Equivalent Circuit?

memphissound · 24th February 2010, 11:59 AM

For the tube circuit professionals...

Are these voltage amplifiers essential equivalent? To design the one on the right, what provisions should be allowed for? Thanks.

peace,
memphissound <><

wakibaki · 24th February 2010, 12:16 PM

Yes they are, in an Einsteinian universe. The problem is, memphis, that they're never going to exist in isolation like this, and it depends exactly how you connect up the input and output whether you get equivalent results, and then it'll be results at AC, not DC.

To design the one on the right, one should take a course in electronics design, as the question is beyond the scope of a simple answer on a public forum.

w

memphissound · 24th February 2010, 12:35 PM

I am thinking a VAC amplifier.

I'm very familiar with basic tube audio circuits, a journeyman of tube guitar amp designs. But that type of voltage amplifier (+ and - voltage) is not something that I've ever seen in that application.

But I stumbled across a design for a "tube op-amp" that used something similar with + and - voltages at the anode and cathode instead of using ground below Rk. (BTW, I'm not really interested in the op-amp application.)

But if I can make it work in a MI tube pre-amp, feeding a MOSFET power section, it would make designing the PS that much easier.

peace,
memphissound <><

torrence · 24th February 2010, 03:38 PM

As long as the inputs and outputs are not DC coupled to anything else in the circuit (which should be obvious) I don't see a reason why these two shouldn't be equivalent in any universe.

The other connection to watch out for is the heater, which would be elevated if it were still referenced to ground. Some tubes have a rather stringent limit on the voltage difference allowed between heater and cathode.

memphissound · 24th February 2010, 05:42 PM

Heater to Cathode voltage...
Let's see for a 12AX7 it's listed as 180V -- so that should be fine.

Any special tips for a coupling capacitor? Or would the ubiquitous .022uf - 400V work?

peace,
memphissound <><

kavermei · 24th February 2010, 07:33 PM

The coupling capacitor combined with any resistance in the grid circuit forms a first order highpass filter, so its value will have an impact on the lower bandwidth limit of the stage.

You may want to read the PDF document at this web site, for example:
http://www.freewebs.com/valvewizard1/gainstage.html

Best,
Kenneth

torrence · 25th February 2010, 07:22 PM

Quote:

Originally Posted by memphissound

Any special tips for a coupling capacitor? Or would the ubiquitous .022uf - 400V work?

Indeed as kavermei says, you still have an RC circuit and must pay attention to the critical frequency, but this is no different in the two cases. If you are working from an existing design, the same capacitor will work equally as well (or poorly) in the two circuits.

planet10 · 25th February 2010, 08:42 PM

The juggling of the rails on a circuit like this is used to create circuits that allow DC coupling. Taking care of the grid bias is a part of the art, as well as watching any heater issues wspecially with any dual section tube.

dave

torrence · 25th February 2010, 09:06 PM

Regarding Wakibaki's original point, you do need to be careful to ensure the input isn't DC coupled at all. One easy way to do this is to use a transformer, but if you are using a typical grid resistor, this needs to be connected to the negative voltage rail (not ground) and capacitively coupled to the input. Make sure, however, that you don't inadvertently attach the input ground reference to the negative rail. That would be bad...

wakibaki · 26th February 2010, 12:10 PM

Regarding wakibaki's original point, the inputs require to be DC coupled to somewhere. This is what determines the Operating Point, and the quiescent or DC conditions. The circuits can only be considered functionally identical at AC if they share a common operating point, although this is not the only necessary condition.

This is why I said 'in an Einsteinian universe'. The answer should probably have been 'No, not in an Einsteinian universe', since this is, after all, an Einsteinian universe. Or is it? I mean, probably? That makes it quantum mechanical. No, these circuits are not identical. Except on the days of the week that Schrödinger lets his cat out.

Really? You want me to teach you electronics design by correspondence on a public forum? You must think I'm a glutton for punishment.

w

24th February 2010, 07:33 PM	#6
kavermei diyAudio Member Join Date: Jan 2005 Location: Lokeren, Belgium	The coupling capacitor combined with any resistance in the grid circuit forms a first order highpass filter, so its value will have an impact on the lower bandwidth limit of the stage. You may want to read the PDF document at this web site, for example: http://www.freewebs.com/valvewizard1/gainstage.html Best, Kenneth __________________ Never send a human to do a machine's job. --Agent Smith

25th February 2010, 08:42 PM	#8
planet10 frugal-phile(tm) diyAudio Moderator Join Date: Oct 2001 Location: Victoria, BC, NA, Sol III Blog Entries: 4	The juggling of the rails on a circuit like this is used to create circuits that allow DC coupling. Taking care of the grid bias is a part of the art, as well as watching any heater issues wspecially with any dual section tube. dave __________________ community sites t-linespeakers.org, frugal-horn.com ........ commercial site planet10-HiFi p10-hifi forum here at diyA

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24th February 2010, 12:16 PM	#2
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	Yes they are, in an Einsteinian universe. The problem is, memphis, that they're never going to exist in isolation like this, and it depends exactly how you connect up the input and output whether you get equivalent results, and then it'll be results at AC, not DC. To design the one on the right, one should take a course in electronics design, as the question is beyond the scope of a simple answer on a public forum. w

24th February 2010, 12:35 PM	#3
memphissound diyAudio Member Join Date: Aug 2005	I am thinking a VAC amplifier. I'm very familiar with basic tube audio circuits, a journeyman of tube guitar amp designs. But that type of voltage amplifier (+ and - voltage) is not something that I've ever seen in that application. But I stumbled across a design for a "tube op-amp" that used something similar with + and - voltages at the anode and cathode instead of using ground below Rk. (BTW, I'm not really interested in the op-amp application.) But if I can make it work in a MI tube pre-amp, feeding a MOSFET power section, it would make designing the PS that much easier. peace, memphissound <><

24th February 2010, 03:38 PM	#4
torrence diyAudio Member Join Date: Feb 2008 Location: Oregon	As long as the inputs and outputs are not DC coupled to anything else in the circuit (which should be obvious) I don't see a reason why these two shouldn't be equivalent in any universe. The other connection to watch out for is the heater, which would be elevated if it were still referenced to ground. Some tubes have a rather stringent limit on the voltage difference allowed between heater and cathode.

24th February 2010, 05:42 PM	#5
memphissound diyAudio Member Join Date: Aug 2005	Heater to Cathode voltage... Let's see for a 12AX7 it's listed as 180V -- so that should be fine. Any special tips for a coupling capacitor? Or would the ubiquitous .022uf - 400V work? peace, memphissound <><

25th February 2010, 09:06 PM	#9
torrence diyAudio Member Join Date: Feb 2008 Location: Oregon	Regarding Wakibaki's original point, you do need to be careful to ensure the input isn't DC coupled at all. One easy way to do this is to use a transformer, but if you are using a typical grid resistor, this needs to be connected to the negative voltage rail (not ground) and capacitively coupled to the input. Make sure, however, that you don't inadvertently attach the input ground reference to the negative rail. That would be bad...

26th February 2010, 12:10 PM	#10
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	Regarding wakibaki's original point, the inputs require to be DC coupled to somewhere. This is what determines the Operating Point, and the quiescent or DC conditions. The circuits can only be considered functionally identical at AC if they share a common operating point, although this is not the only necessary condition. This is why I said 'in an Einsteinian universe'. The answer should probably have been 'No, not in an Einsteinian universe', since this is, after all, an Einsteinian universe. Or is it? I mean, probably? That makes it quantum mechanical. No, these circuits are not identical. Except on the days of the week that Schrödinger lets his cat out. Really? You want me to teach you electronics design by correspondence on a public forum? You must think I'm a glutton for punishment. w

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