Zener diodes to drop voltage in Rocket

[LarryLarry]

The Baldwin organ PT I used to build my Rocket was a bit high at 290-0-290 so I picked up 3 10V 5W NTE5125A Zener diodes from Fry's in hopes of dropping the voltage 30vdc or close to it. It ended up dropping the plate voltage on the EL84s from 345 to 318. It dropped the voltage across the cathode resistor (100 ohm 25W) to 12.3vdc.

I mounted them on a tag strip staggered help with heat issues.

Does this look satisfactory? Should I get a couple more and drop the voltage any further?

The amp sounds fantastic, and the hum issues I had are 98% gone...

[Cliff Schecht]

Those Zeners look much too small wattage-wise. What are they rated? Did you calculate the power dissipation through the Zeners to make sure you weren't overdissipating them? I prefer to use the chassis mounted studs if I'm using a Zener to drop voltage, otherwise they will eventually cook themselves. I think only going 2x under the dissipation (i.e. 2.5W through a 5W Zener) is not acceptable with them completely encased inside of a tube amp with elevated temperatures.

I don't like using the high powered Zeners to drop my B+, I've had a few of them fail in different ways. I've had them open up completely more than once because the little stud you solder to is easy to spin free. I prefer to use the amplified Zener circuit like what I posted in my 5E8A thread. R.G. Keen uses a MOSFET in his design but I've been using BUS14 BJT's (rated 1000V @ 30A) and am yet to have one of these fail. Even after hours of playing you are still able to touch the case of the transistor (I use the chassis as a heatsink). I've got the amplified Zener circuit in four different amps right now and am yet to have a problem with them (other than the B+ drifts a bit as the transistor heats up, but not by more than 5%).


FWIW I noticed my Rocket got a lot quieter with lower voltages too. 320V may still be a bit high, I made mine hit spot on with what the voltage charts of a real Rocket read (300V exactly IIRC at the highest point). Check your plate dissipation, if you are running the stock 50 Ohm bias resistor you could be cooking the EL84's.

[LarryLarry]

I mentioned in my post, they are 5 watt 10 volt Zeners (NTE5125A) and that I have a 100 ohm 25W cathode bias resistor (in place of the 50R).

I'm getting 12.3vdc across the cathode resistor and according to the Weber bias calculator, it's about 9 watts per EL84. I can hold my hand on the PT after playing it for an hour, so no heat issues that I can tell...

[RJ Guitars]

I like the idea but I would go for something with more beef (wattage). It sounds like you found the sweet spot voltage wise.

These are my own preference. http://www.mouser.com/Search/ProductDet ... -NTE5265AK You can chassis mount them and they should never give you trouble... but they are not cheap!

rj

[Cliff Schecht]

I did reread your post after I re-read mine and saw answers to my queries. Sorry about that, I was rushing!

So you are running 10V 5W Zeners with a 12.3V bias reading into a 100 Ohm resistor. This means your quad of 84's is drawing 123mA of RMS current at idle. 9W is probably close enough to 8.4W to be happy but I didn't stop until I hit 70%. This amp was for a friend and I didn't want it frying tubes and costing him more money in output tubes. Anyways back to the Zeners!

Your Zener diodes see more current than the 123mA. Since you are elevating the center tap of the HV secondary, all of the current drawn by this winding is fed into the Zeners. So you have 123mA of power tube current at idle, plus whatever current the preamp/PI is drawing that is seen by the Zeners. Let's say that when you dime the amp and dig in you are drawing 180mA of current now total from the PT. This puts you at 1.8W of continuous power through each Zener diode and maybe 2.5W peak power (assuming a perfectly sinusoidal waveform from the HV secondary tap).

I feel like I'm being conservative in my 180mA number and again you might be pushing those Zeners pretty close to their limit. They are rated at 5W but don't really have any place to dissipate the heat except in the leads and the junction. At least with the Zener studs you can dissipate the heat very well with an aluminum chassis. IME the smaller Zeners don't work well for a permanent solution when stressed like this continually in a hot environment. The inside of a tube amp chassis does get pretty toasty after a few hours.

FWIW I've fixed more than one new model Fender amp that had a Zener go bad in it (they were cheaping out on the voltage regulation!).

[diagrammatiks]

well those zeners are rated at 75C with derating above that...

Usually with those components if you are at about 1/3 of the max wattage, free air will be able to keep the component cool enough.

However as you get near the max rating heat could be a problem.

[Cliff Schecht]

Maybe I'm being overly worrisome but think about the peak power those Zeners see. Even if it's only for a small moment (microseconds) the peak currents can reach well over an amp, especially right when you flip that standby switch.

IME with silicon devices even a tiny pulse that exceeds the rating of the device will derate its life dramatically. I've blown up many pieces of silicon in all stages and understand the mechanisms that destroy them (read: heat!). Zener diodes operate under a high electric field with a thin depletion layer. They don't like stress and make terrible voltage regulators. If you feed them just the right amount of current, however, they do make a great reference voltage for non-critical applications. But voltage references are not good at supplying much current at all, this is what bulkier regulators are for. The Zener elevating trick is a reference circuit, the amplified Zener is a simple voltage regulator. In the end I just don't trust Zeners to handle power, I've had way too many go south for me to do that.

[cbass]

They've got high power zeener for cheap here http://www.electronicsurplus.com/Item/8 ... 2010Z30T5/

I have never orderered from them though

[LarryLarry]

Thanks for all the awesome help! Sounds like I should go to stud-mount Zener(s) for the long term.

Can someone explain to a novice (me) about the reverse-polarity factor of a Zener? I've got regular Zeners in there but I've read alot about reverse-polarity.

TIA

[surfsup]

Why not use an irf820 on the PT centertap to drop voltage?

[John_P_WI]

Why not use an irf820 on the PT centertap to drop voltage?

+1 Cliff referenced RG Keen's amplified zener above. IT is here bottom of the page as "B+ Reducer":http://www.geofex.com/Article_Folders/m ... tfolly.htm

[LarryLarry]

Thanks guys.

Cliff, do you have a layout and/or schematic for your take on the amplified zener circuit?

[greekie]

From the page itself - just use the values given. R.G. is a great guy!

Geofex.com

[img:931:401]http://www.geofex.com/Article_Folders/m ... osfol4.gif[/img]

[hans-jörg]

Hello,
I had the same problem to reduce the voltage of my B+ from 365 VDC to 316 VDC. So my screens will see exact 300 VDC - what I wanted.
As you can see on the picture below in the left upper corner is a green plate. Under it, there are 7 x 10V5W Zener diodes which brings me down 49 Volt (per diode / 7 V).

The plate where I mounted them I turned down so the Diodes laying flat at the chassis. The screws on both sides of the plate holding them down right on place. But to dissipate the heat I placed the alu rips on the top side of the chassis. And this works very well.
If you don´t play the alu gets warm for shure - but only warm. When you start playing it becomes cooler and cooler. Thats a physical law with kathode biased tubes. As mor power as less current - and the Zener gets hansome cool. The heat raises when you let the amp on and do not play.
As I understood: cathode biased amps you can go to 100% of power because if you cranc the amp the current at the cathode goes down.
Fixed biased amps need the 70%.

Conclusion: a rocket or Liverpool - as harder you play as cooler they stay.

Friend of mine played the liverpool over on e houre cranked at 70%-80% - I controlled the heat in the rear. I had the whole time my hand at the alu rips and chassis.
But: there are 7 pieces of them, and if one fails ...
So if I have the possibility to get one of this bulks Zener - I´ll change them. Only to be shure, not because it doesn´t work.

Hasns-Jörg

[LarryLarry]

That's a sweet looking cabinet Hans! Did you build it?

Not sure if this is a big deal, but my PT HV secondaries don't read the same. One side reads 279vac and the other reads 286vac. Is that anything to worry about?