blowing fuses with no tubes

[vigs]

hey all,

i was enjoying my 36 watt tmb for some time (i purchased it thirdhand, i have no knowledge of who the original builder is). it had some oscillation issues when i picked it up which i addressed by cleaning up the lead dress. i also put in a half power mod. at practice the other day, it just gave up on me. mains fuse was blown and testing with no tubes, it still blows fuses. i've read this most likely implicates a shorted PT or filter cap. i tried testing the PT with everything i could do from this page (http://www.geofex.com/ampdbug/pwrtrans.htm) without having it powered up and it tested fine. also, poking around the chassis, i don't see any leads that have come loose or anything that should be shorting out.

where should i go from here? i figure next step could be desoldering the PT from it's connections and seeing if it pops fuses connected to nothing. i assume that could definitely implicate a blown PT. anything i can do to suss out if it's the filter caps?

thanks for the help!

[kd]

Leave the tubes out of the amp and put the standby switch in the "standby" position(not play). Turn the amp on and see if the fuse blows.

Does the amp have a tube rectifier or SS rectifiers?

Ken

[rjgtr]

Without the tubes, the OT shouldn't provide enough of a load to be a problem. As Ken asks, what kind of rectifier is it? If it is tube and the tube is out, then you very well might have a bad PT. But don't rule out a bad wire or other simple issue. It only requires one strand of wire to ground or cause mayhem. Also check the power light, make sure it's not shorting or otherwise having an issue.

[vigs]

it has a tube rectifier.

it blows fuses when powering on with the amp in standby (or in play, not that that means anything). the first time it happened, it was in the middle of playing. since, it pops the fuse immediately after powering on whether in standby or play.

[rjgtr]

Sounds like it is likely a PT issue. To be totally sure, disconnect and tape off all the PT SECONDARY wires, except maybe the grounds, and try again. If the fuse blows then the PT is bad. Leave the primary wires connected.

[vigs]

i'll give that a shot this weekend. presuming the PT is blown, what might be going wrong in here that would make that happen?

when i first got the amp, one of the el84s was in wrong, it had one pin bent all sideways and it was inserted a way it shouldn't be. however, that was months ago. could any strain that put on the PT somehow manifest now with it suddenly shorting out?

[rjgtr]

If it had weakened the PT it might take some time for it to ultimately fail.

An easy way to tell if you're drawing too much current it to make a light bulb bases current limiter to test with. It's basically a light bulb in series with the AC wall voltage. If it glows bright, then you're drawing too much current. If you're doing amp building it is cheap and easy to build and will keep you from blowing fuses and can tell you if your wiring it meesed up.

[Chubsman]

Here is another test you can do with the PT completly out of the amp (or atleast disconnected). You will need a 9V battery and an indicator light.

You can test each winding of the transformer,

http://www.premierguitar.com/magazine/I ... ester.aspx

Try not to kill yourself.

[vigs]

thanks guys. i'd been meaning to build one of those light bulb limiters. now seems like a good time to do so. that little battery trick is intriguing too. i'll post results this weekend.

[vigs]

i disconnected the secondary entirely and it still popped a fuse so i assume that definitely implicates the PT. guess i'll be ordering one tomorrow. unfortunately, i won't be able to play it at my gig monday night. oh well. [/img]

[vigs]

fyi, even though it was possibly user error, weber is sending me a new PT for free. i was thinking i'd upgrade to a mercury or something similar but at that price, it's hard to pass up.

[rjgtr]

I would still check all of the wiring from your power cable to the input of the transformer. Makes sure none of the wires has cut or worn area that's making it ground. Also check the power switch and fuse connections. It would be a shame to wire in a new PT just to have the issue continue.

I know this sounds a little crazy, but have you tried it in a different room in the house? Can you make sure your wall plug is wired correctly (with a ground/fault detector plug)?

[vigs]

I gave all of the wiring a once, twice and thrice over. i could have missed something but don't see anything that should be shorting out. since they're sending me the PT gratis, running the same test with the new one, (only the primary wired in) should be a pretty good test and not a significant time suck if, in fact, you're right and it's a bad wire or something similar.

as for trying in a different outlet, i haven't tried that and i'll give it a shot. i think i have 2 separate 20 amp circuits in my rehearsal space where the amp currently lives. not that is definitive in any way, but i've run at least 5 other amps on the same circuit, same jack in the outlet, with no issues.

[vigs]

with the new PT, the amp is singing again. thanks all.