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 Post subject: Transistor Clamping
PostPosted: Sat, Sep 08, 2018 7:43pm 
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Ok so I've noticed MPSA06s wired up base-to-emitter actually clip LESS than 2N5401s. But the MPS clamp at 4v and the 2N5 clamp at 5v. I thought the MPS would be more compressed and gainey, with the most volume drop. But it's the other way around. The MPSA06 sound like 12v zeners even though they clamp at 4v and the 2N5401 (5v) are more compressed and yield a larger volume drop. Is there some other aspect of these things that affects how they clip?

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 Post subject: Re: Transistor Clamping
PostPosted: Sun, Sep 09, 2018 10:58am 
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I’ve found the same thing. The MPSA06 clips way less than 5v zeners.

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 Post subject: Re: Transistor Clamping
PostPosted: Sun, Sep 09, 2018 2:59pm 
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psychodave wrote:
I’ve found the same thing. The MPSA06 clips way less than 5v zeners.


Some folks on SLO Clone measured theirs and seems like the MPS clip at about 11v. So weird.

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 Post subject: Re: Transistor Clamping
PostPosted: Tue, Nov 13, 2018 7:40am 
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BJT's don't "clamp" on voltage. What you're reading is the maximum safe forward bias voltage they're rated at. They aren't designed to work like a Zener, so there's no real rating on how far they'll go when you push them beyond their rated specs. The PN junction of a BJT is far more similar to a PN diode (rectifier diode) than to a Zener diode, so really what you're doing here is looking at the breakdown voltage of the base-to-emitter junction. The life-expectancy of BJT's used in this manner will probably be pretty short on average. This is really outside their area. The Zener is designed to run high reverse bias voltages all day, every day, with (relatively*) predictable results.

* Zeners are good first-approximation, inexpensive voltage regulators. However, there's a reason you go to 78XX/79XX for more precise voltage regulation.

It looks like you got a bit of coverage on this on the SLO Clone forum when you brought it up. I usually don't read that subforum.


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 Post subject: Re: Transistor Clamping
PostPosted: Tue, Nov 13, 2018 9:43am 
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rstites wrote:
BJT's don't "clamp" on voltage. What you're reading is the maximum safe forward bias voltage they're rated at. They aren't designed to work like a Zener, so there's no real rating on how far they'll go when you push them beyond their rated specs. The PN junction of a BJT is far more similar to a PN diode (rectifier diode) than to a Zener diode, so really what you're doing here is looking at the breakdown voltage of the base-to-emitter junction. The life-expectancy of BJT's used in this manner will probably be pretty short on average. This is really outside their area. The Zener is designed to run high reverse bias voltages all day, every day, with (relatively*) predictable results.

* Zeners are good first-approximation, inexpensive voltage regulators. However, there's a reason you go to 78XX/79XX for more precise voltage regulation.

It looks like you got a bit of coverage on this on the SLO Clone forum when you brought it up. I usually don't read that subforum.


Yeah, been sorted out. Thanks for the info. In any event, MPSA06 wired as clippers are close to 12v zeners and 2N5401 are close to 9v.

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 Post subject: Re: Transistor Clamping
PostPosted: Tue, Nov 13, 2018 12:36pm 
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FourT6and2 wrote:
Yeah, been sorted out. Thanks for the info. In any event, MPSA06 wired as clippers are close to 12v zeners and 2N5401 are close to 9v.


I assume you were trying those because someone (Jose?) in the 80's did those before going to Zeners? It is interesting what's been come up with so many various electronic bits used in an unorthodox manner.

It's interesting to follow the current interest in using diodes/transistors for internal amp clipping, after it was such a maligned practice for so long.


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 Post subject: Re: Transistor Clamping
PostPosted: Tue, Nov 13, 2018 12:36pm 
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Double post.


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 Post subject: Re: Transistor Clamping
PostPosted: Wed, Nov 14, 2018 10:47am 
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rstites wrote:
FourT6and2 wrote:
Yeah, been sorted out. Thanks for the info. In any event, MPSA06 wired as clippers are close to 12v zeners and 2N5401 are close to 9v.


I assume you were trying those because someone (Jose?) in the 80's did those before going to Zeners? It is interesting what's been come up with so many various electronic bits used in an unorthodox manner.

It's interesting to follow the current interest in using diodes/transistors for internal amp clipping, after it was such a maligned practice for so long.


Don't know much about what Jose or anybody else did in the '80s. But today, some amps do use transistors (along with zeners). Friedman, Fortin, Cameron, Gower, etc. So I gave it a try.

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