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Post subject: Adding standby switch to a Princeton Reverb reissue - PRRI
Posted: Wed Nov 24, 2010 3:48 pm
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Would anybody be kind enough to tell me how I could add a standby switch to a Fender Princeton Reverb reissue? I intend to use a progressive off-on(1 circuit)-on(both circuits) toggle switch (an example is part #SW-520 here http://www.amprepairparts.com/switches.htm ).

I asked the same question elsewhere, but most of what I got were suggestions that I do not need one, or that it could be done, but nobody could tell me how/where I should cut into B+ line. I have an inkling I should insert the switch into the circuit at connector P1, a connector on a yellow wire coming from the power transformer. I don't think I should fool around and unsolder the wire at leg 8 of the rectifier tube.

Any suggestions/help would be appreciated. I intend to carry out the mod unless someone demonstrates that this modification would blow up the amp.

Thanks in advance!


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Posted: Wed Nov 24, 2010 4:16 pm
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One thing to note before proceeding is the voltage and current rating of that switch. These days it's hard to find a switch that has both voltage and current ratings that it will see in most tube amps (400+ VDC on B+, without load & 1-2 amps).

The consequence is early switch failure which may lead to very annoying loud pops when used. And perhaps even severe arcing. Arcing maybe somewhat controlled with a non-polar high voltage cap across the contacts.

Anyhow, here's a link to one suggestion:

http://www.netads.com/~meo/Guitar/Amps/ ... andby.html


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Post subject: Standby switch for PRRI
Posted: Wed Nov 24, 2010 8:40 pm
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Hi Serge_L,

I used that exact switch for a standby on my BJr, it works very well. This Carling switch is rated at 15 amps and should be heavy duty enough for the PRRI. Billm sells these same switches, and he says on his site specifically that they will work just fine in a PRRI, but may be a tight fit. I agree with BMW2002Ti, if you hear loud popping whenever you turn off the amp with the new switch, you may need a cap across the high voltage contacts. Occasional light popping noises are normal when switching off though. Looks like P1 (or P2) would be a perfect place to insert the switch. The switch has two circuits (circuit A, circuit B), two terminals for each circuit, one circuit on each side of the switch. In the off position both circuits are off. In the middle position circuit A is on, circuit B is off. In the up position both circuits are on. Use your ohm meter to make sure which connection is which, and mark the two circuits on the switch. Disconnect the wire at P1 and connect the wire to one terminal of circuit B on the switch, the other terminal of circuit B on the switch connects back to the terminal that you removed P1 from. For the power side of the switch, use the existing black wires to connect to the new switch circuit A, and then just connect the two left over white wires together. You may have to fashion some push on connector wires to reach the switch from P1, and one to connect the two white wires together. Keep us posted on what you decide to do. :)

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Posted: Wed Nov 24, 2010 10:10 pm
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Thanks, for the detailed step-by-step. BTW... I usually use 0.001-0.005 mfd/1200 VDC non-polar caps across contacts to reduce arcing. Use good quality caps (like Sprague OD's). Also, there are MOV's designed for this purpose. Check out link:

http://www.diyaudio.com/forums/power-su ... ded-4.html


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Posted: Wed Nov 24, 2010 10:16 pm
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Wouldn't a 600VDC be adequate, B-man?

Arjay

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Post subject: Standby switch for PRRI
Posted: Wed Nov 24, 2010 10:29 pm
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I couldn't find any amps that had a standby wired as described above, so I'm thinking that way might not be the best way to do it. I was just looking at the 57 Deluxe schematics and found another way to do it that might be better. Use P11 for the switch connection points instead of P1, in the same way as described in my previous post. P11 is the ground connection for the center tap of the transformer section that powers the rec tube. The switch would lift the ground connection and stop current flow through the rec tube. I think that using P11 would be a better way to do it. :idea:

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Posted: Wed Nov 24, 2010 10:56 pm
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Arjay, yeah you culd use 600 VDC rated caps, but I have a bunch of 1200 VDC rated ones (Sprague Mylars and Black Beauties) ...so might a well do overkill here and try to avoid future issues.

As for the wiring of the switch... this was in the linked article. Whether it's an issue or not, I dunno. I'm couldn't find specfic reference on AX84 site:

``For this, I drilled a hole in the chassis and put in a big switch. Cut the red & yellow wire that runs from your power transformer to the rectifier socket and insert the switch.''

"This is a common way to handle standby. But as someone at ax84.com pointed out, this can be a safety issue as you are interrupting a ground connection. It's somewhat safer to add the standby in the B+ line, either immediately after the rectifier or after the first cap or two, assuming that's still before the power tube[s] B+ line. If you put it after any caps, make sure those caps have a voltage rating high enough to handle 50V to 100V over the normal B+; without a load the voltages will be higher."


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Posted: Wed Nov 24, 2010 10:56 pm
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Thanks guys for your help and insights. If I do go ahead with the mod, P11 seems the way to go (by the way, I have the Billm's switch, originally purchased for a Blues Jr).

Again, thanks a million to all.

BMW2002ti: really like your nick!


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Posted: Wed Nov 24, 2010 11:36 pm
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BMW2002Ti wrote:
Arjay, yeah you culd use 600 VDC rated caps, but I have a bunch of 1200 VDC rated ones (Sprague Mylars and Black Beauties) ...so might a well do overkill here and try to avoid future issues.


Ah, got it. Thanks.

My '65 Princeton Reverb has a standby installed but I'm filling this thread away lest one of my clients request the mod on a re-issue. I don't have one of those amps to examine so this data is invaluable to me. I'd hate to drill a hole into a chassis though......

Arjay

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Posted: Fri Nov 26, 2010 2:44 pm
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Totally OT: Serge, Thanks for moniker kudos. You may like this factory restoration of a 2002Tii (injected model) YouTube vid. They used Kugelfischer mechanical injection unit.

http://www.youtube.com/watch?v=FR6Am76o ... re=related


I have my 2002Ti still. Used to do weekend slalom stuff. Replaced the stock Solex PPH side draft carbs with twin Webers 45 DCOE's.

Ran 3/4 cam, Alpina suspension, Bilsteins, 4:11 rear end. 9.3:1 Mahle pistons. 15-inch wheels with Pirelli P7's. Thing was a real Q-ship. Fun as hell to drive. Hopefully, will restoe the bad rubber parts this next summer. Go to 9:1 pistons & milder cam--- as the thing pings and knocks like crazy on 91 octane gas.

You have one of these old Beemers? Friend has this BEAUTIFUL 3.0CS he's redoing. Anyhow thanks, again.


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Posted: Fri Nov 26, 2010 8:40 pm
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Awesome!

No, unfortunately I don't own one. But it's the first image that comes to my mind whenever I hear these three letters in that order.


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