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Zener Diodes in a 5F1 Champ to lower B+?

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10/16/2004 10:57 AM

Zener Diodes in a 5F1 Champ to lower B+?

Hi,

I noticed that some of you use zener diodes on the HV-Center Tap of the Deluxe PT to get the B+ voltage down to "vintage" levels.

Would that work in a 5F1 Champ, too ? (maybe that's a stupid question, but since the Champ is class A and runs the 6v6 at max dissipation all the time, I am not sure if I can use the diode like in a 5e3)

What power rating should I use?

My Champ's volatges are way higher than specified on the ampwares.com layout diagram (390V off Pin 8 of the rectifier instead of 340V, 180 V on the first preamp plate instead of 150 V).

O.K., these values are within the 20% "tolerance", but still...

BTW, I am using a Mojo 760 Export PT, wired to 240 V (I am in Europe), our AC supply is 230V.

10/16/2004 1:18 PM
Chris @ CMW amps	Hi Peter A buddy of me did build a 5f1-kit-Champ and did have a B+ from 390 Volts. I did suggest putting 4 10V/5W zeners between HT-centretap and ground to get a B+ from about 340 Volts... wayyy nicer tone and wayyy healthier for the 6V6. Chris - also in Europe

10/19/2004 4:11 PM
Peter	Hi Chris, many thanks for the information! How did you mount the series of 4 zeners: did you put them in shrink tubes? Can I buy any 10V/5W zener or are some products better than others? PS: Have the zeners been in the 5F1 for a longer period of time? Were there any heat issues?

10/19/2004 4:52 PM

Bruce /Mission Amps

I think I started the whole 5w zener thing about 10 years ago and I really mst tell you the only time I've had one overheat (if it was the right wattage or group of zeners with the right wattage) was when I wrapped them in shrink tubing!!

Let them run in free air across a couple terminal strips if needed but try to keep a large portion of the thickleads intact, they act like a heat sink.

Also, here's a free little trick I use to do...
Drill a 1/16" dia hole, dead center in a couple pennies.
Slide one penny up the lead to the zener body (from both ends) and solder them to the zener leads.
Now you have a great radiator for the 5w zeners.
Looks odd but is way trick and yes the pennies do get hot.

Bruce

11/27/2004 2:00 PM

@Chris - Re: Zener Diodes in a 5F1 Champ to lower B+?

Hi Chris,

>buddy of me did build a 5f1-kit-Champ and did have a
>B+ from 390 Volts. I did suggest putting 4 10V/5W
>zeners between HT-centretap and ground to get a B+
>from about 340 Volts...

how did you mount the Zeners in the 5F1 - there is not much room in there.

Another thing (probably stupid question): I have this cloth covered wire for wiring the heaters (but it's not pushback wire).

How can one easily and cleanly remove the isolation?

Using a wire stripper always leaves a mess of fringes. (Maybe my wire stripper is not good - but it works good for normal wire.)

Cheers
Peter

11/27/2004 8:20 PM

Anytime, you're dropping more than 20 volts, you're talking about a lot of heat.
I've heard from other people trying to drop 40 or 50 volts using a series of 5 watt zeners & having problems.
The proper (but relatively expensive) way to drop that kind of voltage is to use a 50 watt DO-5 zener.
The body of the zener is actually a large bolt that comes with the mounting hardware.
If you order an NTE5275, it drops 50 volts.
If you order an NTE5275K, it drops the 50 volts & is reverse polarity.
In other words, you bolt it to the chassis & connect the B+ center tap to the solder lug on the top of the DO-5 & you're done- nice & neat & it'll hold up.
You can order them from Mouser.
The bad news is that they're $15 each.
-Ed

11/27/2004 6:34 PM

Bruce /Mission Amps

How much heat are you expecting there Ed?
Personally in medium power amps, I agree that dropping more then 36v-40v with zener diodes means you have the wrong PT and are flirting with a minor disaster.
Most diodes short across their junctions so the B+ rises if they short.
However in a little Champ, which will pull much less the 80ma at any given time, dropping 50v would mean:
50 x .08a = 4 watts.... not a big deal.
Use three or four 9v-12v axial 5 watt zeners.
Each will only drop their respective 9v-12v and be responsible for less then 1 watt each!!
They'll run forever like that.
Bruce

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