Zener diode

[stokes]

Using a zener on the CT of my PT to drop the voltage.I have a 62v 50 watt zener,but its only dropping 29v.Do I have a faulty diode?Or maybe its not grounded properly?Is this normal?Maybe I need a bigger diode?

[R.G.]

There are footnotes and special cases for zeners, like other parts.
- it is always possible that the actual zener was mis-identified
- it is possible that there is so little current through the zener that it's not getting all the way up to full break-over; this is not a common problem, but neither are 62V/50W zeners common
- wiring issues are always possible, as are rectifier problems.

What's the actual schematic of the power supply? CT B+ winding into a rectifier (tube? SS diodes?) into a first filter? Where does the transformer CT wire go? What is connected to (a) signal ground and (b) chassis ground?

[stokes]

The zener is bolted to the chassis.I'm using a 5V4 rectifier into a 30uf main filter.The CT is connected to the diode.I also have an 82v 50 watt zener which is acting the same way,only dropping about 29v.I wonder about the grounding because I am using a black steel Hammond chassis which I had to grind off the black coating to expose the steel and wonder if I didnt take enough off,although it does appear to be sufficient.Without the zener I'm getting about 450v on my plates of the 6BQ5's,about 420v with either zener.

[mozz]

A 5Y3 will drop a little more volts than a 5V4. Same filament draw so no worries.

[stokes]

Tried a couple different rectifiers,including 5Y3,doesnt drop enough.Would like to get it under 400vdc on the plates.

[R.G.]

I think it might be very interesting for you to shunt about 50K-100K across your first filter cap. That would divert some more current into the zener. I'm suspicious that there's not enough current flowing in the zener to bring it up to full zener voltage, so it's just wiping off a little before it gets into full limiting.

May not be what's happening, but I think it's worth the test. 100K on 400V is about 1.6W, 50K is about 3W, so either do this as very short test or use power resistors if you try it.

It also bothers me that the other zener acts the same way. Still thinking about that one.

[stokes]

I'll give the shunt a try,R.G.,what should I be looking for with this test? A little more background,I bought these zeners about 10 yrs ago when I was trying to drop some volts with a PT back then.I bought 3 different values at the time,a 52v and the 62v and 82v I have now.I believe I tried all 3 and settled on the 52v at the time,but all 3 worked.Do you think a test with a signal running thru the circuit would show something? I'm using a vintage Standard Coil PT that seems to be okay,although the DC voltage after the rect seems to be a bit higher than I expected.The AC with no load was 340-0-340.I wasnt expecting to see 450v with the rectifier.

[Dave H]

Is anything connected to the PT center tap other than the zener? The zener should be the only connection to the center tap. You could test the zeners by connecting them across the B+ supply with a suitable resistor in series.

[Jazz P Bass]

Here is a schematic provided by fellow MEF member Tubeswell.



Note: I have seen references to NOT using over wattaged zeners as it may take more current through the zener to get them to operate correctly than a tube amp draws at idle.

[stokes]

Here is a schematic provided by fellow MEF member Tubeswell.

[ATTACH=CONFIG]42994[/ATTACH]

Note: I have seen references to NOT using over wattaged zeners as it may take more current through the zener to get them to operate correctly than a tube amp draws at idle.

Maybe thats the problem,Jazz.I'm using 50 watters cause thats what I have on hand.The amp I'm using these on is a 12 watt Brown Princeton clone with 6BQ5 power tubes.I think I have a bunch of 5 watt zeners somewhere,I'll try stringing 4 or 5,I think they are about 10v's each.

[R.G.]

Yeah, that's another way of looking at the "shunt". Zeners do not go from zero current below zener voltage to rock-solid zener voltage above it. The "knee" where it makes the transition from non-conducting to conducting it a rounded curve. The bigger (which translates to "higher wattage) the zener, the more current it takes to get it to fully conduct and limit at the zener voltage.

A low current power supply drain may not pull enough current to pull the zener past the "knee" where the voltage starts limiting more.

The resistor shunt was a baldfaced attempt to shuffle more current into the zener and bring it up into limiting.

[stokes]

Ok,I found the smaller 5 watt zeners I have,they are 18v.I got them many years ago from a guy in Lithuania,they are Russian military surplus.So,anyways,I hooked one up to see and it dropped the exact 18v's one would expect.So I am going with the presumption that the 50 watters are just to "big" for the lower current I have in this amp,as R.G. and Jazz both referenced.I guess I could string a few of these,but I'm working in a small chassis,gonna check Mouser and get a 15 watt or so of the appropriate voltage.Thanks guys,I'll post back when I get the new zener.

[DRH1958]

[QUOTE=stokes;451592The amp I'm using these on is a 12 watt Brown Princeton clone with 6BQ5 power tubes.I think I have a bunch of 5 watt zeners somewhere,I'll try stringing 4 or 5,I think they are about 10v's each.[/QUOTE]

Did you retain the fixed bias method when changing over to 6BQ5 tubes? The Brown Princeton (6G2 model) shows that it has a fixed bias supply is taped off the main secondary winding. Since this voltage will be lowered using the CT zener method, your bias voltage will also be lowered. Make sure to re-check your bias current after the mod is done if you kept the fixed bias method.

[R.G.]

At the risk of further getting myself to being "Mr. MOSFET", I feel that I ought to point out that a $1 MOSFET plus a much smaller power rated zener makes for a big-power zener equivalent.

[stokes]

At the risk of further getting myself to being "Mr. MOSFET", I feel that I ought to point out that a $1 MOSFET plus a much smaller power rated zener makes for a big-power zener equivalent.

Never played with Mosfets,I'm listening.