• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Please can you identify this part

Hi, Here is a source follower from a Melos SHA1

The source followers have a double diode connected between G and S. I need this double diode but can't identify the part number, Do someone knows what it coule be ?
Also, what is the purpose of this double diode and how it works ?
Capture d’écran, le 2022-01-16 à 09.20.48.jpg


Paid Member
2007-09-15 8:14 am
The diodes are for gate/source protection. I assume these are lateral FET's

I seem to recall 8v2 Zener's being a typical value and they are used in series with a 1N4148. Note the polarity of the diodes with respect to the N channel and P channel FET's they protect. They clamp the G-S voltage to 8v2 +0.6 in one direction and have no effect in the other direction (appear open).

This shows a typical arrangement.
Screenshot 2022-01-16 150608.jpg
Might be a TVS, but I am confused by your schematic symbol,internalZeners are in series, not diodes in parallel.
That said, I already found on comment about proper symbols being oftn ignored,so ...

In any case, you want something which clamps voltage to less than 20V (which is a typical Vgs breakdown voltage), both ways for 100% safety.

Personally I use a 1N4148 in series with an 8.2V Zener across GS in the IRFP250/240 I use in my Musical Instrument power amps and it works fine, but of course would be as happy with a symmetrical array.


Paid Member
2010-10-06 12:39 am
Tanunda SA
As I would like to use what's inside in other places, here is a picture. The part has a gold line in the center
That is a diac. It is a by directional avalanche breakdown device. Threshold typically 30v but also ranging down to around 10v, either direction and only several volts conducting with a holding current of a few mA. The primary use is as a AC mains trigger for thyristors.


Paid Member
2007-09-15 8:14 am
That symbol is a Diac as Johno mentions. Looking at the circuit in post #1 you can visualise it actually says Diac 30v under the left FET.

BR100 and BR101 are numbers that come to mind (ancient TV PSU's)


  • datasheet.pdf
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Paid Member
2007-09-15 8:14 am
Diacs are fundamentally different to Zeners. A Zener begins to conduct when the voltage across it reaches its marked value. The Diac also does this but once conduction starts the device drops to a low impedance condition and conducts heavily until the current flowing falls below some predetermined value. Diacs are also bidirectional.

So if they are Diacs then you can not really make those from Zeners.

Are you sure the circuit is drawn correctly because G-S Zener protection is common but I've never seen or heard of Diacs used with FET's.
^^^^^^^ that.

Using a DIAC there gives me the shivers: uncontrolled, BRUTAL negative impedance clamping action, quite undefined trigger and clamp-to voltage, ugh!!!!
I´ll take a Zener any day of the week.

You use, say, a 12V one, you know it will not let anything above 12.1V, will remove itself from the picture at 11.9V ...or thereabouts.

A Diac is completely unpredictable, unless, say, pretested before use.

But hey, Designers often have their own pet ideas.