Insight into "U47-inspired" with EF12 tube?

[soapfoot]

Can we look at an example?

Looking at this U47 PSU schematic, what calculations would I need to do to determine the voltage on the secondary of the power transformer?

 

[Gus]

You need ohms law.  You can figure out what you need from that.  Voltage Current Resistance If you have two you can find the third.

The words I posted before to search for were not a joke, they are things you need to understand how a power supply works.

You should just know things looking at a simple circuit like the PS above 40ma(about the u47 current) 1k 40VDC drop per resistor 1Ks in series.  105VDC 40ma is marked   so you are dropping 160VDC in the supply

Simple fast    1V,1K,1ma   Do you understand the relationship?  A teacher told me >3 decades ago 1K 1V 1ma is a useful thing to know for DC circuits

Do you understand series and parallel what voltage and current do? T and N stuff.

EDIT you are not trying my patience.  Often I will not just give an answer.  I would not be posting if you were trying my patience.

When I look at a tube microphone schematic for the first time I often just use ohms law even if there is no voltages given.  I take a guess at the plate current or look at the voltage divider string and calculate my first guess.  After you work with microphones you can guess capsule voltage or plate current and check it by the calculated grid bias voltage if it is in a workable area of the tube curves.

 

[soapfoot]

Right.  I have done searches and I do know Ohm's Law.  I get the feeling I'm trying your patience, and I'm sorry if that's the case.  I'm doing my best over here.  I did read the first several volumes of the NEETS manuals in their entirety a few years back, but as I said, I have no formal training.

My problem is that I'm not necessarily sure how to apply Ohm's law in this situation.  I do very well with examples and reasoning from that, but I don't do as well with abstractions.  Thanks for all your help so far.

 

[zebra50]

Which power supplies did you build for your existing microphone projects? I'm sure we can pick an example from those?

 

[soapfoot]

The two microphones I built so far were cases of "recipe following."  I built two of the Royer modifications of the MXL 2001, using the 5840 tube.

The PSU for that was a voltage tripler design running off a 24V transformer... would not exactly apply in this case.

 

[zebra50]

Well, it's a fine microphone circuit design - very well respected and many people here, including myself, have started out with that as a successful first tube microphone build.

If I recall right, it will give you around 105V for B+, and 5 to 6V for the heater. With a chunky enough transformer, you might make it work.

 

[soapfoot]

A very good point, and duly noted.

However, for purposes of learning, I think I'd really like to learn to do it the more 'conventional' way.  For me most of the fun of doing it myself is the learning part.  Additionally, while I'm at it, I'm more keen to do it the way that would make for the very best sounding microphone I could possibly make.  I don't want to spend good money on components only to sabotage myself with a power supply that might hold me back.

 

[boogietube]

I learned a lot from studying the Gyraf PSU in this thread, There's also some great math there:
http://www.groupdiy.com/index.php?topic=25597.0

 

[soapfoot]

A couple of questions:

1) How do I determine the approximate amount of current that a tube microphone will use?  Filament current would be 200mA, if I understand correctly.  How do I determine the approximate current draw of the HT supply before having built the microphone?  Or at least make an educated guess?

2) Once I determine the approximate current draw of the HT circuit, how do I, for a given schematic, work backward to determine the necessary PT secondary voltage?  Using the U47 schematic above as an example, would I take the current draw, multiply it by the resistance, and get the voltage drop across that resistor?  Then add that to the total and proceed down the line until I get to the output of the bridge rectifier?

3) I assume, from looking at other viable schematics, that I can tweak the final voltage with a voltage divider (perhaps even variable) at the output?

Would love to know if I'm on the right track here.  I don't have the good fortune to have someone I can ask in person, a tutor, or anything like that.

 

[tommypiper]

Simple fast    1V,1K,1ma   Do you understand the relationship?  

No.  Care to explain or point us to more information?  It's otherwise too oblique to know what you have in mind.  I never had a teacher, so I never came across anything like that.  I have no clue what you're referring to.  ???

A teacher told me >3 decades ago 1K 1V 1ma is a useful thing to know for DC circuits

Sounds useful ?  If I knew what it meant or what it was referring to? 

T and N stuff.

This has got to take the cake for obscure references!  I've been hanging out on this board and built some things and I'm bright and a quick learner, yet never have I heard anyone say, "dude, T and N stuff, you know?" 

A lot of us have no electronics training, OK?  We just do DIY to the best of our ability and try to learn as we go.  We're not dumb, we're just not used to obscure "hints" from people who have been at this for, what did you say, 3 decades, and had a teacher...?  Ten times longer than many of us here!!!!!

I really don't get the whole obscure, hint from a master approach mentality that means nothing without explanation or at least referencing and context or some guidance...  please.  If you insist on only hinting and not explaining or helping in a usual fashion, please guide one to further contextually applicable material that will reveal these hidden gems of wisdom. 

 

[MagnetoSound]

Simple fast    1V,1K,1ma  Do you understand the relationship? 

No.  Care to explain or point us to more information?  It's otherwise too oblique to know what you have in mind.  I never had a teacher, so I never came across anything like that.  I have no clue what you're referring to.  ???

This is Ohm's Law!

One Volt into 1K gives 1mA, simple as that. 

(I have no idea what is meant by T and N stuff either.)

 

[zebra50]

I assumed that was Thevenin & norton equivalent circuits.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/thevenin.html

 

[Gus]

I posted words to search for in a post in this thread.  If you typed them in google you would find things to read.
Tommypiper did you read my other posts in this thread?

IMO one needs to know at least Ohms law to understand simple things like passive power supplies.  Just google bing etc.

The 1V 1K 1ma stuff makes it very simple.  I knew how use Ohms law >30years ago.  The1V 1K ohm 1ma relationship lets you do math fast in your head for simple resistor circuits.  Example follows

Look at the PS schematic 40ma at 105 VDC is marked as what the tube microphone "needs".
Series circuit of 1Ks I "see" 40VDC across each of the the 1Ks.
1ma and a 1K is 1 volt
40ma across 1k = 40VDC, 4 in series 160VDC

Or use a calculator and do the simple math.  .040A X 4000 ohms = 160VDC

Then do the math for DC power to find the power rating needed for the resistors.

 

[0dbfs]

So, with 160V of drop across the 4 1k's...

Add that to the 105V that is left-over/indicated/specified...

Gives us 265V..

The mic is a divider = 2625 ohms (in order to provide 105 at the node).

Start plugging in some number here (and/or elsewhere):
http://www.bowdenshobbycircuits.info/r2.htm

Looks like the 4x 1k's dissipate a total of about 6.4W. P=IE, P=.04A*160V, 6.4W=.04A*160V

I see the PSU schematic specifies 2W. I guess watts add in series because if they didn't, it wouldn't work and they wouldn't have sold many mics.


Now, with 265V at the bridge what does the BV03 need to provide, and what about those 50uf caps?

I see an indication of 250V/75mA at the bridge. Looks like additional info that might be helpful.

Best,
jonathan

 

[gary o]

This is interesting thread.....Im afraid Im just a builder too & struggle greatly to understand stuff like ohms law or more how to apply it....I also find the criptic clue type approach doesnt help me tho I appreciate anyone trying to help me  I no Im a pain.....also the mathmatical way some of the wesites explains stuff also blows my mind.....I used to get ohms laws....about 15 years ago Saturn sounds Ashly Styles told me how to apply ohms law to C28A PSU I was building, I got it ,it worked he had a great way of explaining to me that I understood...

So, with 160V of drop across the 4 1k's...

Add that to the 105V that is left-over/indicated/specified...

Gives us 265V..

The mic is a divider = 2625 ohms (in order to provide 105 at the node).

Start plugging in some number here (and/or elsewhere):
http://www.bowdenshobbycircuits.info/r2.htm

Looks like the 4x 1k's dissipate a total of about 6.4W. P=IE, P=.04A*160V, 6.4W=.04A*160V

I see the PSU schematic specifies 2W. I guess watts add in series because if they didn't, it wouldn't work and they wouldn't have sold many mics.


Now, with 265V at the bridge what does the BV03 need to provide, and what about those 50uf caps?

I see an indication of 250V/75mA at the bridge. Looks like additional info that might be helpful.

Best,
jonathan

Is the 6.4W shared between the 4 1K resistors 1.6W each ?

As said great thread.....

I have a transformer puts out 240VAC, 250 ma I want to run a MK47 mic kit 105VDC 40ma and so gonna try to apply all this to make this off to the shop to get some resistors

 

[0dbfs]

I found this in the PSU META:
http://www.kpsec.freeuk.com/powersup.htm#trsr

And going through that gives me about 190V at the transformer secondary referring to the U47 PSU schematic listed above. Max's mk47 specifies a 200V secondary and is slightly different but mostly the same. So, we are in the ballpark here with the theory and a known good implementation anyway.

240V at the secondary would require more dropping across the CRC filter stage.

Rectified / CRC'd 240V = 340V.

Need to drop it about 235V for 105V @ 40mA.

235 across 5k8 @ 40mA = E=I*R = 232 = 5800R*0.04A

5k8 across four resistors in series is a little less than 1.5k per resistor.

I would start by looking at some of those values and plugging it into max's mk47 PSU which also has an inductor and a tweaker pot/rheostat for fine tuning.

Also need to figure your dissipation in watts for the dropping resistors and overrate them so they don't burn up.

Not sure yet exactly how the inductor fits in WRT resistance so I would need to look at a datasheet for the inductor max specifies and probably look up more equations to understand it a little better. But the inductor generally opposes AC and passes DC based on the mH, and apparently has wattage/resistance ratings that play into it.

Best,
jonathan

 

[soapfoot]

Thank you all so much.

The "missing link" was right under my nose the whole time-- the 40mA listed right on the U47 schematic.  I understand the math now.  As for the question of the bridge rectifier-- that circuit used selenium diodes, correct?  Selenium rectifiers had a higher voltage drop than silicon diodes.  I suppose this voltage drop would depend on current/load, but in any case it would be greater than with silicon diodes, and it would be multiplied by 2 since two diodes in a bridge rectifier are always conducting.

Now comes the hard part, for me.  I have to figure out how to parse how much of the U47's 40mA current draw is due to the inefficient manner in which the filament supply happens.  I don't know how to do this, but maybe I can reason it out.  

Looking at the one limited data sheet for the VF14 I can find, I see a specification called "Heizstrom."  Plugging this into my little translator here, that is German for "filament current."  The specification given is 0,05, which I assume means 0.05A?  But wait... that's 50 mA, and the whole mic supposedly draws 40mA.  And the heater supply in that mic is inefficient and generates a lot of heat across the 1k78 resistor in series with the heater, which drops 69 volts according to the schematic (which would seem to add another 38.76mA, doing the math according to Ohm's law) and we haven't even supplied B+ or capsule polarization yet.

what am I missing?  I really am trying hard, I promise.  

 

[MagnetoSound]

Current across resistors in series is constant at each point (the same through the heater as through the dropper resistor).

Plate voltage drops from 105v to 34v across two resistors, and polarisation is negligible.

 

[soapfoot]

OK so if I understand you right--

Most of the current drawn by the mic is via the heater supply, correct?

Since it's under-heated, perhaps it draws less than the full 50mA... hence the 40mA number.

Do I have that right?

So in your opinion, for my project-- with a regulated 6V supply for the EF12 heater, I can assume a very small load for the 105V supply for the rest of the mic?

 

[MagnetoSound]

Yes, yes and yes.