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**Enzo** · 06-06-2007, 06:49 AM

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** · 06-06-2007, 07:18 AM

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** · 06-06-2007, 08:01 AM

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** · 06-09-2007, 04:18 AM

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** · 06-09-2007, 05:03 AM

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** · 06-09-2007, 05:59 AM

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** · 06-09-2007, 07:12 AM

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** · 06-09-2007, 08:55 AM

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** · 06-09-2007, 02:13 PM

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** · 06-09-2007, 02:51 PM

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** · 06-09-2007, 03:37 PM

Originally posted by MitchK View Post

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** · 06-09-2007, 06:21 PM