Zener Diode, Stud Cathode or Anode?

[JamesHealey]

To do the B+ dropping mod on a bridge rectifier on this type of zener
http://www.farnell.com/datasheets/49120.pdf

do I want the stud as the cathode or anode?

I normally just face the band to ground, just I have no idea which way that actually is.

[jaysg]

do I want the stud as the cathode or anode?

cathode...do you know how many watts you'll be dissipating? The NTE models 5240AK through 5296AK are 50W.

[Structo]

I used a 25 watt one to drop the voltage on an amp.

I used a 125v 25 watt stud mount and mounted it through a hole I drilled in the floor of the chassis.

I connected the center tap of the HT winding.

If you do this make sure you buy the one with the cathode connected to the stud.

I also soldered a .01uf cap from the anode to ground to quiet any switching noise.

I also used heat sink compound between the stud and chassis.

Works great but man it gets hot!

[M Fowler]

Reverse anode.

[JamesHealey]

Reverse anode.

now you're saying the opposite of the previous two.

now we're back to square one, can anyone confirm 100% they're right.

[M Fowler]

Yes and I'm sticking to it too.

If you are drilling a hole and bolting the zener diode to the chassis.

For power reduction you want a reverse polarity zener diode and on NTE the K indicates reverse polarity. Such as voltage 50v 5275AK

Mark

[M Fowler]

http://www.electronics-tutorials.ws/diode/diode_7.html

Here is some reading for you.

[M Fowler]

In the reverse zener diode AK the anode is connected to the screw on the stud and is bolted to the chassis without isolation. A normal A zener has the cathode to the stud.

[skyboltone]

Mister Fowler is of course correct but that doesn't mean that buying a cathode stud won't work. Thing is, at the center tap of the transformer is AC. Alternating Current. So it's only going to conduct on half the cycle. And it's going to buck which ever winding it's polarized to conduct.

A bigger problem as I see it is who wants regulation of a guitar amplifier power supply? Sag and cap stiffness and transformer coil inductance all work together to shape the sound. It's a band aid that we often use to utilize transformers originally designed for 5U4 rectifiers with SS full wave rectifiers. I think it's better to just to stick a tube in there OR put a fairly high DC resistance choke in series with the center tap.

If you search long enough you'll find me talking about doing this very thing with darlington transisitors sourceing regulation voltage through strings of 1 watt zeners. In the end it's just better to order a PT with the voltage you want from Phil at Heyboer.
Still, have fun. Do it Mark's way.
YMMV
Dan

[M Fowler]

Dan,

I agree with you get the right transformer or at least try a different rectifier tube.

Oh and I did read your post about alternative ways to lower voltage other than zener diode.

I was only trying to help him with his selection.

[skyboltone]

Dan,

I agree with you get the right transformer or at least try a different rectifier tube.

Oh and I did read your post about alternative ways to lower voltage other than zener diode.

I was only trying to help him with his selection.

Absolutely, and your link is a good one. I marked it for next time the discussion comes up.

[skyboltone]

Actually he's talking about a bridge rectifier. Worse yet.

What are you going to do? Put a zener between the negative of the bridge and the chassis? What is the B+ you are shooting for? Have you found a zener that size? I'm not getting where you're going here. So you'll isolate the chassis 25 or 50 or 75 volts below B-? Above B-? Where you going to get your bias voltage from? How are you going to bias the pre amp tubes?
Let's see here....No you can't do that. Gimme a scheme. I have to do the thinking part with pictures and arrows.

You can't ground the negative of the bridge and run the center tap of the transformer through a zener. No. And you can't just put a zener between B+ and ground. You'll draw unlimited amps and the output voltage will drop to zero. You can put a series zener in the + rail. But you won't like the way that sounds. At. All.

I see troube on the horizon......

[JamesHealey]

http://www.chambonino.com/carlsbro/carl60tc.html

this is the schematic of the B+ and the bias circuit i've used because this is the amp the transformer is from.

It's NON CT transformer, and the zener goes on the tail of the bridge rectifier to ground with the cathode to ground. like in this.

[skyboltone]

As drawn the diodes are reverse anode as Mark said. If they had one whisker on the polarity band they would be cathode polarized. You are operating the zener at it's breakdown voltage.

As the bias voltage is 12.5 I'm going to make a wag that you are using either 2 or 4 El-84. Off the top of my head I think you'll be biased at 22-26ma per pair. So let's say you've got 4 of them. You'll be operating the zeners at around 50ma for the power tubes and another couple three for the rest of the circuit. Say 55ma for grins. In full roar feedback sustain you'll be in the 150 to 200 ma range.

So if you need to drop 95V normally on these zener dump circuits you need to up the voltage about 10% or so. I'd look for around 100-110 volts of zener breakdown voltage.

We always size the zener for maximum current and then some so let's see how much power you'll be dissipating.

P=I X E therefore .2 X 110 = 22 watts. Make all the zeners as close to the same voltage as possible and figure a way to heat sink them unless you can find a 100V 25 watt stud mount and just settle for the voltage you get. If you use 5 watt 20V there is a resistor heat sink clip that is almost the right size listed in the mouser catalog. You have to squeeze them a bit to make them work for a 5 watt zener. In free air, I personally think they will have a limited life.

Once you have a stable power supply you'll have to experiment with the bias on your pre-amp tubes. If you're working from your own scheme you know about where you want them to run so you'll have to substitute in values from 870 to 2.2k ohms until you get the 1.2 or 1.5 or whatever. The phase invertor bias resistor will change the most and you'll be playing with the tail also to acheive the balance between headroom and breakup that you are looking for.

I assume from looking at the PS scheme that you are providing a filament transformer also.

Use the reverse anode as shown in the tutorial supplie by MFowler (A) and you'll be fine provided you can find the size you need. I know of no way to series string stud mounted diodes but I'm sure it can be done with a little ingenuity and a milling machine.

Heyboer would probably charge you about $85 for a custom wound 200ma tranformer with filament winding and the works. Ask for a 300V center tapped secondary HT winding. With the need for a 240V primary shipping will of course probably kill that idea but if you are building the amp for sale or intend to use it gigging you have to factor in something for reliability. 410VDC on EL-84 or even 7189 is a bunch. I wonder what the transformer was designed for with all that HT voltage? EL-34? If I'm wrong on the tube type then the bias supply is not drawn right and you'll need to derive it from the AC side of the HT winding using a .2uf bipolar cap and a 220K resistor in series before the adjustment/filter portion of the bias circuit. This is tough to design because the capacitor will only source a certain current then it will collapse. I've been there. It works but there is not much literature out there to help.

I hope all that helps
Dan

[skyboltone]

Ok so I finally get around to looking at the donor amp scheme. That clears things up. The bias shown on the donor is -37V so that's what you want for the Express. Not -12.5V. Ok, so everything else up above should work out fine. Like I said the pre tube bias will be all wrong because of the 100 volt dump. The bias string will also have to be adjusted for the same reason. You are operating the chassis ground and hence valve cathodes independend of the bias winding. See? Not bad, just you gotta play with it.

Good luck and report back with your findings.
Dan