Neumann M49 Clone : D-M49c and D-M49b Tube Microphone Build Thread. (+Sample)

[gary o]

Hi Johnathan not sure how to measure/ calculate what your asking .... sadly I lack lots of basic electronics knowledge but would like to learn ..... I can measure B+ plate voltage  & then need to calculate is that right ......

Edit ..... Ok I now have B+ 115.7 plate 53.1V  I think polar voltage is 53.3V if measuring from correct node

thanks

 

[0dbfs]

Easy thing about having a 100k in that position is that you can do the math in your head.

B+ = 115V
Plate = 53V

Drop across 100k = 115-53 = 62V Voltage drop across 100k and that means the tube is running at .62ma

We would want about 45V on the plate to get to the specified 0.7ma

Some tubes will differ and heater voltage can also effect this so worth experimenting with the tweaker positions and such to see how your tube is working and adjusting accordingly.

Are you using a 5840?

Thx,
jb

 

[poctop]

Hi Johnathan not sure how to measure/ calculate what your asking .... sadly I lack lots of basic electronics knowledge but would like to learn ..... I can measure B+ plate voltage  & then need to calculate is that right ......

Edit ..... Ok I now have B+ 115.7 plate 53.1V  I think polar voltage is 53.3V if measuring from correct node

thanks

Hi Gary,

Normally the polarization voltage in cardiod should be exactely half the B+ voltage minus a hair for current limiting resistor inside the psu , so 53.5V would be on the low side should be more like , 58V,  also the heater operating voltage will affect those conditions a little where the plate voltage may vary a tad,
did you built the PSU with the pcb i supply or it is home brew,  Wich capsule did you ended up doing ,  if you have a chance you might want to post a quick Krooner sample of yours  , 

did you make the c or the b version of the circuit, 

LEt me know,

Good Job,

Best ,
Dany,


Let me ko

 

[gary o]

Hi Jb Dany & anyone thanks for help ....how do I change plate voltage...... change plate resitor value ?...... Yes using 5840 tube Jb thanks

My PSU is a homebrew I made years ago before internet day made it for a C28A really didnt know what I was doing then (still dont now) Ashley Styles Sarurn sound help me out over the phone its based on AKG C28A PSU really work great with C28, 120V B+ adjustable LT adjusted to 5.7V for my 5840 ....Capsule Im testing with is Tim Cambel & Violet Vin67, the circuit im messing with is the Oliver A TLM149 conversion wich I think is most like the M49C

Yes I can do a quick youtube trashy croon with the mic ....

thanks for help fellas very interested to learn 

 

[gary o]

So I need a 70V drop across plate resistor .........doI raise my B+ to 123V to achieve the .7ma ....

 

[poctop]

So I need a 70V drop across plate resistor .........doI raise my B+ to 123V to achieve the .7ma ....

Try this , put B+ to 120V and then adjust the plate voltage to 50 this will give you 0.7ma plate current ,  this is the most important aspect of the circuit.
the bias is the results of this operation and will be dependant of the heater voltage used and tube type and also the final plate voltage set for this circuit ,
the bias with the 5840 tube might not be exactely 1.6V but should be pretty much nearby this value the plate current has predominance over the bias,
what is actually calibrated is the plate current not the bias try it out and let me know what is the final value of your bias , and make sure you do have 58-59V on  the R6-R7 node.  and then try the mic out and let me know, with only one trimmer the bias and plate current will be a compromise over the best results 

if you want to vary the range of both condtition you may want to tweak the plate resistor value    this will get you exactely where you want to be on the tube curve

Best,
DAn,

 

[0dbfs]

What Dan says and the CT12 doesn't want to see more than 60V.

Which transformer are you using? Sorry if I missed that

I did two of Oliver's 5840 circuits with ct12's and those are great. I'll try to measure my voltages and post but I think I had about 43V or so on the plate with 120V b+. Slightly different on both mic's but close.

Cheers,
-jb

 

[poctop]

if you want to vary the range of both condtition you may want to tweak the plate resistor value    this will get you exactely where you want to be on the tube curve 

Best,
DAn,

 

[poctop]

Based on a B+ at 120V ,    Exactely 47V at the plate would give the calibration plate current 0.73ma, 
Hope this helps,
D

 

[gary o]

Thanks Dan I have been tweeking I now have 130K plate resistor B+ is now 122V plate is now 52V I have -1.6V cathode & cathode resistor is 2K8 instead of 2K2 I have 56.5V at R5 6 7 node for capsule ?......

Soooo 122V B= 52V plate = 70V drop = .7ma is that right ...... edit Im just thinking Noooo cant be right cant it coz my plate res is 130K.......my brain hurts it late well early sleeeeeeep now

jb Im using T14/1 from a AKG C28

 

[gary o]

Cant sleep.... trying to do maths so 70V drop divided by 130k is .538ma Im further away now ......can I put DMM in series with plate & plate resistor ......

 

[poctop]

Cant sleep.... trying to do maths so 70V drop divided by 130k is .538ma Im further away now ......can I put DMM in series with plate & plate resistor ......

to measure the plate current the easiest way is to measure the voltage drop acrosss the plate resistor and divide it by the plate resistor ,
U/R = I

Try a lower Value of plate resistor like 80K and it will probably get you statifiy both condition at once while trimming for the bias value, 

Let us know what you end up with

Best,
Dan,

 

[gary o]

Just tried 91K plate res volts across 59.6/91000R = 0.00062A is that right 62ma Im still down my B+ went down to 114.3V Plate was 54.V ....._1.6V cathode

I think I need more power captain !! PSU Im using was designed for a AKG C28A  doesnt wanna give me 120V when I use 100K plate res with this mic, will see if i can get more power.....

 

[0dbfs]

Adjusting the cathode resistor in a self bias should set the current through the tube. Try adjusting that one and see how it affects current.

Cheers,
j

 

[gary o]

I have canged some resistors in my homebrew PSU .... I now have more power .....

Adjusting the cathode resistor in a self bias should set the current through the tube. Try adjusting that one and see how it affects current.

Cheers,
j

Sorry J I didnt answer this earlier ..... here are some results

with 100K plate res B+ measured 124V plate 56.8v drop across plate res 67.3V cathode was at -1.653V capsule 58.1V
I adjusted cathode pot to -1.6V & that changed the B+ 123.2V plate 55.1V drop across plate res 67.8V capsule 57.5V    sooooo 67.8 ma not far off .....

I tried 91K plate res next cathode was at -1.65V  B+ went to 121V plate 56.9V drop across plate 64V ( 70.3MA ?) capsule 56.6V
I adjusted cathode to 1.6V B+ went to 120.2V plate 55.6V drop across plate res 64.5V ( 70.8MA ?) capsule 55.5V

 

[0dbfs]

That looks pretty close with the 91k & -1.6V cathode.

The DMM readings at the backplate polarization divider may be off due to higher impedance so best to calculate the resistor values With 120V b+ the polarization should be 60V if you have matched the resistors pretty close in that divider section.

Cheers!
-j

 

[poctop]

I tried 91K plate res next cathode was at -1.65V  B+ went to 121V plate 56.9V drop across plate 64V ( 70.3MA ?) capsule 56.6V
I adjusted cathode to 1.6V B+ went to 120.2V plate 55.6V drop across plate res 64.5V ( 70.8MA ?) capsule 55.5V

Yes Go For it  ,

Best,
dan,

 

[gary o]

So with 91K plate resistor I will tweak cathode pot to get 63.7V/91000R = .00070A ..... BUT the cathode voltage wont be bang on _1.6V BUT close & plate current more important right

 

[poctop]

So with 91K plate resistor I will tweak cathode pot to get 63.7V/91000R = .00070A ..... BUT the cathode voltage wont be bang on _1.6V BUT close & plate current more important right

Correct , Since we are not using the exact same tube the resulting plate current adjustment at 0.73ma will results in a slighlty different bias condition it should be close but will not be necessarly be 1.6V precisely,  since we are using a fixed plate resistor change in cathode resistor ( trimmer)  will have an effect on the final bias value,  all you have to do after this is try the mic out and let me know. 

After you inital test with the mic :
a nice addon experiment would be to replace the plate resistor with a 250K pot and tweak the value of the bias trimmer from 1.6V to 1.9V and then variying the plate resistor trimmer to get about 0.8-0.9ma while readjusting the B+ at the end at 120V and tweak back untill both conditions are met. and see how the mic react in this zone, if you have time to perform this i would be really curious to see what you can end up withas final plate resistor Value and cathode resistor value , I was to make the experiment myself but the studio wont let me take my mic back  ,  as soon as i have another body to put the my other 49c in i will keep you posted as well.

Let me know,
Best,

 

[Winetree]

M49 Power Supply Round transformer hook-up to main P.S.B.
Yellow & Red
Black & Orange
To P.S.B.