Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 08, 2015, 07:50:06 am
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Concerning my Single Ch. DR build. I have an OT that has 4-8-16 ohm taps and I want to wire them to separate jacks. I have read about this 470k safety resistor and one person said it should be wired across the whole secondary winding of the OT. I was planning on using switchcraft #11 non-shorting jacks but then some say to use #12 shorting. What is the scoop on this? Can I use #11 jacks for each tap and then wire a 470 ohm 1 or 2 watt resistor across the entire secondary? I know the shorting jack is for safety, incase of turn on the amp without a load but how do you implement this with three separate impedance jacks?
I've spent a couple of days trying to find info on this and have had no real success. Can't find an example of the wiring implementation either.
One thread I found on this subject: safety resistor related (http://www.ax84.com/bbs/dm.php?thread=486631)
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470K across the OT secondary is like having nothing across the secondary. I've seen 200Ω and less used for a safety resistor.
You can't use switching jacks (well you can but don't connect the switch terminal to anything) if you plan to use three separate jacks for each OT secondary tap.
IMO, a much more versatile circuit would be to use Hoffman's three position speaker selector switch and two parallel jacks wired Fender style. The MAIN jack would be a #12 with switch lug connected to ground and the AUX jack could just be a #11.
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IMO, a much more versatile circuit would be to use Hoffman's three position speaker selector switch and two parallel jacks wired Fender style. The MAIN jack would be a #12 with switch lug connected to ground and the AUX jack could just be a #11.
I was thinking of going this route however space is at a premium. Still may do it. Thanks for the reply.
Sorry, I meant 470 ohm resistor above and have corrected it.
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Here (http://music-electronics-forum.com/t31452/) is another thread that talks about this safety resistor. Notice Bruce says to connect a 470 ohm resistor to the 16 ohm tap and forget it. But is this kosher with two other taps wired up to output jacks and with a feeback resistor?
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yes
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Ok, thanks, I'm going to try it.
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IMO, a much more versatile circuit would be to use Hoffman's three position speaker selector switch and two parallel jacks wired Fender style. The MAIN jack would be a #12 with switch lug connected to ground and the AUX jack could just be a #11.
Why a 12 for the main out? is this in case the amp is turned on with no cab connected? still i dont understand even in that scenario as I though golden rule was dont switch on without a load.
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Supposedly, if the OT secondary is open (infinite), and the amp is turned on, it in can cause some large voltage spikes that could harm the output tubes or OT. So the 12 jack is shorting and shorts out the secondary so the amp doesn't see infinite impedance. The links I posted talk all about it.
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Since you'll have a safety resistor, just use non-shorting jacks.
If you ever accidentally plugged into the non-shorting external speaker jack of a Fender amp instead of the correct speaker jack, you'll know why I say it. Doing that, you get an extremely weak, distorted output that will have you chasing your tail.
The safety resistor will already do what you might want a shorting jack to accomplish.
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Thanks HPB -thatsa whata I'm a gonna do.. :-)