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Vintage threads from the first ten years

zener diode part #

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10/6/2000 6:47 PM
dave rutherford	zener diode part # Anyone recall the NTE part # of a ~30 to 50 volt, 50 watt reversed polarity zener diode? dave

10/7/2000 1:56 AM

Oooh, I hope you isn't figgerin' on doin' thet cheesy Weber mod to lower the B+ in a fine Fender amplifier. Well, fer starters, I ain't heard of no such thing as reverse polarity zeners. It's just a big ass zener that requires an even bigger ass hole drilled in the chassis. Why? If you must know, use an NTE5278A, but why not just use a resistor? Cheaper and you can easily experiment until you find the value that lowers the B+ the amount you like. Start with 100-ohms/25-watt. Drill one little hole to mount a terminal strip (or solder to the chassis if you have the gun to do it).

10/7/2000 2:37 AM

dave rutherford

Thanks, Randy. While I appreciate the friendly advice, I don't understand your problem with using one of those to lower B+. Your suggestion requires a hole to be drilled also. And I don't want to have the extra sag that a 'big-ass' resistor would create. I guess 'reverse polarity' is the wrong term. I thinks its 'cathode case' - add a 'K' to the end of your part number - to be technically correct. And don't worry, it ain't no vintage fender I'll be drilling!

dave

10/7/2000 3:21 AM

The Zener diode is not a "Weber" mod.

It's been around a long time, long before Weber heard of it. I've known about it for years and years. It works really well when called for.

I call it the "big ass zener diode" mod.

A resistor is a bad choice for a number of reasons. The biggest of which is that it increases impedance of the power supply. That makes it impossible for the for the amp to supply stable voltages when it comes under load. Too much sag.

Sounds like ass.

The zener on the other hand drops the voltages by a stable, predictable amount without incurring increased power supply impedance. Result is reduced power supply voltages with good regulation.

The best place I've used this mod is in the later silverface twin reverbs which often show up with well over 500v on the plates. Dropping the B+ a bit is well called for in these amps, resulting in improved tone, longer tube life and retention of the clean, punchy, sparkly tone a Twin amp should have.

Depending on the design of the power supply, the bias supply will probably need adjustment of values as the zener will reduce the available bias voltage if the bias is ground through the center tap. (most Fenders are).

Drilling a single-hole to mount a stud-mount zener in an unseen place is not exactly hack-butchery.

10/7/2000 3:48 AM

Nuke:

Thanks for your support of the semiconductor industry...

    It seems to me that off-the-shelf PT's are available only with specific secondary voltages and adding in a zener to the PT CT or FWB ground connection is a great way to dial in the exact B+ voltage you want on the plates... And even if you custom order trannies from Hammond the voltage they produce could be considerably off-spec (ask Benjamin about that one!)

    I haven't used the stud mount zeners yet, but I have been experimenting with the 9.1v/5W zeners that Bruce Collins told us about. (You can get them for about 50 cents each from Mouser.) With the currents normally seen in a 50W to 100W guitar amp, the 5W rating should handle any voltage drop up to maybe 12V just fine.

    With them being so cheap you can experiment until you find the voltage that sounds best with your power amp. And you can wire up a noiseless switch to select between different B+ voltages (dealing with the lower voltages in the CT/ground circuit rather than the high B+ voltages). The transformer windings seem to act like a choke in squelching any noises when I toggle the zeners in and out of the circuit... [Maybe R.G. can shed some light on why these switches work so well- no pops, no clicks, nothing but the silent switching of the B+ voltages!]

    IMHO power supply resistors work fine in dropping voltages to the screens, and to the preamp and PI tubes... but not for the plates of the power tubes. Ohm's Law rules here... (the voltage drop across the resistor will vary with the current drawn).

--Thanks!

Steve Ahola

10/7/2000 5:07 AM

Benjamin Fargen

Steve,

Hey there!!!

You are correct about the hammond trannies and the spec voltage being a little different than the actual voltage you end up with. Bruce and I both come up with similar voltages with the same trannies and they always seem to be a bit higher on the high voltage taps.
Bruce nailed the fact that they are so well made and efficient that a 275vac-0-275vac tranny usually yields a true voltage of 285-290vac on the secondaries. I use multiple 5 watt zeners in line with the center tap to drop voltages all the time. I don't see a problem with reliability of the zener as long as you displace the heat that is generated properly with some sort of heat sink.
It would be nice to afford custom wound trannys for all the different amps I build....but lets face it........at this point in the game it's just not cost effective to me or the DIYer.

See ya,
Benjamin

10/8/2000 4:24 PM
Gus	Benjamin, Bruce do you solder Copper fins to the legs as heatsinks it seems to be a good way to get rid of heat in small devices more direct than thur the epoxy DANGER IS THE HIGHER VOLTAGE as you go "up" the zeners from ground. I don't Know if this is a safe way to heatsink.

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