Zener diodes

[richard1986]

Hi all,

I've got an amp I made a few years ago which has a 350-0 primary so makes about 500v after a bridge rectifier.

At the time, I could only get 450v capacitors for the screen grid and preamp supplies, so if you turn the standby switch on too soon you risk blowing them up. I'm not happy with this, so I'm going to change it.

I can get 500v capacitors easy enough now, and I'd like to use some zener diodes to drop the voltage a bit more, for a margin of error and because I know mains voltage is often higher than it really should be around here, and I don't need the power. I know I could just use 600v caps, but they're 5 times the price.

If I put a string of zeners between the zero tap and ground, does that reduce the voltage the filter caps see before the output section conducts? I'll be aiming for about 475 I think.

Thanks,
Rich

 

[CirrusBand]

Yeah, if you want to drop 25v then give or take a few volts 25v zener will do it as you describe it, as long as you're drawing enough current through it to reach its rated voltage drop - they have spec sheets showing how they act.

They do produce a lot of heat, so you either get several 5w zeners in series - say five volt zeners, and work out how to dissipate their heat, or you get a 25/50w zener which mounts direct to the chassis with heat sink paste to transfer heat away.

When I tried dropping 25v in my ac30, I attempted both methods and they both generated too much heat to feel like safe long term solutions, though they did work. But others have reported success...

 

[Ten Over]

If I put a string of zeners between the zero tap and ground, does that reduce the voltage the filter caps see before the output section conducts? I'll be aiming for about 475 I think.

Zero tap?

 

[dan40]

I have used this reversed bias, stud mount zener when I need to drop 50v or so. The higher wattage rating and stud mounting prevent it from running excessively hot. You can find these in any voltage and wattage rating that you like. If you decide to use one of these, make sure to use the "reversed biased" type. The letter "K" at the end of the part number indicates "reversed biased". Installing one of these in a cathode biased amp is as easy as mounting it to the chassis and connecting the centertap to it. If your amp is fixed biased, there are some considerations that need to be taken into account depending on where your bias voltage is derived from.

 

[richard1986]

Zero tap?
View attachment 1044781

Yeah, you're right. Was half asleep when I wrote that. It's a bridge rectifier, so I think it'd have to be between the rectifier and reservoir cap.

 

[wangdaning]

I have used this reversed bias, stud mount zener when I need to drop 50v or so. The higher wattage rating and stud mounting prevent it from running excessively hot. You can find these in any voltage and wattage rating that you like. If you decide to use one of these, make sure to use the "reversed biased" type. The letter "K" at the end of the part number indicates "reversed biased". Installing one of these in a cathode biased amp is as easy as mounting it to the chassis and connecting the centertap to it. If your amp is fixed biased, there are some considerations that need to be taken into account depending on where your bias voltage is derived from.

I can't see the link. I am interested. Need to drop about 80v and it looks like a good way. I checked it out and you can get them here fairly cheap, but the way specs are listed here are a bit confusing to me.

 

[2L man]

I don't like the idea to use Zener on AC voltage because it cut voltage from "low end". I recommend installing series Zener after the first filter capacitor where it cut DC. Then peak current thru Zener comes lower as well.

I would use a Mosfet circuit to waste excess voltage because nowadays they are cheap. It can be used as adjustable voltage dropper so PS "sag effect" stay or adjustable voltage source which keep the voltage regulated. Possibly both way just installing a switch to the circuit?

 

[sds1]

350 * 1.41 = 493.5

Minus 1.4VDC Vf in the diode bridge.

Even if mains were to come in at 122VAC, you're still less than 500VDC after the rectifier

Even if you somehow idled unloaded at 505VDC....

The filter caps aren't going just go up in smoke at 501VDC. The cap rating is for continuous operating voltage at a higher temperature than what they'll see in your tube amp.

Indeed you can run them that close to their rated voltage.

I don't mean to discourage the exercise, but just pointing out I think the zeners are technically unnecessary.

 

[Ten Over]

I recommend installing series Zener after the first filter capacitor where it cut DC.

Wouldn't that allow the first capacitor to initially charge up to a voltage that the OP was trying to avoid?

 

[mfratus2001]

Zener diodes basically have one failure mode... they short! I would not trust a zener diode that gets that hot.
Instead, you can use a large-wattage resistor, maybe 20W, to dissipate some of that energy. A zener would be doing the same thing, drawing off excess voltage. Ohm value to be left as an exercise for the reader. You can use a resistor in a metal housing designed to be heat-sinked.

 

[2L man]

Wouldn't that allow the first capacitor to initially charge up to a voltage that the OP was trying to avoid?

You're righg it would! I did not notice this was a capacitor volgage rating issue and thougt this being more common too high voltage issue

 

[dan40]

I can't see the link. I am interested. Need to drop about 80v and it looks like a good way. I checked it out and you can get them here fairly cheap, but the way specs are listed here are a bit confusing to me.

This is a different link to a different online supplier. Hopefully this one will show up for you.

NTE 5275AK Zener Diode Equivalent | NTE Electronics Parts

Buy the NTE 5275AK Zener Diode at www.Arcade-Electronics.com. We are a NTE Electronics Distributor with a large inventory of NTE Parts in stock ready to ship. Shop our secure online store for more Electronic Parts.

www.arcade-electronics.com

 

[duaneo]

Zero tap?
View attachment 1044781

Yeah, you're right. Was half asleep when I wrote that. It's a bridge rectifier, so I think it'd have to be between the rectifier and reservoir cap.

A safer bet would be to do what @Ten Over suggested in the schematic, place the zeners on the - side of the FWB and / or use stacked caps with ballance resistors. When putting caps in series it is like parallel resistors: C total = 1/(1/C1+1/C2)