I want to drop 30V on the B+, help with math!

[exmaxima1]

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[exmaxima1]

This is the simplest way I know to get B+ exactly where you want. I've done it on 2 amps now and it works great.

Do you know if it would work on a power supply that has no tranny CT? For example, connect it between ground and the (-) on the full wave (4 diode) bridge? My Trace Velocette has only a 2-wire HV transformer, and I'm worried about 383 VDC which equates to 18 watts dissipation on each EL84.

[ant]

You can put zeners between the transformer center tap and chassis ground (or the bridge rectifier ground). Get 30V worth of zeneers perhaps 2x15volt 10watt zeners. Piece of cake.

[exmaxima1]

You can put zeners between the transformer center tap and chassis ground (or the bridge rectifier ground). Get 30V worth of zeneers perhaps 2x15volt 10watt zeners. Piece of cake.

I had tried a string of zeners and they did not work on this amp. The string measured 30V on a bench supply, but did not drop anything when installed in the amplifier between the star ground and the bridge rectifier ground. I verified correct polarity and the PCB cut was thorough (I drilled a hole).

I guess I am stumped why the zeners did not work on a Velocette...

Thanks!

[Firestorm]

You can put zeners between the transformer center tap and chassis ground (or the bridge rectifier ground). Get 30V worth of zeneers perhaps 2x15volt 10watt zeners. Piece of cake.

I had tried a string of zeners and they did not work on this amp. The string measured 30V on a bench supply, but did not drop anything when installed in the amplifier between the star ground and the bridge rectifier ground. I verified correct polarity and the PCB cut was thorough (I drilled a hole).

I guess I am stumped why the zeners did not work on a Velocette...

Thanks!

I'm stumped too. That should work, There are no other connections from the negative side of the bridge? And you did connect the zeners cathode to ground?

[exmaxima1]

I'm stumped too. That should work, There are no other connections from the negative side of the bridge? And you did connect the zeners cathode to ground?

Right next to the star ground point, I drilled a hole and brought out a pair of wires for the zener string. This was at a point directly between the ground and the (-) of the bridge rectifiers. I actually measured about 10 volts across it, though it measured 30 volts on my bench supply. And when I measured across B+ to chassis, I got 384 volts (no drop at all!).

Wierd. I have used zeners on tranny CT to ground with success, so I thought this would be a piece of cake. I don't see any other ground paths other than the star ground. Stumped....

[Firestorm]

I just checked the schematic for that amp. No reason the diode trick shouldn't work. You sure about the polarity of the diodes themselves? They make 'em both ways: cathode case and anode case. Either way you just have to make sure the "pointy" end goes to ground. Probably ought to keep the case insulated from everything to in case they sent you the "A" version.

[exmaxima1]

I just checked the schematic for that amp. No reason the diode trick shouldn't work. You sure about the polarity of the diodes themselves? They make 'em both ways: cathode case and anode case. Either way you just have to make sure the "pointy" end goes to ground. Probably ought to keep the case insulated from everything to in case they sent you the "A" version.

I thought that zeners "point" against the current and breakdown (pass current) at their rated voltage. Pointing them to ground would only have a .7 volt drop per diode, wouldn't it?

In any case, I will look again at the board for a parallel ground path. I noticed a CT on the filament winding where it connects to the PCB, and maybe that is the culprit.

Thanks!

[Firestorm]

The center tap on the filament winding won't affect the high tension winding so don't worry about it. A Zener diode conducts in the reverse direction when the voltage on the cathode exceeds the voltage on the anode by more than the Zener voltage. If you put the anode to ground (especially in your case where the negative end of the bridge has mucho negative volts on it), that won't happen, so the Zener will behave like a regular diode. Cathode to ground, on the other hand, and the cathode voltage will be higher than the anode voltage all the time, and the breakdown voltage will be exceeded and you'll get your B+ drop.
The Zener diodes used most commonly for B+ reduction are the "AK" versions which have the cathode on the case. Just bolt 'em on and the chassis acts as a heat sink. The regular polarity ones ("A") can be used, but you can't bolt them to the chassis, so you need an ungrounded heat sink or a higher wattage diode or both.

[exmaxima1]

The Zener diodes used most commonly for B+ reduction are the "AK" versions which have the cathode on the case. Just bolt 'em on and the chassis acts as a heat sink. The regular polarity ones ("A") can be used, but you can't bolt them to the chassis, so you need an ungrounded heat sink or a higher wattage diode or both.

My zeners are apparently "A" version, cuz the symbol points to the lug (not the stud). It takes a magnifying glass, but I can clearly see the arrow pointing away from the threaded end. I must have them wired backwards!

I already had them on a separate board with 2 heatsinks, so I just need to reverse the wires.

Thanks!!!!

[grindomatic]

Doug H wrote:

Wire up a MOSFET like in this schematic: http://www.aronnelson.com/gallery/main. ... alNumber=1

This link does not seem to work, the site is active - and a tremendous resource! I tried using the gallery search for "mosfet, b+,drop voltage, etc." and can't find this, schematic, but I imagine it is there - probably just need the name of the amp it was used in

Thank You