Replace Caps on 12AX7 in RD50?

inertian

Doing a cap job on an 11/83 (Rev E) RD50, replacing all 12 electrolytic caps on the board. Does anyone also replace the two 20mfd/25v caps on the 12AX7 tube used for the Distortion Mode? Or replace with 2mfd/25v as spec'd in the Rev E schematic?

Thanks.

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mm210 Mon, 01/12/2015 - 14:16

If it's an electrolytic, I would replace them. They dry out. Will they kill your amp? Probably not but for and extra $4.00, why not.

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Ian Azrikan

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inertian Tue, 01/20/2015 - 18:28

That was my first instinct as well, so I did a little test. Ran the amp will all caps changed except the two on the 12AX7. Then clipped in new caps and disconnected the old. Had a friend with exceedingly good guitar amp ears test before and after. The verdict: no difference. Go figure. Either the old caps weren't totally done (hard to believe for a 32 year old amp) or the role those caps play in the distortion circuit is minimal. Or I suppose, both the old and new caps could be equally no good...

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mm210 Wed, 01/21/2015 - 10:07

The problem though is that the test is invalid. They can be OK right now but in ten minutes, short out. You are better with new caps to risk your amp on. I'm just sayin............. Mike.

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Mike Griffin
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mgriffin155 Wed, 01/21/2015 - 17:42

Hi Inetian,

That cap helps hold the cathode bias voltage constant on the 12AX7. Much like the big 1 Farrad caps kids (and some adults) use on hi powered car stereos, it stores a reserve of current so when bass frequencies hit the tube, the cathode bias voltage will hold steady. From a reliability stand point, that cap lives near a hot tube and in a hostile environment. At 30 years old, it's tired. Like Ben Franklin said, "Don't be penny wise and dollar foolish." I'd go with the 20uF.............. mgriffin

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inertian Fri, 01/23/2015 - 09:37

I purchased enough caps to replace those along with the rest of the cap job, was just curious if it would impact the distortion voicing in any noticeable way. The schematic calls for 2mfd/25v, I've now seen three Rev E RD 50s with 20mfd/25v instead.

Appreciate the input as always.