BD139/140 Overvoltage

[B.I.G]

can i go overvoltage for some BD139/140 ?

I have a +/-60V supply and they are rated 80Vceo ... I have used BD237/8 on +/-60V rails and had no problems and they are also rated 80Vceo

[JensRasmussen]

Hello,

I'm almost certain that you will burn them, Are they in the VAS of an amp the might "see" almost twice the railvoltage. In this case it will be 120V.

They might not burn though. I used 139/140 in by Leach at +-55V rails before I changed to MJE340/350, the transistors played, but distorted like mad because of overvoltage.

\Jens

[sam9]

Even if they are used in a way that they should never see the rail-to-rail potential but only-to-common, some people routinely try to choose ratings that are twice what is "needed" just to allow for unforeseen circumstances. Some call this a "belt and suspenders" approach.

You could try 2SD669/2SB649 or 2SC2690/2SA1220A (KSC1690/KSC1220A, if Fairchild). If you can find them. . .

[B.I.G]

My MJE340/350 are comening in a week or so but i was looking for something to put there until they arrive !


Thanx for the replys

[JensRasmussen]

Here is a sim of the VAS transistor in a Leach, I did not have the time in my first post - sorry.

Green is output, red is voltage C-E on the transistor

\Jens

[Eva]

I've repaired amplifiers with BD139 & BD140 used in VAS and as pre-drivers with up to +-50V rails with no problems. I had to replace dozens of output devices but never had to replace a blown BD139 or BD140. At low currents [ie: 10mA] they appear to be able to reliably withstand 100V [te ones I saw were from Philips]

Anyway, for VAS or predriver with +-60V supplies I would look for 150V or higher Vceo transistors. BD139 & BD140 would be OK for other applications where they are exposed to a single 60V rail


JensRasmusen :

BD139 & BD140 are much faster than MJE340 & MJE350. That probably caused your leach amp to be unstable and oscillate thus producing very audible distortion

[JensRasmussen]

Hi Eva,

The distotion was about 0.3% when measured at 0dB. "like mad" was just words that came to mind when I compared it with the result using the 340/350 pair instead. The distotion went down to about 0.02% at 0dB into a 8 ohm load if my memory serves me right.

\Jens

[sam9]

This is not the result of a carefully controlled test but I have gotten the impression the MJE340/350 may be a contributing factor toward oscillation in a CFB amp using 2sc5200/2sc1943 or similar. 2SD669/2SB649 "cured" the problem. While there is no reason to believe that this is directly applicable to the Leach, I mention only to support the observation posted, above, under some circumstances MJE340/350 may not be a good drop-in substitute.

In the above case a simiulation (not the most sophisticated in construction, I'm sure) showed a lot of high order harmonic spikes when the MJEs where in but not with others having higher fT, so there may be something to this.

[B.I.G]

They did not blow there us no audible distorsion but i am still going to replace them when the mje340/50 are going to come .... just to be sure .

Ps: my BD`s are made by ST

[Eva]

sam9 :

Slow transistors in a VAS produce almost the same effect as increasing the value of C-B miller compensation capacitors. When the dominant pole of the amplifier is located on the VAS, lowering that pole reduces open-loop gain-bandwith product and increases phase margin thus improving stability [and increasing closed-loop THD due to less feedback]

In the other hand, slow transistors in a CFP contribute to oscillation, not to say that most CFP transistor combinations are inherently unstable when driven simulstaneously at high Ic and low Vce [ie: when 'big device' starts to saturate, at clipping threshold]


Jens :

Did you check wether this THD increase was due to transient RF oscillation [usually at zero-current crossing or output-device Ic/Vce dependent] due to too low phase margin, or due to C-B reverse breakdown leakage? [only above a certain output level threshold]. In case of reverse breakdown, the amplifier should show pretty low THD up to at least -6dB