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Dirty-South Blues Harp forum: wail on! >
Tube Retainers
Tube Retainers
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lor
159 posts
Sep 18, 2012
4:58 PM
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My heavily modded Blues Jr has the power tubes held in place by a metal bar across the top of the tube (upside down) held to the chassis by springs. Problem is, it feels like the bar is glued to the tubes. I can't pull it off. At least, I haven't tried too hard, lest I break the tubes.
Any thoughts on how to loosen the retainer and maybe pull the tubes? They're fairly new so I don't want to break them until they need replacing.
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tmf714
1267 posts
Sep 18, 2012
5:08 PM
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Like this-
Remove the springs from the upper mounts-then you should be able to remove the large bottom shield.
Article here-http://www.fender.com/community/forums/viewtopic.php?f=13&t=64115
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lor
160 posts
Sep 19, 2012
10:20 AM
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Pulling straight down on the retaining bar should stretch the springs and the bar would come away from the tubes, I believe. Seems obvious. But I cannot push the bar away from the tubes. It's stuck to the tubes somehow. Any clues?
Also, where do you get those rubber (silicone?) collars?
Thanks.
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Rick Davis
707 posts
Sep 19, 2012
10:22 AM
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lor, how is your BJr modified?
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-Rick Davis
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lor
161 posts
Sep 24, 2012
9:27 AM
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@ rick
I got all these mods from Bill Machrone at billmaudio.com, a very good guy to work with. He can do the work although he's backlogged, or he will provide great instructions and all the parts pre-tested and partly assembled, so you can do them yourself as I did. You will have to muck around with a soldering iron in the chassis wiring.
These are helpful for both guitar and harp sounds, making the amp a lot more versatile in general, with noticeably better volume and tonal range.
As for harp specifically, I am contemplating a mod of my own design to be able to directly manipulate power stage distortion without having to run the whole rig in high volume overdrive. I'll keep you posted on that one. I'm super busy so it will be a while.
Note, I have the "cream board" March 2001 schematic version. The PC board itself is dated 2003.
From billmaudio.com:
Added a parallel 47uF cap to the power supply filter cap.
Modified the tone stack by replacing the mid and bass caps with "Orange Drops".
Replaced the 1st and 3rd pre-amp stage forward coupling caps with "Orange Drops".
Replaced the cap between the master volume pot and the PI stage with a jumper.
Revised the mids control in the tone stack to allow grounding all the mid frequencies by connecting the mid pot's wiper terminal to the terminal connected to the lead from the bass pot. This allows getting totally scooped, or, flat across bass mid treble, responses, as well as all bass or all treble, and everything in between.
Replaced the voltage divider resistors providing power tube grid bias with a 50k trimpot in order to adjust the bias to approx. 8 watts idle dissipation, which is a good value but less than the tube roasting stock value.
Added a 100pF cap across the plate circuit grounding resistor for the PI stage to drain potential supersonic and RF oscillations which would consume power in the final amp stage.
Replaced the output transformer with a bigger one to provide more robust tone, especially bass, overall.
Replaced a resistor in the negative feedback circuit from the output transformer with a pot allowing control of the feedback, at the input to the PI stage. The resistor is the one controlling the capacitive bypass to ground shunt. This mod affects the "attack" response of the power stage, ranging from mellow bloom to icepick stabs. Probably the most obvious audible control in the whole set after the tone controls.
Added a pot in the grounding lead from the cap which drains very high treble signal off the input to the master volume control, so that those frequencies can be passed or drained as desired. Passing them makes the amp brighter. For harp they're better drained.
Next.
Last Edited by on Sep 24, 2012 9:30 AM
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