#1
So my JTM45 build is having some issues.

First off, the power tubes are smoked, but I already know that. One of them looks like a hot coal when the amp is on. Those will have to go out immediately. I'm replacing them ASAP.

But something else I've noticed is a buzzing kind of sound coming through my speaker on low notes, its a faint (?) but very noticeable crackling fuzziness, almost like tube rattle being amplified (sort of) under the note I'm playing, at all volumes, and on high notes I get pretty bad ghosting, seriously as if a discordant note is being played over every high note I play.

Could this be due to a microphonic preamp tube? Or is it a sign of bad caps. I've heard of problems with the coupling caps raising these sorts of issues. Or bad filter caps as well? The last thing I really need is for a filter cap to short out and explode all over my amp. But those are new, I built the amp over the summer. Regardless the first thing I will do is swap out the power tubes as I know those are problematic.
Last edited by al112987 at Dec 22, 2008,
#2
well since power tubes are definitly bad, I qouldn't judge the amp on it's sound until you actually have new and working power tubes in there. If the filter caps are a few months old, i wouldn't think of those yet, although you can't rule them out. When filter caps fail, they do let a bit of hiss and hum into your circuit.

if you're replacing power tubes, why not replace power tubes for a couple of bucks more?
Quote by Invader Jim
The questions people ask here makes me wonder how the TS's dress themselves in the morning and can shower without drowning...
#3
Quote by al112987
So my JTM45 build is having some issues.

First off, the power tubes are smoked, but I already know that. One of them looks like a hot coal when the amp is on. Those will have to go out immediately. I'm replacing them ASAP.

But something else I've noticed is a buzzing kind of sound coming through my speaker on low notes, its a faint (?) but very noticeable crackling fuzziness, almost like tube rattle being amplified (sort of) under the note I'm playing, at all volumes, and on high notes I get pretty bad ghosting, seriously as if a discordant note is being played over every high note I play.

Could this be due to a microphonic preamp tube? Or is it a sign of bad caps. I've heard of problems with the coupling caps raising these sorts of issues. Or bad filter caps as well? The last thing I really need is for a filter cap to short out and explode all over my amp. But those are new, I built the amp over the summer. Regardless the first thing I will do is swap out the power tubes as I know those are problematic.

That's a serious issue. Sounds like red plating. Your going to have to be careful when you replace the tubes not to do the same thing for the next tubes. Check the bias

Is this the JTM45 Metro amps built fot you?
#4
Not exactly, I built it from Metro's parts. Either way, I know the power tubes are fried. Back when I first built the amp I pretty much fried the power tubes the first time I biased them. Funny story actually and a really bad decision on my part, my bias voltage was -35V when it should've been in the -45V to -50V range, but was a little too impatient and decided to run the set anyway. Well first thing I do when I turn the amp on with the tube installed, I check the idle current, 115mA on the plates!

Anyway, I eventually fixed it and got it back in range, but I pretty much knew those tubes were going to be on their way out very soon. Well, low and behold, here they are redplating again. Either the tubes are on their way to their imminent death or the bias just drifted a ton (which it shouldn't have as I checked it periodically the first few hours I was using the amp) I've been meaning to switch them out of el34s but can't until I change the screen grid resistors to a higher value.

My biggest concern has to do with the fuzziness and ghosting, I haven't played an amp old enough that really needed a cap job before so I can't really tell what bad caps sound like. It worries me most because it started very recently, the redplating has been going on for months (the amp hasn't been seeing regular use as I've been away at college). But it seems that the general symptoms of bad caps are ghosting and fizziness. Regardless, I saw THIS picture a few days ago and I'll have to say it's really been making me nervous.



Last edited by al112987 at Dec 23, 2008,
#5
Yikes that looks terrible...

What filter caps are you using? They could be old and could have been sitting on the shelves for a long time. It is quite unlikely that you have a failing coupling cap, a bad filter cap is much more likely.
#6
F+T filter caps. A 16x16 450V and a 32uf x 32uf 500V. I know a lot of people go on about forming filter caps before using them and I never formed these. But shouldn't they be pre-formed from the factory? I'd imagine it would be a good regardless to form new filter caps if I replace the old ones. I mean, it's a pretty cheap and simple fix.