zener diode to decrease high voltage....

[Chuck H]

It would be asking a lot of the diodes. Dropping 30V in a 120W amp would tax the zeners with something like 10 watts with the amp running wide open. So even with a string of three you'll be over the zeners max dissapation by 1 watt for the string. Also, in a hot amp the zeners max dissapation will drop. I'd probably rate the wattage of the zener string for about double the watts it should dissapate. So you could use six 3W 5V zeners in a string for that 30V drop.

I do like a string of DO package units better than one big chassis mount thing. Much easier to deal with. Just make sure to over rate generously so the diodes don't get so hot as to unsolder themselves. And keep them away from any electrolytic caps.

[acorkos]

is this the correct location and orientation for the string of 6 x 3W/5V zeners?

[jmaf]

would this work with a 120 watt amp w/525 volts on the plates? the guy plays at pretty much full tilt...

120 Watts at 525 volts would be roughly 230 mA. Add a few mA for the preamp. That current through a 30 V Zener would dissipate 6.9 watts. If you divide it into three Zeners would be less than 2.5 Watts per Zener. I use twice the power in all my assemblies just to be safe and for long lasting builds. So three 5 watt zeners would do.

[Chuck H]

Right. But the guy is cranking the amp. AFAIK zeners don't have a "nominal" rating like a slo blo fuse. So I added a little safety margin and called it ten watts. As for the five watt zeners, can you get those in the DO package?

[Tom Phillips]

120 Watts at 525 volts would be roughly 230 mA...

However, that is just a straight power calculation. If the amp is putting out 120 Watts to a load then the HV power supply will be required to provide substantially higher power to the power amp section. I would at least double that 230 mA as a first order approximation.
Regards,
Tom

[lowell]

And don't forget about this too.

http://www.geofex.com/Article_Folder...ly/mosfol4.gif

I've incorporated this quite successfully in series w/ the center tap of the OT to lower plate voltage. I also use one in series w/ the screens too so that when these "reducers" are IN circuit the ratio of Vscreen/Vanode remains constant. In my design I use a 100v zener and bring the voltage down alot to get pentode overdrive. In my higher power 5E3 design this is great for switching between a high-headroom 5E3 and your stock sound.

THANKS RG!

[trobbins]

And don't forget about this too.
http://www.geofex.com/Article_Folder...ly/mosfol4.gif
THANKS RG!

I've used that 'zener' for HV regulation (for mulitple oscillators) - if the operating current through the zener is within a known range, then a resistor can be put in series with the fet drain so as to lower the dissipation requirement of the FET. Using a TO3P device can allow something like 10-15W dissipation, but you have to go through the standard thermal design process. Certainly more robust than trying to use a series string 5W radials in free-air.

[acorkos]

ok...i installed a chain of 6 x 6.8V/5W zeners between the HT fuse and the top of the 1st caps in series. 525v on the plates dropped to 485v, w/482v on the screens. so far, so good. i'll play-test it tomorrow and see how it sounds.

[acorkos]

play tested it just now....high volume/heavy playing caused the amp to go dead. fuses intact...looked at my diode chain: blown apart at one of the 'links'....hmmm

[Tom Phillips]

...blown apart at one of the 'links'...

By that do you mean at one of the solder joints or that one of the diode cases blew up?

[Chuck H]

Arranged like that the diodes need to float in free air to stay cool. But at 40V and what is probably 250mA that's pnly 10W across a 30W string of diodes??? They shouldn't be getting hot enough to unsolder themselves.

[acorkos]

By that do you mean at one of the solder joints or that one of the diode cases blew up?

solder joint

[hasserl]

Like I mentioned before, they tend to get real hot, even when they are over size. You can use copper pennies in between the diodes as heat sinks, drill a hole in the pennies and run the leads thru them and solder. (a trick I learned from Bruce Collins)

[Prairie Dawg]

That's a neat trick. I used a zener in a Vox to get a 50v drop and with one of those infrared thermometers it was about 135-140 degrees at the base of the diode but a couple of inches away the chassis had cooled down considerably. If mounting them to a chassis allow for a couple inches of metal around them for a heat sink.

[Tom Phillips]

...at 40V and what is probably 250mA that's pnly 10W across a 30W string of diodes??? They shouldn't be getting hot enough to unsolder themselves.

I still dispute the 250mA figure per my post #35