simple microphone tube tester

I am building a simple tube tester for checking tubes to use in microphones and piezo amps. Just took a break most is done being built on a piece of plywood and parts I have

I am building close to some "standard" operating points in tube microphones

What is is so far a zener regulated 150V or 75V B+ switchable fixed 100K plate R

two DC heater voltages switchable

Fixed bias voltage I picked about -1.5V DC to start.

I wire up sockets for different tubes with 100K grid to ground(for now need to order some high value Rs). Plan add a grid current section,. switch in and out the 100K across maybe a 1gig

The idea is cheap but hopfully good

set the supply to 150V B+ to start and A heater voltage

let tube warm up for some time measure voltages across the plate R and Plate to cathode

switch the voltage to 75VDC B+ same heater voltage and grid voltage measure again

Knowing the plate is 100K cal the plate current (V drop across the plate R/ 100K)

Rp should be tube voltage plate to cathode / plate current at both 150VDC and 75VDC

rp (dynamic plate R) should be delta tube voltage/delta plate current (this is at a fixed -1.5V a close value for a lot of tube microphones) I might make the bias voltage switchable in the future (delta changes from 150VDC and 75VDC at a fixed bias voltage)

I then want to do this again at underheating voltages.

Why switchable and not pots? maybe a microcontroller measuring setup.

What do you people think? Anything I got wrong with this so far?

Yes more B+ and bias voltages might be good add, but maybe two B+ might be enought. G3 to ground or plate switch etc.

I just did some tests old magnavox 6AU6, G2 and G3 to plate triode mode,5.56VDC heater.

fixed -1.5VDC bias

B+ 149.0VDC
resistor drop 90.2VDC for .902ma (100k)
Plate to cathode voltage 57.1VDC

B+ 74.9VDC
R drop 31.0VDC for .31ma
Plate to cathode voltage 42.2VDC

57.1 - 42.2= delta 14.9VDC
.902ma -.31ma = delta .592ma

14.9V/.592ma = rp of 25.1K

anyone have a uf ef vf 14 that they can test????????????????? I want to check the web stories that low plate R triode mode is part of the VF14 magic in u47s.

hey gus,

thats interesting. i could test a ef12. did you try a circuit with it? its a very nice tube for akg ck3 (http://www.groupdiy.com/index.php?topic=10714&highlight=ef12)

I just tested some more tubes.

RCA 12ax7 rp 275K yuk

12at7? rp 66K

GE 5* 6072a rp 45K

12au7 GE? rp 20K DDG microphone tube?

tested with the same tester as above.

My circuit 150VDC 75VDC B+

100K plate

Fixed -1.5 grid voltage

> RCA 12ax7 rp 275K yuk
> 12at7? rp 66K
> Fixed -1.5 grid voltage

Fixed grid voltage is NOT right when trying triodes of very different Mu.

Be fair. Grid bias is about the easiest thing to change. Your supply voltage may be limited by Safety; your total power by Heat. Your load impedance may be fixed by the transformer you can find, or by the output power you need. But you can change bias on a whim. Set up a reasonable supply and load, fiddle bias for best result, then check for excess grid current.

Try grid bias of about 0.6 * (plate-cathode voltage) / Mu.

For 12AX7 with 150V supply and 100K plate resistor, plate-cathode voltage should probably end up near 50V-100V, say 75V. 75V/100= 0.75V, 0.75V*0.6= 0.45V. Biasing at 1.5V, you are forcing the poor 12AX7 into starvation. It is technically "beyond cut-off", though tube cut-off is soft and you got some current happening. But that current may mostly be electrons taking end-runs around the grid, rather than flowing through and being controlled by the grid.

At "reasonable" current, 0.5-1.0mA, and reasonable voltage, the 12AX7 should be showing Rp in the 60K-100K range, not 275K.

I agree that 12AX7 may not be the queen bee of cap-mikes. It may show high grid current, has a lot of gain, isn't real keen to pass current. You may get stuck between too little output current and too much input current.

Of the super-common types, 12AU7/6C4 is a good bet. In fact 12AU7-type tubes were the standard way to drive small audio transformers, when we tranny-coupled instead of R-C coupled. The 12AT7 12AX7 in tight layouts make great R-C coupled amps, but are strained driving transformer parasitics.

The plate voltage of the 12ax7 at 150 B+ and -1.5 was 124.4VDC.
the at7 was 94.4.

The heaters were 5.56VDC.


PRR good points about the operating points

This is for tube microphones with some tubes number found in them.

I picked -1.5 to begin with because most older and clone microphones have bias voltages of -1.1VDC to -1.8VDC.

Often under 1ma plate current.

I can't find way in writing but my guess the bias voltage is for input headroom. The u47 has -1.1VDC and it is Fixed so it has way more current in the 29 ohm than any change in the tube current.

older tube microphone and clone circuits Power supplies run from 105(u47) 140(schoeps IIRC), 210(u67) and little higher in the sony 800 and the church.

I am tring to understand why the designs were often under 200VDC B+ with a -1.1VDC to 1.8VDC bias. The tubes are in a more curved part of the graphs, and if my reading is correct different grid and element spacings will make more of a difference at under 1ma.

I am tring to understand why the older designers designed them like they did.

Somewhere in one of the mic articles (Stebbings/Debenham/Robinson?) there is a blurb on the input leakage current and noise being lowest at some point around -1.5 volt bias, going up in either direction.

-Dale

I wonder if that has something to do with the EV of the oxide (about 1EV) and othe stuff with the oxides

Three people post. Good posts

I guess people just want to clone or they don't get what I am sharing.

I don't realy need any help with this I was hoping to get a discussion going and was hoping to maybe learn something.

Some of the old designers did some cool stuff with one triode tube

I left hints for K2 mods I guess most people did not get it because I have not seen mods more than an expensive NOS tube.

The junk I read at other sites about all the mods to the china microphones makes me shake my head. 12ax7/5751s,6dj8s crap tubes for microphones IMO.

Maybe I built to many circuits and try to understand why almost every thing you do with a LDC microphone makes a difference

I don't care who sells a Gssl etc on ebay. Thats noise. But I guess people care about that more.

Remember this is the web it is very limited in information transfer in some ways.

I hear what you're saying, Gus..

The thing is that much more people read and try to pick up grains of knowledge - than the ones that actually posts. I for one stay out of this topic because I have no clear idea about how I'd do this - but I sure appreciate you sharing your thoughts!!

Jakob E.

All I did was read about ways to measure tubes.

I then built a tester that is a kind of average tube microphone plate out circuit. I wire up a socket for the tube type and solder it in. The cool thing is the fixed bias at a common grid voltage in tubes. Dale thanks for the ref.

I did this because the way tubes are used in most microphones. They are at the bottom of the curves were the curves are more curved should be more harmonic distortion but the circuits sound good sometimes.

Then you read stuff on the web there is nothing like a vf14. Well how do we know? this is a hard thing to test because how is going to let you use there $1,000 VF14 tube?

The junk I read at other sites about all the mods to the china microphones makes me shake my head. 12ax7/5751s,6dj8s crap tubes for microphones IMO.

Click to expand...

Agreed.
regards,
DB

It may be at the bottom of the curves but the signals are pretty small, and there's some amount of negative feedback applied to pretty much all tube mics. Even a few pF of capacitance makes a big difference. Neumann mics often have 4 to 10 pF capacitors from plate to grid - even the KM84 solid-state mic does this. That oughtta help the linearity out where they run them. You hear a lot of people saying they don't like negative feedback in mics, but I don't think you can get rid of it that easily, actually. There's always some plate-to-grid capacitance.

Part of the advantage of a fixed-bias (like the U47) is that, especially in the U47, you don't need a capacitor in the microphone between the cathode and ground. The low cathode resistance coupled with the filament current makes the bias voltage, rather than a cathode resistor. And in the case of a 'normal' cathode resistor (in the 1k or so region), you would loose a bunch of gain unless you bypass it with an electrolytic. In the 1940's, an electrolytic there might have been problematic. That is a really nice design solution, in my opinion.

[quote author="Gus"]I am building a simple tube tester for checking tubes to use in microphones and piezo amps. Just took a break most is done being built on a piece of plywood and parts I have

I am building close to some "standard" operating points in tube microphones

What is is so far a zener regulated 150V or 75V B+ switchable fixed 100K plate R

two DC heater voltages switchable

Fixed bias voltage I picked about -1.5V DC to start.

I wire up sockets for different tubes with 100K grid to ground(for now need to order some high value Rs). Plan add a grid current section,. switch in and out the 100K across maybe a 1gig

The idea is cheap but hopfully good

set the supply to 150V B+ to start and A heater voltage

let tube warm up for some time measure voltages across the plate R and Plate to cathode

switch the voltage to 75VDC B+ same heater voltage and grid voltage measure again

Knowing the plate is 100K cal the plate current (V drop across the plate R/ 100K)

Rp should be tube voltage plate to cathode / plate current at both 150VDC and 75VDC

rp (dynamic plate R) should be delta tube voltage/delta plate current (this is at a fixed -1.5V a close value for a lot of tube microphones) I might make the bias voltage switchable in the future (delta changes from 150VDC and 75VDC at a fixed bias voltage)

I then want to do this again at underheating voltages.

Why switchable and not pots? maybe a microcontroller measuring setup.

What do you people think? Anything I got wrong with this so far?

Yes more B+ and bias voltages might be good add, but maybe two B+ might be enought. G3 to ground or plate switch etc.[/quote]

Hi Gus,

I think this is a great idea. I am in the middle of doing the same thing for a number
of reasons.

Most tubes are not well characterized at capacitor microphone operating
conditions. Even the AC701 data sheet specifies 50M grid leak resistance.

Parameters change significantly at these conditions vs quoted data sheet
figures. Rp can be 2X the quoted number, and mu and gm can be much
less. I want real measured numbers for these values to design circuits to.

I also want to characterize individual tubes and sort for grid leak, noise, and
microphonic susceptibility.

At the end of the day, I'm building a not-so-simple tester but it will have the
ability to be an actual microphone with faraday shielding and switchable
capsules, tubes, and transformers (coupling caps as well...) I will stick to
the basic common cathode triode circuit which I believe gives plenty of
interesting design space to explore.

I have decided to build curve tracing ability so will have fine control over
Vp and Vg and measurement ability of Ip etc. and am thinking about a
serial interface DAQ (data acquisition) module to control and monitor
using some software on a PC.

I decided on fine control of Rp and Rload and support both fixed bias and
self bias with variable Rk all using 10 turn pots and counting dials.

One interesting thing I want to explore is AC701 substitutes. I want the
ability to both approximate electrical characteristics and evaluate the
sound of various alternatives to this unique and expensive tube. I have
a couple of AC701s as spares for Neumann mics that I can measure.

I also have a lot of 100 EF732s I want to do a statistical study on to
look at tube-to-tube variations.

A practical use is the ability to clip onto the leads of an AC701 and test it
in a real M49 circuit before soldering it into a $7000 mic. For example.

So I think it's a great idea and look forward to hearing more about your
findings. I wil share mine as well.

I totally agree with you on the need to explore and understand also.
Is it why they call this the drawing board?

Michael

you read stuff on the web there is nothing like a vf14. Well how do we know? this is a hard thing to test because how is going to let you use there $1,000 VF14 tube?

Click to expand...

When you get this project up and running, I could probably lend you one of my spare VF14's - after all, it would be a very valuable experiment, getting real-life working stats out of that..

Jakob E.

Yes more B+ and bias voltages might be good add, but maybe two B+ might be enought. G3 to ground or plate switch etc.

Click to expand...

Would not curve tracing, or "micro curve tracing' not be a more 3-d way to see the character of each tube? (and store comparisons)

I've seen LabView used as a tracer with some data acquisiton cards.
Or hotrod a transistor tracer=)

Yes I would quess a curve tracer setup more for "standard" tube microphone operating voltages and currents might be the best way to do this.

I have been meaning to scan the schematic of the simple tester.

The VF14

There are u47 schematics on the web with voltages marked that will give one point or the Rp

Now if one was brave enought to jump the 30K from B+ to the top of the plate R that should shift the plate current a little in a u47. The - grid bias voltage should stay the same.

rp dynamic would be
(2nd(30K jumpered) reading of plate to cathode voltage drop - the idle voltage plate to cathode)
divided by
((2nd (30k jumpered)voltage drop across plate R/100K) -(idle plate voltage drop/100K))

rp for a triode plate output is just the change in tube voltage plate to cathode voltage /change in plate current

http://groupdiy.twin-x.com/displayimage.php?pos=-1657


Simple rp tester

About -1.5VDC grid voltage does not move to much with change in tube current close enought to constant IMO for this simple tester.

Operating points close to some tube microphone circuits.

Make your own supply does not have to be 75VDC and 150VDC

Could make the - grid a - supply

I built it with stuff I had.

It is getting boring around here.

:razz: