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If one of your filter caps "bubbled up" I'd replace them all-not just the obvious one. When that went bad, it stressed the other caps.
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Yank all the tubes and disconnect all the power caps including the ones on preamp boards from the rails so all that is left hooked up is the transformer. Turn on the power and check the transformer after 15 mins. or so. If it's a bit warm but you can't smell it it may be OK. Reconnect the first cap can and repeat, then the second one, then the last one. Then the preamp ones one at a time.
Me, I wouldn't bother. I'd change all the power caps 'on general principle' and the rectifier too.
You were lucky your JJ cap just bubbled up - I had one that somebody else installed blow and spew cap guts all over the inside of a JCM900. Geez, what a mess.
KenComment
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If you are popping the meter fuse, it sounds like you are connecting the meter from B+ to ground. Ammeters must be in series with the load.
If I want to check B+ current, a handy place is often the standby switch. leave the switch OFF, then connect the ammeter across the switch. REmember the ammeter has a very low resistance through it, so it completes the circuit, and measures the current flowing through it.
When you measure VOLTAGE across the old one ohm cathode resistor for bias, you might look at the reading as molloamps, but you are actually reading millivolts and translating that into milliamps. The 1 ohm resistor then takes the place of the current path that should have been the meter. Otherwise we would have to wire the meter in series with the cathode and ground
Often a fuse is a handy place. Remove a fuse and connect an ammeter in its place.
And excess draw translates back to the PT primary. My test outlet has a current meter in series with it, so I can plug an amp into the outlet and see what it draws from the mains. If an amp usually draw 120 watts at idle, and this particular one draws 180 watts, that means we are using a half an amp more than normal. It is going SOMEWHERE. If it is cap leakage draw that concerns you, pull the tubes. That removes the power lost to the heaters. A tube amp without tubes should draw very little from the mains. A little eddy loss to the PT. At turn on, a very brief charging surge for the filters, but after a second, they are charged and draw no further current. If they do, something is leaky OR there is a relatively low bleeder resistance. Of course a tube recto means at least its heater is running if you want B+.Education is what you're left with after you have forgotten what you have learned.Comment
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Another way is to pull all the tubes and check the voltage at each filter cap.Assuming the first cap is okay you shouldnt see a voltage drop at the following caps of more than a volt or 2.A leaky cap will show a voltage drop across the resister between that cap and the previous cap.But I would do like Ken says and change all the caps after having one blow,as they all were stressed when the first one blew and most likely will fail sooner or later.Comment
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How does a shorted cap stress its neighbors? A short on the B+ line removes the voltage across the caps elsewhere in the string. The stress is on the rectifier.Education is what you're left with after you have forgotten what you have learned.Comment
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Since the cap didnt actually blow,as he said it was bubbled,had the cap exploded I would think the PT would have had a better chance at surviving.Since the PT did actually fry,I would think the entire power rail was stressed before the cap and PT went south,including the rectifier,but the rectifier didnt short or it wouldnt be working now.Maybe the other caps are fine,but I would change them just in case.Since he is concerned the PT is too hot, we who cant actually tell if it is too hot or not have to go on the assumption that it is and can only offer theories as to what might cause it.My feeling is that since the PT did go it wasnt a fast shorted cap,which would have acted like a fuse,but a slow drain which stressed everything till the PT went.If the new PT is indeed too hot there must be something stressing it,and the old caps arent a bad place to look,I think.Comment
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the rectifiers test good, the amp is sounding very nice. I let it idle for about an hour and the PT still gets hot. This may help in describing-- i can put my hand on it for about 5-6 seconds .Comment
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If by putting your hand on it for 5-6 seconds you mean you have to remove your hand or you will get burned,I would have to think that is too hot.Try pulling the tubes and measure the voltage at each cap to see if you get a voltage drop at any of the filter caps,as I suggested earlier.Comment
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tubes removed-- i get 513v at the first cap and 513v at the next cap (50/50 caps) . This is a marshall 2203 circuitComment
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The schem for the 2203 shows a different cap as the first,but anyway you have to check all the caps in the whole rail down to the preamp.If you see a significant voltage drop at any of them the prior one would be leaky.Comment
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with tubes in i get 468v at the first cap, 465 at the next, 319 and 304v at the last one. The amp has always had these same voltagesComment
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That sounds okay but try it with the tubes out.With no tubes drawing current you should get the same volts all the way down the rail,if you see a voltage drop at any of the caps down the line it will indicate the previous cap is drawing current/leaky.I am not talking about 1 or 2 volts but a significant drop.Comment
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How hot does the PT get with the tubes removed? With the secondaries unloaded?Education is what you're left with after you have forgotten what you have learned.Comment
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That 27 volt drop doesnt look right.What are the values of the dropping resistors between each cap?The schematic I have, that resistor just past the choke is unreadable.I suspect it is less than the 10k downstream but if it were 10k that would be a 72ma across that resistor.Thats too much,like having a pair of 6L6's drawing current.If the resistor is smaller than 10k the draw is that much higher.Comment
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