Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: davidb on February 28, 2023, 09:59:59 am
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I just saw a post from an amp tech talking about how he replaces most center taps with virtual center taps, especially in vintage amps with expensive PTs. The logic here is that if a short between pins 2+3 occurs, it will burn out the PT. I have a Sunn and a Laney that use center taps and I want to preserve them as best I can, is it worth making this modification? I also have a Music Man that grounds one of the 6.3V windings, and I think it would be worth converting this as well since the same problem could occur.
Thanks for the help, I’m still learning here and appreciate the input.
David
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Can't you just fuse the CT if you're worried?
/Max
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Protecting things with a fast acting HT fuse will mitigate numerous failure modes, including this one.
Bear in mind that if the heater circuit gets pulled up to anode voltage, eg by the pin 2 to 3 short described, the heater to cathode insulation of all valves in the amp is likely to be damaged.
I think better mitigation for a pin2 to 3 short by ensuring the integrity of the OT secondary circuit would also a good way forward. eg use of dedicated output sockets for each tap rather than a load impedance selector switch, safety resistor hardwired across the OT secondary in case all else fails.
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... The logic here is that if a short between pins 2+3 occurs, it will burn out the PT. ...
In my limited experience, this doesn't happen.
I had a power tube short Pin 2 to Pin 3, and it popped the mains fuse instantly when I turned on the Power switch. New fuse, new tube, and the amp was 100% again.
We are not protecting the heater winding, because its thicker wires support more current than the high voltage winding. And the high voltage winding already has a mains fuse to pop in the case of excessive current draw.