tweed super, outrageous bias scene!

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tweed super, outrageous bias scene!

Postby centervolume on Tue May 12, 2015 1:36 pm

so THIS is where all the cool kids are hanging out!!!

A tweed super came in with 80ma (375VDC) on the power tube plates.

The bias circuit is set up with a 100K in place of the 56K and a 3.3K in place of the 6.8K in comparison to the dividing network shown on the schematic.

Putting them back to the published/factory values, bias runs up to about 100 ma.

With the power tube side legs of the 2x .1uf caps lifted, there are only -36volts on the grids/pin 5 of the power tubes, that should be closer to -50+ right?

How to get more negative voltage on the grids?

The .1uf caps are newer orange drop replacements, and the selennium rectifier is replaced with the diode ln4007 facing the 100K and PT lead.

I'm getting 127 VAC off the wall, 58 VAC off the red-blue PT lead, and that drops only 2-2.5 volts by the time it gets to the ln4007.

any help most appreciated!



http://www.turretboards.com/layouts_schematics/fender/schematics/fender_super_5f4.gif
http://www.turretboards.com/layouts_schematics/fender/layouts/super_5f4_layout.gif
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Re: tweed super, outrageous bias scene!

Postby Baron Von Machinenmann on Tue May 12, 2015 11:16 pm

The schematic you linked to shows -40 volts, are you running old glass or new production?
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Re: tweed super, outrageous bias scene!

Postby centervolume on Wed May 13, 2015 1:15 am

Hi Baron-

It is old glass, tried with new. But the way it turned out, I was sniffing around in the wrong area. There were some shenanigans going on with the power tube wiring. Apparently someone had installed the replacement OT backwards and did not realize it. Instead they (or some later tech) installed additional components to stop the telltale squealing... which I did not know could be done! I thought I had taken care of that, but there were extra resistors on the power tubes I overlooked.

thanks much for the follow up
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Re: tweed super, outrageous bias scene!

Postby Baron Von Machinenmann on Wed May 13, 2015 4:33 pm

Glad you got it sorted, got any pics? :thumbsup:
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