Ok.
Zener Diode to ground in the centre tap of the high voltage secondary of the power transformer in a valve amp, to lower the voltage going to the valves. You pick a diode with a particular voltage, and it drops that voltage. Practical example: Just as an experiment, I put 3 5 watt 6.8v zener diodes in series, and my ac30 went from 345v to 325v on the plates. Pretty good.
But they are getting very hot, so I'm going to replace them with a 20 watt chassis mounted one.
I don't understand though. How do they do that? Where is that 20v going? It's not going to ground, because ground is still 0v. How does it happen? How does it pull down the high voltage?
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"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The ac30 is happy though, and hopefully it'll help the el84s last a few more months (gets 15.5 watt dissipation down to 13.6). It's way cheaper than getting a new power transformer, too, which was my first thought (and indeed I'm not sure a more conservatively wound transformer with the extra 25v secondary for the IC chips etc exists), and presumably doesn't mess with the natural sag of the circuit in the way a fixed resistor would.
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It turns out that I can't use a chassis mounting Zener, because it's dumping noise onto the chassis. The Zener needs to go between the Transformer and a circuit board, so I need to be creative to make something that works.
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"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I think it's a more serious issue;
On the CC series, the HT Centre tap goes via the standby switch (which I've removed) and HT fuse to the power section circuit board, connecting directly next to the ground side of the first (pre-choke) filter cap.
I read 0r resistance between that point and the amp chassis, so assumed I could just move that wire to the top of the zener connected to the chassis and electrically it'd be as good as the same.
Well I know better now. It might read 0v, but it's actually connected via this little doozy;
And I think that's keeping the rectifier hum off the chassis.
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The way around that is to still use the chassis-mount diode, but with an insulator, and a tag to make the ground connection back to the circuit - so the only functions of the chassis bolt are to hold it and heatsink it.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
My other plan was just to make a bigger string of lower wattage diodes on vero board - 5 x 4.8v would do basically the same thing. But the bolt diode can be made more secure.
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I knew it was a good idea to keep this old Fender Mex Strat trem block/ bridge around after I replaced it years ago
I've got a temp probe to assess the probably longevity of the 20 watter and a 50 watt zener coming today just in case. If that doesn't work I might need to just give up on the idea, and try to source a proper 245v mains transformer.
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