using zeners to lower voltage/different approach

[phsyconoodler]

This Zener diode thing has been hashed out numerous times and what I've seen is using them in the PT center tap.I've been introduced to another approach that seems interesting.Zaphod Phil over at 18watt says he uses them in the B+ line at the first filter cap.
  Seems intriguing but how would they be implemented there?
They are of the opinion that zeners in the PT center tap is to be avoided at all costs.I'm wondering why?And if the B+ line is a better spot,why hasn't it been suggested that way?
  These answers as well as who invented liquid soap and why,are eating away at me.

[Fresh_Start]

The courteous gentlemen at 18Watt flamed me for asking about using a zener on the PT center tap years ago.  Phil & Co insist that anything between the PT center tap and ground is asking for trouble.  Glad to know some things don't change.

Bruce Collins from Mission Amps used to advocate using a string of smaller voltage zeners in series between the rectifier and the first filter cap to drop voltage.  The zeners' cathodes point away from the PT and toward the filter cap (I think - please confirm orientation), and each zener drops a certain specified amount of voltage.  IRRC he also mentioned the possibility of running the connecting leads through pennies to serve as heat sinks.  I seem to remember a discussion about this not too long ago on the AMPAGE board where Bruce advocated putting the zeners between the PT center tap and ground but could be wrong about that.

Bottom line seems to be that either way will work.  I prefer the PT center tap because I can use one big zener the mounts through the chassis which serves as an excellent heat sink.  One solder connection, which you have with a PT center tap in any event, and one mechanical connection with the chassis.  The downside to that approach is cost plus you don't have the opportunity to tweak the voltage down in gradual steps.  There must be a risk of the zener failing, but I don't know if they tend to fail open or closed in terms of voltage passing through.  Frankly, if I were going to use a string of 5 volt zeners to dial in whatever sounds best to me, I might very well do it on the power rail instead of the PT center tap.  More solder joints increases the possibility of a failure/cold solder joint/etc and losing the connection between the PT center tap and ground definitely qualifies as a BAD THING.

I've read various folks theorizing that you need to do something to eliminate zener switching noise.  My suspicion is that this notion comes from the hi fi cork sniffing crowd but I am open to correction on that theory.

Cheers,

Chip

[tubeswell]

All that will happen if the zener goes open when its on the CT is that the CT will lift, which will kill the B+, just like having a standby switch on the CT. The only 'trouble' with that is if you've tapped your bias supply from the B+. Otherwise its fine. I've used a zener or two on the CT in a few amps now to tweak the B+ down a bit.

[jjasilli]

Zeners not only drop voltage but also regulate it.  Unless you want voltage regulation, why not use a bucking tranny?

[phsyconoodler]

Space considerations,simplicity are things that come to mind.

[Willabe]

Zeners not only drop voltage but also regulate it. 

Only the top peak at where you set it, it can drop as much as it wants with/because of the sag in the power supply?

I think thats right?


             Brad         

[Willabe]

Zeners not only drop voltage but also regulate it. 

Only the top peak at where you set it, it can drop as much as it wants with/because of the sag in the power supply?

I think thats right? Oh wait, maybe thats only when it's used as a shunt.


             Brad         

[sluckey]

Zeners not only drop voltage but also regulate it.  Unless you want voltage regulation, why not use a bucking tranny?

Zeners regulate the voltage 'across' the zener with design limits. When used in a method described above, they will reduce the B+ voltage by a constant (regulated) amount but the voltage applied to the load will still be free to bounce up and down as the line or load changes. So not really  regulating the B+.

[dude]

I take it's the consensus here  to use the Zeners in the B+ line rather than the CT.

Used off the CT I've always run the band toward ground, works great so that's right.

Used in the B+ line the band toward the rectifier?   BTW, which end is the cathode?

al

[Backwoods Joe]

Well I just tried a string of 4-10V 5 watt zeners on the center tap of a amp (2-EL84) with a tube rectifier (EZ81) and had a few surprises. First it does indeed drop the voltage but;
A. had a lot of heat from the zeners
B. Even though I was using 10V zeners I only had a 7.4V drop for each zener
C  Seemingly my EZ81 or PT was also putting off more heat. (remember the EZ81-PT was also having to supply the current load of of the "dumped Voltage" of the 4 zeners in addition to the regular "amp load".
   So I'm not to impressed at the moment but I guess this method would be OK on a small SS rectified amp or at least very small amounts of "voltage drop".

[tubeswell]

Well I just tried a string of 4-10V 5 watt zeners on the center tap of a amp (2-EL84) with a tube rectifier (EZ81) and had a few surprises. First it does indeed drop the voltage but;
A. had a lot of heat from the zeners
B. Even though I was using 10V zeners I only had a 7.4V drop for each zener
C  Seemingly my EZ81 or PT was also putting off more heat. (remember the EZ81-PT was also having to supply the current load of of the "dumped Voltage" of the 4 zeners in addition to the regular "amp load".
   So I'm not to impressed at the moment but I guess this method would be OK on a small SS rectified amp or at least very small amounts of "voltage drop".

You should be alright if you heat-sink the zeners a bit. Even saying that, 2 x EL84s draw about 120mA tops, and 10V x .12A = 1.2W per zener, so 5W zeners should be fine with that.

[Colas LeGrippa]

why not use a 0C3 or so for voltage drop and regulation ? IMO, they don't seem to overheat and are very dependable

Colas

[sluckey]

That's fine if you need 105V or need to drop voltage by 105V. Most of the 0?? tubes operate at 100 or higher voltage. If you need to drop the entire B+ rail by 100V maybe you should consider another PT?

[Colas LeGrippa]

right, but then the voltage is no more regulated, if this is what zener users want as well....

and you could use a switch to turn the tube regulator on/off , then the circuitry becomes kinda complex....

[Colas LeGrippa]

but i thnik it's worth it if multiple voltages are needed in one single amp, since we can't use more than one PT.........

[phsyconoodler]

What is an 'OC3'  ?

[Willabe]

What is an 'OC3'  ?

It's a reg. tube.


                      Brad       

[xm52]

If you want to use a tube on the HV center tap, try a 12AX4GT. On the positive side you get a delayed turn on but it requires 600mA of heater current.

[Geezer]

I have used a string of 5x 10V/5W on the CT & it works perfectly (pair of 6AQ5's). They are mounted on a turret board, put off very little heat, and allow me to use a perfectly good Hammond 300-0-300 where I couldn't have (because of the excess voltage on the little NOS 6AQ5's).
This is in my main amp (Maxim Triple Drive, Dumble inspired, but not even close to a clone). I probably have 100+ hours on the amp now, with not a hint of trouble.
It works, so I don't sweat it....

G

[jjasilli]

Zeners not only drop voltage but also regulate it.  Unless you want voltage regulation, why not use a bucking tranny?

Zeners regulate the voltage 'across' the zener with design limits. When used in a method described above, they will reduce the B+ voltage by a constant (regulated) amount but the voltage applied to the load will still be free to bounce up and down as the line or load changes. So not really  regulating the B+.

Good to know:  Zeners in series with the load drop voltage, but don't regulate it.  Thanks!

[phsyconoodler]

I used a resistor from the center tap to ground on a trainwreck build and added a cap with the positive end to ground to filter out any noise and it works fine too.Lots of ways to skin a cat,just seems to get the audiophiles all upset when you do things out of their comfort zone.
  I dropped 30v and there is no grounding issues either.i guess if it were a lot more voltage that you needed to dump,then it would be prudent to just get a different transformer.

[Colas LeGrippa]

like we say in french : tous les chemins mènent à Rome.

[rdrgtr]

I have a switch in one of my builds that switch between a tube rectifier and a solid state rectifier.  I switch 4 10v zeners in when I am in SS rect. mode.  They copensate for the voltage increase I get with the ss rectifier, therefore, my plate voltages and bias are the same for both modes.  I use the zeners on the PT center tap.  So far so good...

[Tone Junkie]

I love simplicity,     if Geezers amp has a 100+ hours on it that method has my vote.
That much time would surely let you know, It doesnt get better than that
Bill