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Post subject: Need HELP with '68 Dual Showman
Posted: Fri Jun 16, 2017 3:15 pm
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Just re-tubed my '68 Dual Showman with all new tubes and it worked great. All tubes were matched and to spec. Then eight minutes into playing sound just dropped off to nothing. Checked the tubes and only one output tube was functional even the pre amp tubes were not working. I tried some spares to see if it blew the new tubes up but even they had no power going to them. I have a recording session coming up and would REALLY like to have this up in time. Ideas????


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Post subject: Re: Need HELP with '68 Dual Showman
Posted: Fri Jun 16, 2017 4:42 pm
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A defective power tube could show up in an 8 minute warmup.
A blown power tube wouldn't stop the other tubes from glowing. If you have a pilot light but no preamp glow, that sounds like a wiring problem.
If they glow, but no sound, the string of dropping resistors in the power supply may have a blown one, preventing b+ from getting to the preamp.
Check the 470ohm screen grid resistors on the power tube sockets.


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Post subject: Re: Need HELP with '68 Dual Showman
Posted: Fri Jun 16, 2017 4:54 pm
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Tim may be right but it could also be an issue with the HV power supply, possibly the diode stack. Be sure to inspect those as well.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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