zener on ct...again

[Chuck H]

Well, I'd like to try this out to drop some B+ but when I look at whats available in diodes everything seems low on voltage or current. Whats the rules with diodes for series or parallel operation and how do I determine a safe wattage (I think I know, but I'd rather not guess and make fire). And can someone point me toward some bigger diodes.

Thanks

Chuck

[loudthud]

Take a look at this thread. A MOSFET is used to "Boost" a 1W zener.

http://music-electronics-forum.com/s...ighlight=zener

Weber sells a kit to drop the voltage with a large zener diode but some have had trouble blowing zeners.

[tubeswell]

I have successfully used a 12V 5W zener to lower the B+ on my 5F2A with no trouble. I found that it dropped the B+ by about 20 or so volts. I just used a single diode - no other fancy stuff

(If you are having trouble finding some 5W Zeners, I'll make a deal with you for some 47pF, 270pF and 500pF silver mica caps. ;-)

[Chuck H]

Thanks tubes and ...er ah, Mr thud...he he.

Good post on the mosfet booster. It actually gets me thinking about trying a power scaling mod with the regulator at the CT. Your absolutely right on the money about the big cap across the circuit too. If you've ever read any of the old tube books that cover "back biasing" you'll see that in the golden days the engineers understood this stuff. A big cap is bridged across the resistor at the PT CT. And I just found a site that covers a reinvention of back biasing using a zener. Also has a big cap across the circuit, no doubt for the reasons you mentioned.

From what I'm finding I don't see whay the 5 watt Zeners shouldn't work (the best voltage rating I can find is 200 though). You just have to be sure that the voltage drop isn't such that there will be too much current through the device. So if you need more voltage dropped you just need to run a string of them (and a big cap across the circuit). No problem. Would it be a good idea if I'm using a string of diodes to bridge a resistor over each one? The same way series filters are done. Might help keep from flaming the first one in the string on the initial inrush.

Thanks

Chuck

[tubeswell]

Aiken has an article on this BTW

http://www.aikenamps.com/BackBiasing.html

But I gather the parallel reverse-biased electrolytic cap there functions as a reservoir filter for the bias feed (which he has comming off the HT CT)

[TheTinMan]

Chuck - how much voltage are you trying to drop? I think the voltage rating usually applied to Zener diodes relates to how much they drop, not the absolute value of the voltage applied to them. Here's a 10 watt, 20 volt panel mount Zener from Mouser: http://www.mouser.com/catalog/specsheets/1N2970-ssi.pdf

I used a similar Zener by itself on a 5F2-A just like Tubeswell, except IIRC it was rated for 10 watts just in case. Simply bolted it throught the chassis. I don't understand the point of the "kit" Weber sells - the chassis should serve as a pretty good heat sink IMHO.

HTH

Chip

[Chuck H]

Thanks TinMan, I think I found what I need with Mouser. But it's hard to tell because the data sheets for most zeners are incomplete in that they don't list the applied voltage rating. Most, but not all, above 5 watts are rated for 200 volts. These are what most beople have used. It would be nice to know though. Could be why some people have had trouble with the big zeners.

How much voltage do I plan to drop? A whole bunch. Most of it occasionally

I just need to make sure I don't let the smoke out of my amp. They stop working when the smoke gets out

Chuck

[tubeswell]

The only voltage rating you need to worry about is the overall rating that the zener will drop before it goes into breakdown, which is the rating you want for teh particular amount of volts you want the diode to underbias the HT winding for. Say the winding is 320-0-320 and you want to lower it by 12V - you pick a 12V zener and reverse bias it off the CT, so your voltages look like (+308)-(-12)-(+308). That is how they work. A 12V zener diode will pass 12V before it goes into breakdown, that is why you use a zener for these voltage regulator applications, because of the precise low-voltage breakdowns you can get.

5W is probably plenty on the HT because even if you have a 250mA rated winding you are only dissipating (say in my 12V example) 12V * 0.25A = 3W (that is, if your winding is ever going to draw the max current, which it probably won't, unless you want to do somethin' stupid like put in a tube load that draws too much current for what the winding is spec'd at).

[Chuck H]

Thanks again tube

I know the the math to figure the wattage and how many volts I can drop with a given watt rating now. I just had to simplify (zener voltage*max current seemed too simple when your used to dividing voltage drop and yada, yada).

I read that most zeners are rated at 200V. That doesn't make it true. But I did read it...Somewhere. And I did see a couple of specs that indicated a 200 volt rating. Not the zener voltage, but the applied voltage. Not to mention that at some point any diode will fail. Obviously I don't expect to put 10KV on top of a 12volt zener and get 9988 volts on the other side. I'd expect smoke. So why don't they include the info in the data? They do it for every other type of diode. Maybe there is something inherant in thier operation that does make the applied voltage irrelevant. But I don't know enough about it to say.

Chuck

[Steve Conner]

Thanks again tube
Obviously I don't expect to put 10KV on top of a 12volt zener and get 9988 volts on the other side.
Chuck

Why not? If I stand on top of a mountain, I don't need trousers with a 3,000 foot inseam.

[hasserl]

Those 5 watt zeners will get hot, real hot. Hot enough to desolder themselves. The math may say they'll only see 3 watts, so they'll be fine, in reality they'll get very hot. You can help keep them cool with some heat sinking, I used copper pennies (not the cheap zinc ones, use real copper pennies) mounted between them. Just drill the penny and slip it over the lead between the zeners and solder. It's a trick I learned here on Ampage years ago. Probably from Bruce, but I don't recall exactly who it was.

[MattT]

Would it be better to just build a simple Voltage Regulator with a MOSFET? Sort of like an extremely simplified version of Power Scaling, but adjusted/fixed to your target voltage and no voltage tracking or bias voltage scaling. Just 'dial-in' your voltage and then leave it. It would be only a few parts...the MOSFET, 3 or 4 resistors, and a couple of diodes.

On Amp Garage, a guy has made little PCBs just for this. He calls it VVR (or VVR2 if you plan to do anything other than scale the entire B+).

[TheTinMan]

I remember reading somewhere about putting a relatively small ceramic cap in parallel with the Zener on the center tap. The idea was to reduce switching noise IIRC.

Is this a good idea and, if so, what cap value would be appropriate?

Thanks,

Chip

[tubeswell]

Well even though I used the example of 250mA through a 5W diode, in reality you would see less current across it for many tweed amps, so in lots of cases 5W would be ample. A 5F2A would be likely to only see 80 or 90mA tops, and that is only 1W with a 12V. A 5E3, 2W with a 12V.

[Bruce / Mission Amps]

Those 5 watt zeners will get hot, real hot. Hot enough to desolder themselves. The math may say they'll only see 3 watts, so they'll be fine, in reality they'll get very hot. You can help keep them cool with some heat sinking, I used copper pennies (not the cheap zinc ones, use real copper pennies) mounted between them. Just drill the penny and slip it over the lead between the zeners and solder. It's a trick I learned here on Ampage years ago. Probably from Bruce, but I don't recall exactly who it was.

Right, it's an old trick I used for years on original VOX tube amps that were looking for 220v or 110v... not 120v to 126vac! Although those PTs were wired to run with 110vac or 220vac, the big ass things crank out excessive high voltage to the rectifiers when run incorrectly at 120vac to 126vac so a string of three 5 watt zeners (with the pennies as heat sinks in between the diodes) ...holding the high voltage secondary center tap up from ground, .... worked just fine to keep the solder from dropping off.