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Old 20th February 2006, 12:50 AM   #1
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Default Not your average output transformer problem

So I've been playing with my Bogen M60A PA head a little today.

I had an old 8-ohm stereo speaker hooked up to the 8-ohm screw terminal and ground on the back of the amp. I plugged a minidisc player through the "Mag-Tape" RCA input, and got no sound, I tried both of them, as well as the "aux". Nothing.

So I decide to try the guitar. I play with it for about a half hour and suddenly I start hearing it faintly through the speaker - I had it turned down. I had to crank it before I could hear it fairly well. I couldn't have wished for a sweeter-sounding distortion by the way. I tried it with the speaker in my other guitar amp, which is a more efficient speaker. At full volume on the 60W M60A it sounded about like half volume on the 30W solid state amp I have now does.

This is where it starts sucking. I was using it with the old 8 ohm stereo speaker, it wasn't quite loud enough and I noticed it was rattling the amp so I shut it off to put some padding between the head and the speaker. I turned it back on, and nothing came out.

Now it gets weird - I checked all the connections, everything is fine. I plugged my minidisc player back in and after the amp warms up for a few seconds I start hearing the music very faintly. I then realized the sound wasn't coming from the speaker, it was coming from the output transformer! I hardly consider myself an expert on transformers but I've never seen one actually produce sound itself. I can't imagine that could be good for it so I shut it off.

This can't be good, but at least my tubes seem okay . Any ideas?

-Darren
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Old 20th February 2006, 02:18 AM   #2
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Darren,
Had just this problem with a Fender Super Twin. Both 12" speakers on it were blown open circuit and the faint sound I was hearing was comming from the output transformer. That old speaker you mention is almost certainly blown.
Cheers,
Ian
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Old 20th February 2006, 02:43 AM   #3
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Sound coming from the output transformer is common in tube amps. Some are louder than others. You don't normally notice it because the sound from the speaker is far louder.

It should be noted that operating a tube amplifier with no speaker (or an open speaker) can cause damage. Sometimes nothing bad will happen, but sometimes the output transformer or tubes can be blown.
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Old 20th February 2006, 03:03 AM   #4
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It's not a blown speaker, I also tried it with the speaker in my current guitar amp - also 8-ohm, and still got nothing. I checked the wiring inside the amp, it all looks fine. Any other ideas?
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Old 20th February 2006, 03:42 AM   #5
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you could have a blown winding on your o/p xformer...
Did you check it for continuity?
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Old 20th February 2006, 03:53 AM   #6
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What Quad Dot ("...." above) says - check the secondary winding in particular for open circuit
Cheers,
ian
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Old 20th February 2006, 11:26 AM   #7
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Ohh,Also check the little ground strap thingie under the screw on the speaker GND connection,is it connected to the other screw?
IIRC the Bogens had the tranny 0V terminal grounded to the chassis with the little screw-jumper on the speaker terminal strip,so you could let it 'float' if needed.. Check the link/strap,and make sure you used the correct GND terminal for the speaker.
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Old 20th February 2006, 03:52 PM   #8
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Turned out to be a bad wire I was using to hook up the speakers. I checked for continuity and everything was fine. I was about to go to bed last night and figured I'd give it one last run-through, and I'm not sure what compelled me to check continuity on the wires I was using to connect the speaker, but one of them was indeed bad. It's still pretty quiet for a 60W tube amp though. It could just be the speaker I'm using, so I won't really judge it until I can get my hands on a proper speaker cab.

I guess I really have to consider myself lucky, apparently I've been running it a fair amount of time with no load . Thanks for the help.

-Darren
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