Installed 12" Speaker in Pro Jr---Now What?? - diyAudio
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Old 19th January 2009, 07:03 PM   #1
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Default Installed 12" Speaker in Pro Jr---Now What??

A friend has asked me to make his '94 Pro Jr (Tweed version) gig worthy. I checked the tubes, replacing a microphonic 12ax7. I installed a Vox Neo-Dog (same as Celestion Century Vintage), and a larger OT to match this 16 ohm speaker. This is the only speaker which had a small enough magnet to fit, and I still had to trim the heatsink to manage it.

When I play it, it doesn't seem to have much headroom or snap. I don't have another EL84-based amp to compare it to, but compared to a Deluxe Reverb or my Rivera 30-112 (running on 6V6's as well) there is nowhere near the clean output.

The tubes are running stock fixed bias, which I think is about 14 watts per EL84----should this be adjusted?

The circuit looks like it has quite a bit of its gain dumped between stages, presumably to increase distortion (ala Soldano). Should this be revised?

The power tranny is also quite puny--is it just too much to ask of a Pro Jr?
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Old 19th January 2009, 08:22 PM   #2
Trout is offline Trout  United States
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If you could shoehorn in a Blackface Deluxe PT & OT in there, then tweak the fixed bias circuit you could most likely get her pumping out about 22W with a pair of JJ 6V6's instead of the EL84's.

Pretty extensive mods though.

Might be easier to just upgrade to a bigger amp.
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Old 19th January 2009, 10:23 PM   #3
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Default Pro Jr Upgrades

I already installed the OT from a Mesa 50W amp, so it should be adequate iron. I think a DR power trans could be prohibitive unless you have a used one fairly cheap---I see you are in Geneva/Elburn, and I'm in Geneva! Email me if you have something lying around (I 'm too new to this forum to have outgoing emails).

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