Can someone point me somewhere so I can learn about how to use a zener diode?

[elkym]

I need to shed some voltage on the taps from my transformer-- from 6.3 volts to 5 volts, so I can use my 5y3gt... (Fender Champ build-- based on the ceriatone layout)

any help would be appreciated...

[doctorx]

If you want 5 volts for a rectifier and don't have it the 6.3 volt tap is not the place to get it. You should add a 5 volt transformer, they are cheap. (https://taweber.powweb.com/store/magnetic.htm)

[donnyjaguar]

Like the good doctor says. But, if you must run a 5V tube from 6.3V winding, simply add some series resistance. You need to shed 1.3V so if you factor in the 2A filament of the 5Y3, the appropriate series resistance is 0.65Ω. That's not a standard resistor value, but 0.33Ω is and if you put one on each lead that would net you 0.66Ω. A single resistor will need to be 3W, but you could use 2W, 0.33Ω ones. I can supply the math if interested.

I've done this in projects and it works fine. It probably increases filament life by limiting the initial inrush of current during power up too.

[WaltC]

but of course you've just added 2amps of extra load on the 6.3V supply, which is probably over (maybe *way* over) the rated capacity of the 6.3V winding on the PT...

[doctorx]

but of course you've just added 2amps of extra load on the 6.3V supply, which is probably over (maybe *way* over) the rated capacity of the 6.3V winding on the PT...

That's what I was thinking.

Or, how about this: If you want the sag of a tube rectifier, add a resistor in the rectifier circuit to give it to you without the hassle of a tube rectifier. I think Ted Weber sells add on units that do this too.

[donnyjaguar]

I was going on the assumption that there were no other filaments running from your 6.3Vac supply. You *can* run a vacuum tube rectifier from the 6.3Vac winding, as in a typical 6X4 circuit, but generally you aren't dealing with the kind of voltages found in a power amplifier.

As for your original question, a zener diode is the same as a regular silicon diode in that it conducts in one direction. However, when the current is reversed, the zener will start conducting at the "zener voltage", or breakdown voltage. Zener diodes are, generally, very low power devices and not suitable for driving a 2A circuit. They are also DC devices and not generally used in AC circuits. Primary services they are used in are voltage regulators.

[BillyJoeJimBob]

I started to suggest just stacking some series silicon and Schottky diodes to get your 1.3 volts... then remembered this is an AC circuit. Duh. Even if it was DC, Zeners wouldn't have been a good choice but here's a link anyway if you're still interested:

http://www.allaboutcircuits.com/vol_3/chpt_3/11.html

[elkym]

shoot... this is no good... say I were to run 5 volts at 3.5 amps or whatever it amounts to... is that going to be better or worse than running 6.3 volts at the regular current?

Or what if I went halfway with some power resistors and ran something above 5v and above 2 amps, but just a little--- a new tranny is out of the question-- $40 is a little pricey at this point-- I'm doing this project on a budget...

[Ronsonic]

*************** WHOA ***************

Stop. Everybody's missing the real problem.

You CANNOT run the heater supply for ANY common 5 volt rectifier off the same heater supply you use for the other tubes.

Look at the circuit around that heater tube. The heater is the cathode, or is connected to it and that is where your B+ voltage is. You'll have full rail on the heaters of the other tubes. This will exceed the heater - cathode spec by more than a little.

If you've got a power tranny with two or more 6V heater windings then yes, drop some V with a couple resistors. Otherwise you need to spend like $5-10 for a 5V tranny.

There are some rectifiers with 6V heaters like the 6CA4 and 6X4 that have the cathode isolated from the heater allowing you to use one heater supply for all the tubes.

[BillyJoeJimBob]

Look at the circuit around that heater tube. The heater is the cathode, or is connected to it and that is where your B+ voltage is. You'll have full rail on the heaters of the other tubes. This will exceed the heater - cathode spec by more than a little.

There are some rectifiers with 6V heaters like the 6CA4 and 6X4 that have the cathode isolated from the heater allowing you to use one heater supply for all the tubes.

Nice catch - the voice of reason speaks!

I vote that all of us who posted on this subject (except the OP) and didn't catch this pay penance by one of the following: soldering iron burn, B+ zap, or painful reminder of your choosing. I don't have an amp open so I guess I'm taking the burn....

[WaltC]

mea culpa, mea culpa, mea culpa... 'course you *could* use a diode to block the DC B+ voltage..., wait, I think I hear Rube Goldberg calling...<G>

sounds more and more like the OP is gonna' have to go w/ silicone rectification and use a resistor to get some sag if he wants.

[mark norwine]

just buy a 5V transformer & bolt it in....

[Ronsonic]

I would just go ahead and build, wire up the rectifier socket with everything but the heater supply and then tack two 1N4007 diodes across the socket from the Tranny leads to where the first cap connects to the socket and play some music. Then when time and finance permits a 2A 5V tranny, pull the diodes, wire it up, plug in the tubes and play some more.

There really should be no significant sag in a Class A amp (when a power tube distorts theory starts getting mushy) and what sag there is gets expressed plenty by the cathode resistor. But a resistor to burn off some excess voltage might be in order. If that added resistor is between the rectifier and the first cap it will have a far greater affect on B+ voltage than simple voltage drop E=IR would calculate (this is a good thing it lets you use a smaller resistor in both Rs and Ws). You might have to experiment, I'd start with 22 - 47R.

No punitive burns or jolts are in order. Just don't ask me about the time I advised a buddy who was putting a MM power tranny into some weird 90s Vox AC30 variant that used a solid state rectifier bridge and failed to consider that this thing was going to give him a >700V B+. Smacked my palm on my forehead hard enough to leave a long lasting imprint.