Zener and TIP50 regulated B+ with a twist. Will this work?

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Kingston

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making regulated B+ supplies seems quite easy with zeners and TIP50. But what about user selectable B+ using series zeners?

I've been toying with the idea of a preamp with ideal B+ voltages, but also a user selectable "starved plate" setting for crunch and farts.

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Will this work, or is it plain dangerous and just kills parts and why? I just can't see it. If it's dangerous, what could be done to make the idea work?

SW1 is basically a DPST switch or a relay.

Thanks for the help,
Mike
 

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abbey road d enfer

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Do you think the TIP50 will handle 220V Vce? That's what happens when you drop B+ to 150V...

When you turn on, the unreg voltage ramps up very quickly, but the reg voltage has to follow the slower reference voltage (delayed by R3/C5) so during 1 second, TR1 is subjected to 370V Vce.

You need high-voltage transistors.
 

Kingston

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I thought TIP50 is a high voltage transistor. Max rating for Vce is 400V. Let's say I take the input to 350VDC (doesn't have to be 370VDC like in the picture), it should be within safe limits, if only barely. Also, I don't have to drop a whole 150V, it could use a 100V zener instead.

That's still leaves us with the problem of dropping from 350VDC to 170VDC. TIP50 has a max dissipation of 40W.

Let's say my worst case scenario drop with TIP50 is 180V with 0.1A. It's going to dissipate 18 Watts.

Doesn't all this mean that I just need a big heat sink with good thermal coupling?
 

mad.ax

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If you want a more comfortable margin, you may consider the MJE18004G. Vce= 450V. 75W. same package and pinout. 0.38€ at Farnell

Axel
 

Kingston

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MJE18004G is better indeed. Thanks for the tip. I will test it on bench sometime. Any other safety precautions, or tips on efficiency etc. feel free to drop in.

Although it could be argued that dropping 180V while dissipating 20 watts is as inefficient as it gets and that it's past the point of any meaningful optimisations. Then again feeding that to a preamp that is biased to a whole different B+ it'll be completely out of wack, pretty far from efficient and optimised as well.

At least it's for a good reason.

I hope.
 

emrr

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Try it and report back. 

You know me, I'd rather use a variac on B+ AC input, or a power rheostat in the B+ line, old style. 
 

Jarno

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Regulated B+ sounds like a great idea, because you can then eliminate big filter caps and still have very low ripple. But I would use this with small filtercaps so you can build a really compact and high quality power supply.
On the other hand, you can also try the Supertex high voltage regulators, for very low parts count / high compactness.
 

Kingston

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I almost got excited about that supertex chip, but it's quite useless for tube applications due to very low maximum current. Only 10mA. Last relatively simple project I built needed 20mA per channel. A new one I designed needs 22mA for the output stage alone...

[edit]

here are their linear regulators http://www.supertex.com/pdf/misc/inductorless__ics_SG.pdf

not really catering for tube amps.
 

mad.ax

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According to: http://www.supertex.com/pdf/app_notes/AN-H40.pdf

those Supertex "operate like any other 3-terminal adjustable linear regulator."

So, maybe one can use an external transistor to boost the max current, like it's done with other 3 terminal linear reg...?

Axel
 

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