OK, I goofed.
I turned my guitar amp on the other day and didn't hear anything. I strummed a few chords, TURNED THE VOLUME UP (*forehead slap*), and let a few question marks appear above my head before turning it off. I then realized that I had unplugged the speaker earlier and forgot to plug it back in. Now there's no output coming from the amp. Balls.
So I damaged something, although I'm not sure what. From what I've read, it's likely I destroyed the output transformer.
Any tips you guys can give for troubleshooting this kind of problem? The amp is a homebrew similar to the Princeton Reverb.
I turned my guitar amp on the other day and didn't hear anything. I strummed a few chords, TURNED THE VOLUME UP (*forehead slap*), and let a few question marks appear above my head before turning it off. I then realized that I had unplugged the speaker earlier and forgot to plug it back in. Now there's no output coming from the amp. Balls.
So I damaged something, although I'm not sure what. From what I've read, it's likely I destroyed the output transformer.
Any tips you guys can give for troubleshooting this kind of problem? The amp is a homebrew similar to the Princeton Reverb.
Belongs in I & A so I'll move it there.Sorry to hear about the amp.. I suspect Wavebourn is right and the transformer is shorted, but check for blown fuses.
I see you say, homebrew Princeton Reverb. If you're lucky, and your output jack is arranged to short out with nothing plugged in, as per the original design, you might just be talking about the HT fuse, or maybe output valves.
I've only ever done this once. Recently. Into an open circuit speaker. The amp's not completely dead. Now distorts with a sound like an arc welding set.
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Simon:
That sounds like typical OPT damage from such an incident. Use some sort of stethoscope and listen to the tranny while running signal into a dummy load.
Inspect tube bases and sockets for signs of arcing. Better, just toss the power tubes.
fingersoccer, there's no telling what you took out. If you're reasonably lucky the thing went nuts and blew the screen resistor. That would explain silent and is a fairly probable fault. Completely opening an output tranny isn't that likely, damaging it severely yes, but they rarely just go completely open. You're going to have to break out the meter and start measuring things. Start with the voltages around the power tube. Oh, and start with a new one.
That sounds like typical OPT damage from such an incident. Use some sort of stethoscope and listen to the tranny while running signal into a dummy load.
Inspect tube bases and sockets for signs of arcing. Better, just toss the power tubes.
fingersoccer, there's no telling what you took out. If you're reasonably lucky the thing went nuts and blew the screen resistor. That would explain silent and is a fairly probable fault. Completely opening an output tranny isn't that likely, damaging it severely yes, but they rarely just go completely open. You're going to have to break out the meter and start measuring things. Start with the voltages around the power tube. Oh, and start with a new one.
Thanks for the advice. There's a repair shop nearby with a tube testing machine. I'll take them by and have them checked out, grab some new ones if need be, and measure the voltages around the tubes. I'll come back with results.
Oh yes, Ronsonic, it is indeed OPT caput. Hence the non-smilies!
.......😀My stethoscope has a bit too much metal at the probe end for me to be comfortable using it for this particular application. I use a bit of rigid plastic tube when HT is involved, plenty of insulation, pressed to the bone in front of my lughole it works fine.
Fingersoccer, hope you were lucky and get away with a fuse, screen grid resistor or valve socket - anything other than the OPT in fact.
Just thought if we had a schematic or parts list to go by, particularly info on the output transformer we could give better advice.
I would remove the output tubes and ohm out the transformer, the leads and then to the iron. Gross faults to be found that way. Then if that was inconclusive I would take a small wall adapter transformer, about 6V, crack it open and use the 6V AC to feed the secondary. Measure the voltages across all the terminals and to the iron. Don't forget that there will be (hopefully) a high voltage across the secondary.
I would remove the output tubes and ohm out the transformer, the leads and then to the iron. Gross faults to be found that way. Then if that was inconclusive I would take a small wall adapter transformer, about 6V, crack it open and use the 6V AC to feed the secondary. Measure the voltages across all the terminals and to the iron. Don't forget that there will be (hopefully) a high voltage across the secondary.
Here's the schematic.
The OT is made by Weber Speakers, and is model W041318.
The OT is made by Weber Speakers, and is model W041318.
An externally hosted image should be here but it was not working when we last tested it.
Nah, pictures seem to take a while to get resized when I look at them with my browser too - look again, you might find it shrunk now.
Well your schematic shows a shorting jack speaker output. If your amp's wired that way there's a good chance that you won't have wrecked the OPT, more likely something else. Running into an open circuit produces very high voltages on the windings, which arc over.
Assuming your trafo was shorted, the damage will have been caused by current rather than volts. In my experience OPTs are way more resilient to this than over-voltage. I've seen them cooking to the point of smelling strongly and they've been fine, but arc that primary over just once.....
You have checked the fuse by now right? 😀
Yeah, the fuse looks good. When I turned it on the other day, the pilot light was glowing and I could hear a really (really) faint oscillating noise. If the fuse was gone, it would be entirely dead...right?
OK, so that's potentially good news about the OT not being goofed. I'll see if I can swap out the tubes tomorrow. Here's a question though; if I switch out the tubes and that's NOT the problem, then won't I run the risk of damaging something again when I turn it back on to test it? Or: is there a better way to root out the problem before turning it on?
I appreciate the help.
OK, so that's potentially good news about the OT not being goofed. I'll see if I can swap out the tubes tomorrow. Here's a question though; if I switch out the tubes and that's NOT the problem, then won't I run the risk of damaging something again when I turn it back on to test it? Or: is there a better way to root out the problem before turning it on?
I appreciate the help.
Nope, secondary. Six volts across the primary will produce nothing for voltage in the windings. Fed into the secondary you should get a 100+ volts on the primary if all is good. Transformers can test good with an ohm meter but bad with high voltage (not that you would hook up an ohm meter with high voltage on a transformer).
I agree with the shorting output jack saving the OT for a brief period of time if it was fitted with one. Takes a while to heat up the windings while excessive voltage takes a mS to punch a hole through the insulation. I would start by taking voltage measurements on the output section of the amp to see if all is where it should be.
I would not swap in some good ones yet. I would measure the voltage at the secondary center tap, at the plates of the output tubes, at the screens and across R21, R22, and the bias voltage.
To start with I'd carefully check the amp cold and unplugged, checking that the psu caps have zero volts on them.
Check the appearence and resistance of the screen grid resistors, R23 & R24, 470 ohms, pin 4 on valve socket. With the valves cold they're non-conducting, so you can put your meter straight across them, but take the output valves out anyway, just in case they're permanently shorted somehow.
Also the cathode resistors, R21 & R22, on pin 8. They're each 1 ohm, but are in parallel, so you should see 0.5 ohm here. For a low ohms measurement like this, touch the two test probes together first, and subtract any resistance you see from your actual reading.
Whilst you're at it, check R44 & R45, 1k5, on pin 5, and R19 & R20, 220k, connected to them.
You mentioned getting the 6V6s checked? This should catch a really bad fault, but generally tube testers treat valves very gently - they often put far less voltage on than an amp will, so they don't tell the whole story. Worth doing anyway, if you can.
Would be good if you told us what test gear you have and are you experienced with this stuff at all? Safety-wise, this stuff can be a bit unforgiving of mistakes...😀
Check the appearence and resistance of the screen grid resistors, R23 & R24, 470 ohms, pin 4 on valve socket. With the valves cold they're non-conducting, so you can put your meter straight across them, but take the output valves out anyway, just in case they're permanently shorted somehow.
Also the cathode resistors, R21 & R22, on pin 8. They're each 1 ohm, but are in parallel, so you should see 0.5 ohm here. For a low ohms measurement like this, touch the two test probes together first, and subtract any resistance you see from your actual reading.
Whilst you're at it, check R44 & R45, 1k5, on pin 5, and R19 & R20, 220k, connected to them.
You mentioned getting the 6V6s checked? This should catch a really bad fault, but generally tube testers treat valves very gently - they often put far less voltage on than an amp will, so they don't tell the whole story. Worth doing anyway, if you can.
Would be good if you told us what test gear you have and are you experienced with this stuff at all? Safety-wise, this stuff can be a bit unforgiving of mistakes...😀
According to the schematic, there is no extra HV fuse, so thats too bad:
If there were, it would have been blown, saving everything else.
I once saw a guitar amp that had low wattage cathode resistors (typically 1 ohm), all fried.
Replacing the resistors restored the amp to new.
Often these 1 ohm cathodes are just cheap fuses:
If there is over-conduction they blow out, saving tubes and transformer.
A few brands of guitar amp did this, I can't remember which now.
You may only have to replace the cathode resistors.
Remember, if they are only 1 ohm, they are not bias resistors,
and only provide a cheap fuse and a way of measuring current (inaccurately) going through the tubes.
If there were, it would have been blown, saving everything else.
I once saw a guitar amp that had low wattage cathode resistors (typically 1 ohm), all fried.
Replacing the resistors restored the amp to new.
Often these 1 ohm cathodes are just cheap fuses:
If there is over-conduction they blow out, saving tubes and transformer.
A few brands of guitar amp did this, I can't remember which now.
You may only have to replace the cathode resistors.
Remember, if they are only 1 ohm, they are not bias resistors,
and only provide a cheap fuse and a way of measuring current (inaccurately) going through the tubes.
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