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Post subject: Blowing fuses Showman 65
Posted: Fri Aug 20, 2010 2:39 pm
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Hi there, this is my first posting.

I'm starting a little repair vintage amp shop here in montréal, Canada. For now is really the beginning. I just started repairing amp for people around me and try learning the much I can.

Here is my probleme.
I've got a fender bassman showman 1965 that keep blowing fuses at high volume. I did check a lot of things around the power supply section; Power tubes, fuse value, bias circuit. And with a little research I'm wondering if its the output transfo who start arcing at high current.

I did the resistive measuring test and the ratio test with a signal generater. I dont see anything special.
Ive got a lamp in series with the amp but I cannot really see where the problem come from.

Is there anyone who as already see a problem like this one who can help me?

thanks

Rémi


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Post subject:
Posted: Fri Aug 20, 2010 9:50 pm
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Welcome to the forum, Remi ! Not sure which model amp you mean, a Bassman or a Showman ? If you suspect that the ouput transformer is arcing, the only definitive way to check is a hi-pot test. I guess you could try substituting another trans for a test. Sounds like you've done the other tests, but I would go back over everything in the power supply and bias circuit; diodes, resistors, caps, solder joints, and ground connections. The tubes are always suspect. It's tough, when the failure only occurs in a high demand situation. This condition would make me suspect a faulty filter cap. I would do a thorough voltage(pop) test, before sending the transformer out for a test. Art

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Posted: Sat Aug 21, 2010 6:27 pm
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Did you check the power transformer, too? I'd first check the rectifier. Pull all power tubes and see if fuse pops. Then, you can pull secondary leads off power tranny and see if the fuse still pops. If it does, could be PT.

I hope not. Good luck!


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Posted: Fri Aug 27, 2010 6:18 am
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Hi Remi,
If your output transformer is arcing I would expect that you would hear
loud popping from the speakers. Does this happen only when the
lamp is connected? I have used a lamp in series with the amp but
only when charging new caps.
I also live in the Montreal area, Saint-Lambert, and enjoy working
on old amps.
I recently used the the services of Michel at Amptek (514) 501-1186
in Laval. Il est un 'bon gar' aussi.
You can email me direct at barry_tl@hotmail.com. Maybe we can
meet sometime. I have a '62 Super and a '65 Deluxe (no reverb).
Barry


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Post subject:
Posted: Fri Aug 27, 2010 2:49 pm
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Hi Everyone

I worked on my showman ab763. I try a lot of things/tests to see if it was the power transfo or the output transfo. Ididnt see anything special.

But I forgot too said that the amp was set with 4 x 5881 sovtek with a tube current of 33mA each.Someone told me that these tubes was really not good tube for a blackface fender with 450 Vdc on plates.

So I try a test with 4 6l6gc. and surpisely no problem. no fuse blowing when a push the amp.Even if a set the volume at 10 with a thd hot plate.

I check in the datasheet and find that the support max voltage on 5881 is 400 vdc and the 6l6GC is 500Vdc. Am I wrong?

..... this week a receive a new amp for repair. A fender vibrolux reverb reissue 1992. I notice in the cabinet , the tubes diagram paper in the amp said 6l6gc / 5881 for power tubes.But this amp run there powers tubes at 450 vdc ?

do anybody can explain me a little bit about these observation?


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Posted: Fri Aug 27, 2010 5:42 pm
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Glad to hear that you found the problem, and that it was only the tubes ! As for the 6L6GC/5881 designation, the current production tube makers are not held to the same standards as the older tube companies( 1950-1970), so the tube's capabilities can be determined by the manufacturer. It's not the same market condition, as when the military and medical fields were the main buyers. There is no way to generalize on the new tubes. Each particular tube's spec sheet must be checked for it's performance parameters. Art

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Post subject:
Posted: Sun Aug 29, 2010 2:11 pm
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Aclempoppi, I read in the original (1968) RCA receiving tubes manual about the 5881:
Plate voltage, maximum rating : 400 volts

This mean that those olds tubes are tough enough to work with exceed their max voltage. Not the new ones.


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Posted: Sun Aug 29, 2010 3:24 pm
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nitrosonique wrote:
But I forgot too said that the amp was set with 4 x 5881 sovtek with a tube current of 33mA each.Someone told me that these tubes was really not good tube for a blackface fender with 450 Vdc on plates.

So I try a test with 4 6l6gc. and surpisely no problem. no fuse blowing when a push the amp.Even if a set the volume at 10 with a thd hot plate.

I check in the datasheet and find that the support max voltage on 5881 is 400 vdc and the 6l6GC is 500Vdc. Am I wrong?


Oh ye of such little faith......!

I warned everyone about this -- several times.

How many good 5881's must die?

Go my son, and sin no more.

:mrgreen:

Arjay

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