Hey amp gurus: Marshall in distress

[Tadpui]

I may have had some amp maintenance go awry tonight...

I've been dissatisfied with how my Marshall JCM800 2204S has been sounding. It just sounds strained, like I can hear the transformer distorting as much as the tubes. So I sat down tonight to bias the amp, which I haven't done for several years. I've still got the same set of JJ 12AX7 in the preamp and JJ EL34L in the power section as I've had since I biased it last (5 years ago, maybe?).

I broke out the tube probe from Eurotubes, got it all set up and got it biased to the best of my ability. The last hands to be inside of it were by an amp tech in Kansas City. He'd commented that it was biased cold (which is how I'd set it) so he rebiased it. I never checked it to see where he set it.

Tonight when I first turned it on, it was running at about 470 volts and 26 mA on the tube that I was measuring. I did the Eurotubes calculation for my 25W EL84L tubes ((25W / 470V) * .7) to set it at 70% of its max. That worked out to about 38 mA, which brought down the plate voltage to about 460W, give or take.

I let it settle in for about 30 minutes, made another adjustment to keep it at that 38 mV value after it'd risen to about 42 mA, then reassembled the amp and started playing. It sounded great, not quite as strained as before, but still a little.

Then...nothing. It just quit. It briefly made that fuzz-pedal tone that it makes when I play it as it's powering down. Then it went quiet. I tried both inputs, made sure it wasn't my guitar or cable or attenuator that went kaput. Power tubes were still glowing, pilot light was still on. I don't know what happened. My adjustments were well within spec, as far as I know.

To add insult to injury, I decided to check the fuses. Both the 500 mA and 3A fuses came apart inside the sockets. The 3A fuse spilled what looks like silicon all over the place. I can't even see a filament in the remainder of the 500 mA fuse.

So now I have 2 problems: a) an amp that suddenly quit working mid-strum, and b) two fuse ends stuck inside the fuse plugs inside the amp.

What the hell do I do now?

[Armistice]

Ouch!

I wouldn't be so reckless as to venture an opinion, and if I did, you probably shouldn't act on it.

Prayer?

Sounds like one for our resident amp builder to me...

Good luck.

[Lt. Bob]

I'm gonna say the deteriorating fuses would make the sound go away.
I imagine I'd be able to get those out with some small needle nose .... prolly with a lot of cussing.
greg'll let you know about the sound but I think that 500 ma fuse would act sorta like a standby switch when it failed removing needed voltage from the output tubes.

[JD01]

Yeah, this is one for [mention]Greg_L[/mention]
Like most things electrical, you'll need to get the fuses sorted though, its pretty hard to test stuff if you can't turn it on!

[Greg_L]

The tubes lighting up is a good sign, but it doesn't tell you a whole lot.

The simplest guess is that you had a power tube go bad. You said yourself the amp was sounding weird so you went it in to check it. You bumped the bias up hotter and then things went kaput. Maybe that tube could no longer handle the higher current. Just a guess. Always always ALWAYS suspect tubes before anything else. Get some new EL34s.

Obviously the first thing to do is fix the fuse situation. Unplug the amp and drain the filter caps. You see those big cans on your amp? Those are filter caps. They're probably blue. Maybe black. You can drain them by grounding the positive terminals to the chassis with a jumper wire. They're all connected, so pick whichever terminal is easiest to deal with. It only takes like 15-20 seconds. Use a multimeter to probe the terminals and see how drained they are. < 10v and you're good to go. These things probably won't kill you, but they can really shock the ever loving shit out of you, and there is the freak accident potential for death. So anyway, try like hell to dig the fuse remnants out of there cleanly and safely. Spray some contact cleaner or whatever into the fuse holders to break up the crud. Steal your wife's tweezers if you have to. And of course, order a bunch of new fuses so you have some backups. Make sure you get the right values.

With the fuses sorted, power the amp on with no power tubes in it and see what happens. All of these next steps happen with no power tubes. If the amp stays alive, with your multimeter carefully check the bias voltage at the grid pin of the power tube sockets - pin 5. It should be a negative voltage. Probably like -50vdc or something like that. If that looks good, turn your bias pot so you get the most negative number you can get on pin 5. Turn it all the way full negative. And verify that both tube sockets are getting the same voltages. So now you have the bias set so cold that tubes probably will barely even work, if at all. Turn the amp back off, drain the caps again, and check the screen resistors. These things like to pop in old amps. These are the big resistors across the power tube sockets connecting at pins 4 and 6. Pin 4 is the actual screen, pin 6 is unused and just used as a convenient tie off point. These should be 1k ohms....give or take 10% or so. If they're open or shorted, replace them. If they measure anything anywhere near 1000 ohms, you're fine.

You can go around the power tube sockets and check for expected voltages.
Pin 1 - suppressor grid, connected internally, tied to pin 8 to ground. Nothing here.
Pin 2 - heater 6.3v AC
Pin 3 - plate, very high voltage, probably around 450-500vdc
pin 4 - screen, also very high voltage, should be a little less than plate voltage
pin 5 - grid, negative bias voltage
pin 6 - no connection in an EL34 but this socket pin is often used to tie the other end of the screen resistor
pin 7 - heater
pin 8 - cathode to ground. 0v.

If all that checks out okay, then you can add the new power tubes in it and re-bias. It was probably just a bad tube all along!

It's also maybe a good idea to change out all of the electrolytic caps if they haven't been done already. They're old now. All of the filter caps and definitely the bias caps could probably stand to be replaced if they're still original.

[Tadpui]

Thanks [mention]Armistice[/mention] [mention]Lt. Bob[/mention] [mention]JD01[/mention] for the words of encouragement.

[mention]Greg_L[/mention] so a "bad" tube could still glow when the amp is turned on?

Thanks a ton for all of this info. I'm honestly not sure how brave I'm feeling. I guess that I need to start getting comfortable with all of this stuff if I'm ever going to build an amp. But damn, poking around inside a live amp without really knowing what I'm doing has me shaking at the knees a bit. It sounds like a recipe for a trip to the ER. And I'm currently uninsured until the first of the year

[Lt. Bob]

Thanks @Armistice @Lt. Bob @JD01 for the words of encouragement.

@Greg_L so a "bad" tube could still glow when the amp is turned on?

Thanks a ton for all of this info. I'm honestly not sure how brave I'm feeling. I guess that I need to start getting comfortable with all of this stuff if I'm ever going to build an amp. But damn, poking around inside a live amp without really knowing what I'm doing has me shaking at the knees a bit. It sounds like a recipe for a trip to the ER. And I'm currently uninsured until the first of the year

nah ..... just drain the caps like greg said and take your time and don't just shove your face in there.

The actual biggest real risk is getting shocked and banging your hand into something when it jerks away.
you can cut yourself on something sharp.
But even though it's technically possible for someone to die just take a google and you will find basically no documented instance of it ever happening.

i've disputed this for decades ....... i used to dig around in live amps all the time when i didn't know what i was doing .... did get shocked a few times .... big deal.

I can't imagine anyone that builds computers not being able to be careful about what they touch inside a analog amp where everythings large enough to clearly see.

[Greg_L]

Thanks @Armistice @Lt. Bob @JD01 for the words of encouragement.

@Greg_L so a "bad" tube could still glow when the amp is turned on?

Thanks a ton for all of this info. I'm honestly not sure how brave I'm feeling. I guess that I need to start getting comfortable with all of this stuff if I'm ever going to build an amp. But damn, poking around inside a live amp without really knowing what I'm doing has me shaking at the knees a bit. It sounds like a recipe for a trip to the ER. And I'm currently uninsured until the first of the year

Yes a bad tube can still glow. The filaments (the glowy part) can still light up. They are in close proximity to everything, but they are separate from the actual signal part of the tube. Unless something is literally smoking or on fire, a bad tube is always the first suspect. I once had a TAD EL34 - supposedly great tubes - put on a full 4th of July light show and the amp went BZZZZT. Nothing. There were plasma arcs and sparks and strobe lighting and the terrible smell of electrical fire. The tube shit the bed bigtime but the dumb ol heaters still glowed.

It's good to be cautious but you don't have to be scared. It's like Alanis Morissette said, "I got one hand in my pocket and the other is checking plate voltage".

Just keep a hand in your pocket. What you don't want is current crossing your chest. So if you have one hand not touching anything, current won't flow through you. Clip the ground multimeter lead to a good ground and leave it there. Use your one hand to probe for measurements. It's no big deal. Just be slow and precise with what you touch. Make sure you have good lighting. If you need to move some wires around, use a wooden chopstick or something equally non-conductive. It's really not a big deal. And you'll need to verify some things before you go blowing up a new set of tubes.

And yeah, if you ever wanna build an amp, even a tiny Champ, you'll need to get comfy with this.

[Tadpui]

Well fellas, I'm going to chicken out and take it to a local repair shop for service. As much as I'd love to take this opportunity to learn about the inner workings of my amp, I'm going to have to leave it to a pro. Next time, when I'm fully insured, I'll probably do it myself.

I called a local shop (they're actually a vacuum tube shop that also does repairs on amps and such...a rarity these days!) and their turnaround time is 3 weeks. Not ideal, but at least I can just drop it off and not think about it for a while before it comes back to me. I'll go drop it off tomorrow.

[Greg_L]

Well fellas, I'm going to chicken out and take it to a local repair shop for service. As much as I'd love to take this opportunity to learn about the inner workings of my amp, I'm going to have to leave it to a pro. Next time, when I'm fully insured, I'll probably do it myself.

I called a local shop (they're actually a vacuum tube shop that also does repairs on amps and such...a rarity these days!) and their turnaround time is 3 weeks. Not ideal, but at least I can just drop it off and not think about it for a while before it comes back to me. I'll go drop it off tomorrow.

Yeah maybe for the best. If you aint 100% committed, don't mess with it.

I'm serious though about the filter caps and bias caps. Have them checked. Old amps and old electrolytic caps aren't a good combination. If those are original they're coming up on...35 years old? Swap them out.