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Post subject: Need help with RK Showman jack swap
Posted: Mon Nov 22, 2010 2:46 am
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I'm doing some work on a Red Knob Showman, and in the course of replacing a tube (6L6) socket and all the broken plastic jacks for the effects loop in one marathon soldering session, I cured the problem caused by the bad socket, but ended up with a very quiet (but nice sounding) amp. After much head scratching, I found that when I put an effect in the loop, my volume came back. So I dragged out my original broken jacks (P/N 025929), and compared them electrically to the new jacks (P/N 0990912000). They are NOT electrically equivalent! I'll ignore the fact that the originals were three pin jacks, and the new ones are 4 pins, adding pin 5, which I removed. The problem is that on the originals, pins 2 and 4 are at the same potential without a 1/4" plug inserted, and the new jacks are NOT at the same potential (shorted) without a plug inserted. Putting the old jacks back in cures the problem, but I would like to have a set of unbroken jacks for the effects loop. Does anyone know of an actual replacement for the 025929 plastic jack? Thanks.


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Post subject: RK Showman jack problem
Posted: Tue Nov 23, 2010 11:32 am
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Hi stl-greg,

Are these jacks circuit board mounted with a plastic threaded bushing for the chassis mounting nut? Do you have access to the schematic to be able to see the switching circuit of the jacks? Maybe you could point me to a link with your schematic, and I can suggest some Switchcraft replacement jacks that would be better than the stock ones. :idea:

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Posted: Wed Nov 24, 2010 7:28 am
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Yes, they are circuit board mounted, with the plastic threads. There are no online schematics for the RK Showman, but the effects loop is identical to the RK Twin, which is here: http://www.prowessamplifiers.com/schema ... n_amp.html
Thanks for your reply!


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Posted: Wed Nov 24, 2010 8:56 am
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Edit:
I had the wrong links, I've corrected them, you need the L112AX (Jack schematic III), not the B, sorry.

Thanks, that helps a lot! According to the schematic it looks like all three of the effects loop jacks that you want are the same. The Preamp Out and Power Amp Thru only use two of the three connections on the jack, the Power Amp In uses all three terminals for switching. From the schematic it looks like the sleeve is tied to ground, but I don't know if the jacks needs to be isolated from the chassis with shoulder washers or not, I would get a pair of isolation shoulder washers with each jack just in case. The worst that will happen if it needs isolating washers, and you don't use them, is noise, that's all. To get the jacks off of the PC board, and for better jacks than the original, I would use these Switchcraft jacks from either of these two sites (first two links), or your own source. You will have to run short wires from the jacks to the circuit board. And the Switchcraft site has all of the info, and a phone number, necessary to figure out the connections (third link). Hope this helps, and isn't too confusing. :)

http://www.mouser.com/ProductDetail/Switchcraft/L112A/?qs=sGAEpiMZZMv0W4pxf2HiV6YB0RWJpyofLcleYDZQ8g0%3d

http://www.newark.com/switchcraft/l112a-x-5/hi-d-jax/dp/74R8000

http://www.switchcraft.com/ProductSummary.aspx?Parent=67

Here is a layout of the jack, this one shows a plastic bushing (isolation), but the pin location is the same. Look at the "112A" layout. I had to use this picture, I can't find a link to where I found this document. Obviously from the Switchcraft site, but where? :? :lol:

Edit:
I had the wrong links, I've corrected them, you need the L112AX (Jack schematic III), not the B, sorry.

Image

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Posted: Wed Nov 24, 2010 10:47 am
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That's GREAT information, and I will definitely replace, over time, all of the plastic jacks with the metal. Did Fender save all that much by using plastic over metal, or was there an engineering reason, such as lower hum in the circuit?

I STILL don't have this amp sounding the way I want,the clean channel breaks up way too early (at 3 1/2). I've replaced all the 6L6s with new Tung Sols, and two of the 12AX7s with Tung Sols as well, with no real change to the "grunge." It's biased correctly too. I found a bad 1/4 watt 1.5k resistor across one of the tubes, which I thought might be causing it, but no dice. Guess I'll be living at the bench again this winter!

Thanks for all the advice.


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Posted: Wed Nov 24, 2010 11:24 am
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Yeah, I'm sure they saved a bundle with cheaper jacks. :roll: If you want clean sound, you might consider using output tubes chosen for late breakup, like Groove Tubes (JJ made, not relabels). When choosing the option (pair or quad, label color), the "Red" will be for late breakup, clean headroom. I am not sure how the Tung Sols are, as far as breakup, but I use these GTs and get plenty of clean headroom. Of course, any new output tubes, check bias (and balance if you have that adjustment)

http://www.guitarcenter.com/Groove-Tubes-Gold-Series-GT-6L6-S-Matched-Power-Tubes-100088402-i1125729.gc

You can also send an email to Bob at EuroTubes with your amp model and the sound that you want, and he can put together a Complete set of tubes, or just the individual tubes that you want. There are "Full Retube Kits" available on the site for different amps, I didn't see a Showman listed.

https://ssl.eurotubes.com/cart/

Also, a lower gain preamp tube like a 5751 or 12AT7 in place of the 12AX7 in some preamp positions, will clean up the sound also. Try one in V1 at least. You may find that changing two or more preamp tubes to the lower gain tubes will help too. You might also want a "balanced" preamp tube in the PI position. Just a thought. :)

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Posted: Thu Nov 25, 2010 8:59 am
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Actually, the Tung Sol line does accomodate choices for late breakup, which is what I ordered. My concern is that there is still something wrong in the circuit, and I can roll tubes 'till the cows come home and it won't fix the problem.

But at any rate, I WILL try some lower gain preamp tubes to see what it gets me, that shouldn't cost too much. I just wish that the schemos had better voltage readings at the various junctions. When I work on old Vox amps from the 60's, the voltage reading for nearly every junction are on the prints, and it's really easy to find problems. I guess I've worked on transistors so long, I forgot what tube readings SHOULD look like!

Thanks so much for your help and interest with this amp, it's hard to find help out there for a Red Knob. They're sort of pariahs in the Fender community.


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