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Old March 28th, 2012, 05:34 PM   #1 (permalink)
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Screen Taps

What does a screen tap do and how do you hook it up? I'm guessing that (with a switch) it offers a way to reduce gain. There's a lead on my Triode Electronics 5F1 output transformer with a 33 percent screen tap.

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Old March 28th, 2012, 05:58 PM   #2 (permalink)
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Screen taps are for UL operation. They are placed between 20%( max power) and 43%(min distortion). You have to wind the transformer with them, you can not add them.

What they do is add some signal to the screen grid, its polarity is such that it is Negative Feedback, and that makes the tube act more like a triode, ie lower gain, lower distortion and lower output impedance.

For you, it is worth connecting it and seeing what it does, but the only place I have found SE UL useful was with an ultra high gain amp, and a 5F1 is so not that. But try it, see for yourself what it does, that might come in handy down the road.

To connect it, you just connect the wire to the screen grid with no screen resistor.

Yes you can use it for reducing gain and output, it is less radical then connecting the screen to the plate, both in terms or power reduction and tonal change
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Old March 28th, 2012, 06:16 PM   #3 (permalink)
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Thanks. I suppose I could put it on a switch, right?

Re: 5F1 - 6v6 it connects to pin 4, Is this correct?

With a switch, it could be useful accessory on a 5F1 or similar amp in the home environment.
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Old March 28th, 2012, 09:29 PM   #4 (permalink)
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Yes, pin 4. It shouldn't be a problem on a switch. My feeling is that changing from pentode to UL or triode modes is not a good way to control sound level. Not because it doesn't work, but because it changes so much besides power. To me it is like 3 different amps, not different volume of the same amp. If you don't call it "power attenuation" or something like that, it could be a good thing to add versatility to an amp, but sold as a power cut, I think there will be unhappy customers( no more unhappy then with a pentode triode switch).

I encourage you to give it a try and see for yourself, and from there, at least an option, somebody is going to love it
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Old March 28th, 2012, 10:42 PM   #5 (permalink)
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Thanks Again, celeste. The power supply must be disengaged from pin 4 while The UL Tap engages pin 4. Is this correct?
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Old March 30th, 2012, 12:20 AM   #6 (permalink)
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Yes - correct. and the screen grid #2 must have its screen grid current limiting resistor in place when connected to the B+

And I think she might make one h-e double hockey stick of a pop when you switch her hot, too.
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Old March 30th, 2012, 06:58 AM   #7 (permalink)
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Quote:
Originally Posted by CoolBlueGlow View Post
Yes - correct. and the screen grid #2 must have its screen grid current limiting resistor in place when connected to the B+

And I think she might make one h-e double hockey stick of a pop when you switch her hot, too.
OH my goodness yes!!!! Not a front panel control, for sure.
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