while fuse blows

3. try the amp with 1A fuses



anything else? and can someone point out the bias filter cap for me and the driver tube plate to power tube grid coupling capacitor on the schematic? I think that would be the 2 0.022uf caps that connect to the 5k6 grid resistors (c14 & c16) and the bias filter cap would be the 10uf (c19)??? that connects to the 220k grid leak resistors?

[Don Symes]

Originally Posted by MitchK

so far on the list of things to test is:

1. check the OT but I don't really know how to test for an intermittent short

2. monitor bias voltage while fuse blows

3. try the amp with 1A fuses

for #1 you need to take the OT to a 'Hi-Pot' tester ... I think. Probably take a hair dryer to heat the thing up just before testing? Where you'd find someone who would let you use theirs, I dunno.

#2 should be pretty straightforward. Remember the bias voltage is NEGATIVE.

#3 NO. The fuses are blowing because something is bad wrong. Just raising the current limit can start a fire and/or do seriously expensive damage.

To find the bias cap, look for the electrolytic with its positive terminal connected to GND. There may be a couple of these in a Pi filter arrangement .

One last thought - have you checked the socket tension on these grid pins? On a hot socket, they may open up just enough to let the grid drift high.

Hope this helps!

[R.G.]

Originally Posted by MitchK

I'm sicking of buying 0.5A fuses and watching them constantly blow! Those things aren't cheap at radio shack.

It's frustrating, isn't it?

The frustration disengages your ability to think logically though.

You gotta get it to quit blowing fuses so you can debug it. If you don't do this, you're sunk. So try one of these:
(a) a light bulb limiter
(b) a power resistor calculated to limit B+ current to a safe max value for the PT, and inserted into the HT fuse slot.

With either of these, you can now get it to fail and when it does, measure what's happening.

It's either oscillation or loss of bias when something heats up, but the "something" will be forever lost until you can pin it down without a fuse blowing.

[MitchK]

alright a couple quick questions:

first off with running the amp through my variac, aren't there going to be issues with running the amp with all tubes in at say 50 volts input?

and can you go into a bit more detail on the method of using a power resistor in place of the fuses? how would I calculate this? take the regular HT voltage, lets say 450v divided by 500ma makes for about 1.1k ?? how would I determine the wattage?

And won't the presence of the resistor make the plate voltage drop every time the current draw drops? I'm a little confused here.

basically I would be replacing with the resistor and then of course just watching the bias voltage on the grids to see if they drop at any point?

any information on testing for oscillations?

thanks a lot guys!

[Enzo]

I'd be worried about arcs. Hipot testing is a great idea, take your OT in for test. ANy motor shop ought to be able to handle it, and I don't think it would cost a ton.

Also possible arc on a socket, look them over - over and under.

[MitchK]

http://www.geofex.com/ampdbug/outtrans.htm

What about doing the "advanced" tests on this page? seems like a pretty similar idea to the Hipot test?

Also I know the chain of events began without the speaker cable cutting out but when what seemed to have caused the current problem was playing the amp for an extended period full blast with just two tubes.

and once again word on running the amp through the variac, is this going to be ok even with the tubes in it?

[MWJB]

Variac Q - yes, this will be OK. Start at around 1/3 wall AC.

Fitting 1ohm cathode resistors to ground from the power tube sockets pin 8 will allow you to monitor bias current at pin 8 (in mV) whilst playing. It'll fly around to high readings under normal circumstances but might give clue at the point of failure?

You don't say what kind of plate currents you are running at the moment?

Before removing the OT, i'd check it in situ first. Use a bare 1/4" jackplug to apply 0.5VAC to the speaker jack (secondaries) with your variac, monitor with a voltmeter (dialling in 0.5VAC migt be a bit fiddly but be patient). Use a 2nd meter to read AC voltages, end to end, at the OT primaries (no tubes fitted). Divide the primary's end to end reading by 0.5VAC, then square it, then multiply by output impedance and you should get primary impedance for your tube type, divided by 2 (1800ohms-ish?).

You can also measure from one end of the primary to the centre tap, then compare reading to the other end of the primary to the centre tap to see if one side is shorted.

-37vdc at pin 5 with KT88's? Is it just me or does this sound a bit low?

[MitchK]

Originally Posted by MWJB

Variac Q - yes, this will be OK. Start at around 1/3 wall AC.

Fitting 1ohm cathode resistors to ground from the power tube sockets pin 8 will allow you to monitor bias current at pin 8 (in mV) whilst playing. It'll fly around to high readings under normal circumstances but might give clue at the point of failure?

You don't say what kind of plate currents you are running at the moment?

Before removing the OT, i'd check it in situ first. Use a bare 1/4" jackplug to apply 0.5VAC to the speaker jack (secondaries) with your variac, monitor with a voltmeter (dialling in 0.5VAC migt be a bit fiddly but be patient). Use a 2nd meter to read AC voltages, end to end, at the OT primaries (no tubes fitted). Divide the primary's end to end reading by 0.5VAC, then square it, then multiply by output impedance and you should get primary impedance for your tube type, divided by 2 (1800ohms-ish?).

You can also measure from one end of the primary to the centre tap, then compare reading to the other end of the primary to the centre tap to see if one side is shorted.

-37vdc at pin 5 with KT88's? Is it just me or does this sound a bit low?

Can you elaborate a little more on the equation there? I've really really tried to get the variac to 0.5vac but the closest I can get is 0.643, can you give me the formula so I can just change it, I think I've already got it but I'm confused about the divided by 2 part and I don't know this relates to the primary impedance for my tube type?

[MWJB]

OK, I don't know exactly what primary impedance your OT is wound for, so I would guesstimate that it is wound for between 3.4K and 4K per tube (being an EL34 design, anyone who knows for sure please chime in). You have 2 tubes at each end of the primary, so I guess the the figure you want is between 1750 and 2000ohms. (If its 1695, for instance, don't panic, we are looking for a ballpark reading, if something is really wrong it will show up).

So if we work backwards:

1800 (our nominal target primary Z) /4ohms (voltages with the 16ohm tap will be much smaller and trickier to determine)= 450

Square root of 450 = 21.21

21.21*0.643VAC = 13.68VAC from end to end of the primaries. (13-14VAC with 0.643 at the 4ohm secondary would appear to be within tolerances, if you were using the 16ohm tap I'd expect to see 6.8VAC-ish, end to end on the primary with 0.643 on the secondary ).

Working forwards...

Voltage@Primary, end to end/voltage applied at secondary (ideally 0.5VAC)...

Square the result...

Multiply by output impedance at that tap = Primary Z.

[MitchK]

JESUS H CHRIST.

haha sorry guys just had to get that out. So this thread has been abandoned for a little bit but well this JCM 800 has been through a lot and I'd just make my day to have it back up and running.

So a short as possible recap:

amp got connected with no speaker load and cranked, this blew a tube and some fuses.

replaced the fuses and play the amp with only 2 tubes in it but cranked to 10. works for about a half hour and then cuts. HT fuses are blown

replace the HT fuses turn it on and it blows them again. turns out a tube is dead.

replace the ht fuses and a new quad a 6550 tubes (no el34s available, bad move for the purposes of trouble shooting but what can you do), works for about 10 minutes and the fuses go again.

Replaced all the power filter caps.

find out the bias trim pot is not giving as much as much resistance as it should so I assume the tubes had been over biased and this killed them, but upon testnig them with a tube tester they are ok. I replace the trim pot anyways and giver a try it seems to be fine until I turn it up a little whack out some full chords and the fuses pop again

finally I replace the OT thinking the OT must have gotten somehow wrecked from the original issue of operating it with no load. works ok reasonable volumes, I cranked it for a second just to see if the fuse would blow and it seemed ok. tried it again, this time with a guitar that had a "boost switch" for a little more gain and of course it again crapped out when i turned up to playing volume.

I am basically ready to take this to get serviced. I' started to take apart the original OT to rewind it so I guess I'll put that back together and put it back in. but is there ANYTHING anyone can recommend that I check before I send it in? its not just about paying to get it fixed, I feel defeated. I need something to play though, I'm ampless at the moment. ANYTHING though folks, ANYTHING!

[Enzo]

Well, is it the right fuse? Does it say T1A or T500ma over the fuse holder? (I don't remember the wattage of the amp) If you are using plain old fast blow fuses, they will blow now and then. The T in T1A means "timed," the British way of saying slow blow fuse.

Bias is determined by voltage and measured as current, not resistance. If you set it up for some level of idle current, then the resistance that got you there doesn't matter.

[Bruce / Mission Amps]

Most of the time I've seen this happen over and over, it seemed like it was always a flashed over tube socket hiding it's ugly little head.

[MitchK]

they are for sure the right type of fuses,

500ma T is what it says on the PCB and slo-blo 0.5A fuses are what I've been putting in.

And I know what you mean with the bias, at that point I had just noticed that trim pot which was 22k was only 16k when it was turned all the way, and when i measured the bias it was a little hot for el34s, so I figured it would be best to replace that trim pot.

As for the sockets, I'm willing to try anything at this point, but what exactly can I do to fix this? I hear about people "tightening" them up but I don't really see how I can do that. Also would I not see some scorching either on the socket or the pins of the tube itself?

[mykey]

the more you keep doing this the sooner you will need an output transformer (if you don't already)
you have replaced tubes without rebiasing the amp properly, bad idea
the bias is incorrectly adjusted and should be adjusted by someone who knows what they are doing.
you have a burned screen grid resistor, burned tube socket, shorted diode in power supply or all of the above.
boy, do you need a Marshall tech. stop goofing around and get a technician.

[MitchK]

you know what you need?

to go do a better job of lurking all of the threads I've made.

Most of what you said was to do with my 5150 amp, or you plainly misunderstood me. I don't know where you are getting the part about me biasing the amp incorrectly either, and if if I did, I don't care, I know how to bias an amp now thats for sure. how did I learn? EXPERIENCE.

I don't care about the amp, as much as yes of course I'd love to have it up and running, its not about the amp and like I said if you managed the skill of reading well enough you'd have seen me say that many times before. I'll fucking piss in the amp and mail it to you if that will get the point across, I could afford to go out and buy 10 of them tomorrow if I felt like it, its not about the amp!

I want to learn, about electronics, about guitar amps in specific, and you know what? I'm learning a lot, shitloads, I spent about 2 years trying to learn stuff from reading and deciphering what different acronyms meant on the messages boards, from staring at schematics until I figured out what the hell was going on to some small degree or even just figuring out what all the symbols were in the first place, and since I've actually had an amp to trouble shoot its given context to the things I've learned and I've learned far far far more and much faster.

Don't worry if I somehow electrocute myself I won't have your name etched into my tombstone blaming you, I promise. Stop being such a negative discouraging asshole, if you don't want to read my threads and try to help me out direct me in the right path that you seem to know so well, then GTFO!

[mykey]

you asked a question , I gave you the correct answers.
you don't have much experience working on Marshalls, that obvious.
the fuse just keeps blowing and blowing..................
any experienced person would know better than to be where you are now, you are guessing and badly.
I have worked on Marshalls for 34 years, if you don't want to learn from that it's OK. Its not negative, its TRUE.

[MitchK]

yep 34 years experience milling away in a basement somewhere makes for pretty poor social skills and that what you're displaying.

you're the expert, you know more than me about amps, far more, if you had the amp there i bet you could fix it, I bet you money you could fix it.

I'm making shitty guesses as to whats happening, no doubt. I don't fully understand even whats happening, check!

care to enlighten me o' great one?

you seem to know exactly what the answer is by the sounds of it but just can't share this precious knowledge with me, you've just told me to take it to a technician so far. You? are you having that much of a problem drumming up business you need to canvas message boards?

like I said, if you want to have a pop quiz on guitar amps, you'll win, if thats the only point you've got to prove, someone can give you a gold star and we're all done here, but otherwise if you don't have some suggestions of what I can begin to troubleshoot, just being smug isn't going to teach me anything about amps.

seriously enough of this bullshit though, if you've got some ideas just tell me already.

[Enzo]

Don't let it get to you Mitch. He just discovered the place and ran down the row of threads throwing out a quick response to each opening post, usually ignoring the discussion that ensued.

Wasn't it Mit Romney who recently claimed he was a hunter - or was it fisherman - for some 30+ years? COme to find out he went once in like 1979 and again last month?

SOme of us are trying to help.

An arcing tube socket is best repaired by replacement.

[PerroGrande]

Hi MitchK,

I feel your pain! A couple of quick observations/questions, etc.

Whenever I've got an issue with something blowing a fuse, I always look to whether the fuse blows quickly/immediately or after time. When it blows immediately, I go looking for shorts or opens.

Repairs where a fuse blows after some time, intermittently, or the like are always more difficult because one isn't sure exactly how to replicate the issue. Unfortunately, you've got the latter so it seems.

With the slower failure of the fuse, it is *probably* true that the unit will run without a fuse blowing if the power tubes are not installed. Have you verified this? The HT (B+) circuit *does* operate more than the finals, so it isn't a bad idea to verify that it isn't something happening at an earlier stage (I doubt that this is the case, but it never hurts to be certain).

Secondly, with a slower fuse failure, I always recommend "follow the current / follow the heat" as a troubleshooting mantra. The items most likely to fail (and cause a fuse to blow) are the ones that draw the most current and/or produce the most heat. They are under the most stress. Failing that, whatever controls the high current/high heat devices is my next candidate.

You've got some good candidates identified. The OT is certainly in this category; so are the tube sockets and the 1K Ohm 5W resistors.

Don't forget to check the quality and integrity of the solder connections on the tube sockets, too. There are plenty of cases out there where the heat produced by the tubes caused integrity problems for the solder connections. Never overlook the simple things (all forms of electronics are subject to "physical layer" problems -- cold solder joints, bad connections, bad sockets/jacks, etc). I mention this specifically because running an Amplifier hard for an extended period of time can easily exacerbate (or create) such problems. Running it on 10 with only two finals for an extended period of time strikes me as "running hard". It would be worth it to spend a few minutes at least examining (and/or re-touching) the solder connections in the power amp portion of the circuit.

The OT Transformer tests from Geofex strike me as a reasonable approach if performed carefully. You almost certainly do not have a shorted OT primary as I would expect this to produce a quick (instant) fuse failure. The use of a limiter (light bulb, resistor) is also a good idea.

Good luck! These things can be incredibly frustrating.

[Amp Kat]

Good info from Perro there Mitch and it may come down to you just having to start disconnecting things untill you find it which is what I usually have to do. I like to run the power tranny by itself with nothing connected to it and verify that it's good and then start connecting things down the line and find out what's dragging it down. Another thing Perro also brought up is unless you have a limited supply of fuses the light bulb limiter is a must but I think we discussed it back when but anyway hang in there.

[mykey]

Originally Posted by MitchK

yep 34 years experience milling away in a basement somewhere makes for pretty poor social skills and that what you're displaying.

you're the expert, you know more than me about amps, far more, if you had the amp there i bet you could fix it, I bet you money you could fix it.

I'm making shitty guesses as to whats happening, no doubt. I don't fully understand even whats happening, check!

care to enlighten me o' great one?

you seem to know exactly what the answer is by the sounds of it but just can't share this precious knowledge with me, you've just told me to take it to a technician so far. You? are you having that much of a problem drumming up business you need to canvas message boards?

like I said, if you want to have a pop quiz on guitar amps, you'll win, if thats the only point you've got to prove, someone can give you a gold star and we're all done here, but otherwise if you don't have some suggestions of what I can begin to troubleshoot, just being smug isn't going to teach me anything about amps.

seriously enough of this bullshit though, if you've got some ideas just tell me already.

this is how NOT to work on a Marshall: keep putting fuses in without first finding the real problem. when the fuse blows, you risk damage to the expensive components.
change output tubes over and over without setting the bias correctly.
this causes excessive current draw, again risking meltdown of output transformer.
doing the above will most certainly damage the output transformer, power supply or other expensive and critical parts of the amplifier. If you don't really know how to work on it, its best to hire a real tech who does.
this is really good advice for those who believe they can somehow fix an amplifier by random changing of tubes, random setting of bias, replacing fuses over and over again.
I have seen people do this many times and it never succeeds. next stage: start putting bigger fuses in until the amp smokes, then the repair bill is astronomical. there are some people who will learn from this and others who will not. to set the bias correctly you need: oscilloscope, resistive load, current meter, sine wave generator. if you think you know how to do this without test equipment, you are really dreaming. thats why I said: go hire a real technician, you are passed the self help stage.
one more thing, tube amps contain high voltage that can kill you. even when the amp is unplugged the voltage is stored in the power supply. this is also very good advice.

[Dave Curtis, dB AudioTech]

Originally Posted by mykey

this is how NOT to work on a Marshall: keep putting fuses in without first finding the real problem. when the fuse blows, you risk damage to the expensive components.

Actually, the fuse blows because of a fault in the circuit. Of course feeding an amp fuses is stupid. I use a current-limiting lamp/variac setup for finding shorts/partials.

Originally Posted by mykey

change output tubes over and over without setting the bias correctly.
this causes excessive current draw, again risking meltdown of output transformer.
doing the above will most certainly damage the output transformer, power supply or other expensive and critical parts of the amplifier. If you don't really know how to work on it, its best to hire a real tech who does.
this is really good advice for those who believe they can somehow fix an amplifier by random changing of tubes, random setting of bias, replacing fuses over and over again.
I have seen people do this many times and it never succeeds. next stage: start putting bigger fuses in until the amp smokes, then the repair bill is astronomical. there are some people who will learn from this and others who will not.

READ the whole thread (3 or 4 times if you need to). He seems to know how to bias an amp. You, however, have a lack of reading comprehension.

Originally Posted by mykey

to set the bias correctly you need: oscilloscope, resistive load, current meter, sine wave generator. if you think you know how to do this without test equipment, you are really dreaming.

And if you think you need all that to measure static dissipation you're tripping. I do it with my Fluke 8060a DVOM by the time you're just taking the thing off standby. Go back to puttin' 10,000uF caps on tube rectifiers, phool. "Setting bias" is not "measuring output power".

Originally Posted by mykey

thats why I said: go hire a real technician, you are passed the self help stage.

As you have proven elsewhere on the 'net your advice is off the mark (umm, to say the least); are you going to address any of the OP's specific problems/questions? It would be cool if you could actually help.

Originally Posted by mykey

one more thing, tube amps contain high voltage that can kill you. even when the amp is unplugged the voltage is stored in the power supply. this is also very good advice.

Didn't think so.

To check for arcing sockets: sometimes you can see the arc in the dark. Play the amp with the lights out and look for them around the OPtube sockets (both sides).

"Fried socket" is the most common diagnosis when the customer mentions "open speaker lead" and "cranked it up" in the same sentence. You might be able to see a (conductive) carbon trail somewhere on/near the socket, but occasionally it's in the socket material itself and very hard to find without a megger.

HTH,
-DC

[Steve Conner]

Will you guys stop beating on each other. All I can say is, I'm glad it's not *my* Marshall that Mitch is trying to fix.

In particular, I noticed he was trying to dismantle the output transformer without even being sure that it was faulty. Marshall OTs are fairly valuable, so he ought to have done hi-pot and ringdown tests before condemning it and trying to prise it apart.

If you don't have the gear to perform these tests, one rough and ready method of testing an OT is to feed a few tens of volts back into the speaker winding with a variac. This generates a high voltage on the plate winding, similar to what it would see with the amp dimed. If there are shorted turns or insulation faults, you'll notice a high current draw, and smell burning varnish before long!

When doing this I like to measure the voltage on the plate winding with a DMM on the highest AC volts range, to make sure I'm not overcooking it. If you cranked the variac too high, you could break down a good OT. 500V RMS plate-to-plate is about the maximum I'd care to inflict on a 50 or 100W OT.

BTW, it should go with out saying, but for goodness' sake don't touch the plate leads while you're performing this test! (Yes, those same leads that you're measuring 500V AC on.)

[mykey]

Originally Posted by Steve Conner

Will you guys stop beating on each other. All I can say is, I'm glad it's not *my* Marshall that Mitch is trying to fix.

In particular, I noticed he was trying to dismantle the output transformer without even being sure that it was faulty. Marshall OTs are fairly valuable, so he ought to have done hi-pot and ringdown tests before condemning it and trying to prise it apart.

BTW, it should go with out saying, but for goodness' sake don't touch the plate leads while you're performing this test! (Yes, those same leads that you're measuring 500V AC on.)

thank you. i think the point was that following the methods used, the amp could never be fixed.
readers take note: Steve Conner has a brain and its working.

[mykey]

Originally Posted by Dave Curtis, dB AudioTech

Actually, the fuse blows because of a fault in the circuit. Of course feeding an amp fuses is stupid.

that's the best advice I've heard all day.

[R.G.]

I'm impressed that there's this much smoke around a small fire with so many knowledgeable firemen nearby... 8-)

1. In any fuse blowing problem you have to stop the fuse blowing before you can figure out what is blowing. Make a light bulb limiter.
2. Perro Grande, Steve C, Enzo, and others have good advice. I like the "follow the power parts, then what controls the power parts" advice and the note that high voltage arcing usually follows "without a speaker" and "cranked".
3. Transformers of all stripes can be tested for internal arcing and shorts with the inductive kick test. You disconnect all windings, load any winding up with DC from a battery then disconnect the battery. A resistor/neon bulb is hooked across one winding. If there is an internal short, the bulb will not light. If there is an internal arc-over, you can bound the voltage by which winding has the NE-2 on it. The NE-2 flashes at between 80 and 95V. If you get a flash with the NE-2 on the primary, but not when it's on the speaker outs, there is internal flashover between 90 and 2400V preventing the output winding from getting to flashover.
4. I like the missive to follow what controls the power. A slowly failing bias cap will do much the same thing as has been described.

If it were mine, I'd figure that it's too bungled up by now to ever know the real culprit. I would test the transformer in depth, replace the power tube sockets, see if it still runs away without power tubes in (and a light bulb limiter), test the power transfomer, rectifiers and main caps and only if all that is good look at the bias supply without power tubes, then with power tubes in and a resistive load. If all that checks, run it into a resistive load to try to get it to go over the edge, with a big bulb in the limiter.]

Of course, it MAY be haunted...

[mykey]

Originally Posted by R.G.

I'm impressed that there's this much smoke around a small fire with so many knowledgeable firemen nearby...

the problem here being the person working on it does not want to accept
standard troubleshooting methods.
that is why it should go to a Marshall service center, before it becomes
too badly damaged from all the goofing around.
he could learn real troubleshooting if he wanted to, and i hope someday
he does.

[R.G.]

Originally Posted by mykey

the problem here being the person working on it does not want to accept
standard troubleshooting methods.
that is why it should go to a Marshall service center, before it becomes
too badly damaged from all the goofing around.
he could learn real troubleshooting if he wanted to, and i hope someday
he does.

There are different ways of saying things. Do you know the most effective way to lie? Tell the absolute truth, but in a manner in which it will not be believed.

See my note under "red plating". If one of us pisses him off enough, he will get turned off enough that he'll never learn, and that's a loss for us all. He's a lot more likely to hear and understand what you say if you smile at him instead of kicking him in the shins, even for the exact same words.

Besides, I have a minority opinion... one Marshall less; no huge loss... but then I suspect that not everyone is a Vox bigot, either, so that probably runs both ways. C'est la vie. One many's fish is another man's poisson.

[mykey]

Originally Posted by R.G.

There are different ways of saying things. Do you know the most effective way to lie? Tell the absolute truth, but in a manner in which it will not be believed.

See my note under "red plating". If one of us pisses him off enough, he will get turned off enough that he'll never learn, and that's a loss for us all. He's a lot more likely to hear and understand what you say if you smile at him instead of kicking him in the shins, even for the exact same words.

Besides, I have a minority opinion... one Marshall less; no huge loss... but then I suspect that not everyone is a Vox bigot, either, so that probably runs both ways. C'est la vie. One many's fish is another man's poisson.

well sorry to say that is the truth. advising this person to head for the service center is the best advice possible. encouraging people to work on high voltage circuits with no experience may not be the best advice either. but several have learned from this too: learn some basic measurement, troubleshooting and safety before attempting to repair an amp like this. now the amp is a paperweight, and it all would have been avoided. opening a tube amp and repairing it is not a simple easy thing. maybe the person who did it won't accept that but many other people are learning that from this experience.
start by learning how to use a volt ohm meter, how to solder, how to discharge a power supply, how to properly measure and adjust bias, how to avoid being shocked, etc...
then tackle this project.
Marshall, Vox and others are great amps, but to work on one is not the same as changing the oil in a car engine, changing guitar strings. It's complex and very dangerous, and to encourage people to do this without at least some safety training is very irresponsible.

[R.G.]

Well, you see, you ducked the issue.

There can be no doubt that advising people to be careful, and to refer repairs to skilled personnel is a good idea. But his hands are already in the amp.

However, that's not what I posted about, and is the kind of answer I would expect from the master of reply spin, William Jefferson Clinton.

I was pointing out that he's not likely to accept anything you tell him, correct or not, and is in fact more likely to do the opposite. So let's address one issue clearly and unambiguously.

Do you want to help this guy get his amp fixed? Yes, or no?