Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)

[Phrazemaster]

The folks at TAB are the best!!!!

 

[rmburrow]

His boards are for using a 5840 sub-mini tube. The 5840s use a 6.2V heater.

You'll blow your AC701 very quickly at anything over 4V on the heater.

The good news is that's easy to adjust the PSU. The bad news (for me) was I had a heck of a time setting the other voltages - not sure why. Dany told me I had a gremlin in the mic after he helped me troubleshoot, so I rebuilt it with a new board and got it working. You'll have to figure out how to wire the AC701 because Dany's instructions are for the 5840.

Do NOT wire up the AC701 and turn on the mic without  having first gotten your PSU heater voltage calibrated with a dummy load to 3.8 ~ 3.9V. If memory serves the dummy load was 40K but look through this thread and Dany already answered this one.

Good luck!

Mike

For the AC701 dummy load, use 40 ohms.  Heater is 4 v at 100 ma DC;  ohms law 4 v/0.1 A = 40 ohms.  A 2 watt resistor should be ample; the dissipation is 0.4 watt.  The attached should help.

 

[micaddict]

Do not overheat.

 

[e.oelberg]

someone is selling 65mm diameter  plexiglas  boards.  They are cheap and usefull for this build
http://www.ebay.de/itm/Acrylglas-Rund-4mm-10-100mm-Scheibe-Kreis-Zuschnitt-Plexiglas/192015813481?_trksid=p2481888.c100651.m4497&_trkparms=aid%3D777001%26algo%3DDISCO.FEED%26ao%3D1%26asc%3D20160801204525%26meid%3D688e7c908f324c31b9a8afe66631ae04%26pid%3D100651%26rk%3D1%26rkt%3D1%26&_trkparms=pageci%253Ad65888cb-5f53-11e7-b6df-74dbd180e37f%257Cparentrq%253A048cf93a15d0aa1a2c5022effffbe191%257Ciid%253A1

 

[adrian]

hi!
I'm a little bit confused... just need to be sure what is the good point to measure bias for the M49b?
between R6 R7      /    HG
or between R6 R7        /        H+ 
  ????

thanks!

 

[rmburrow]

I am looking at a schematic of the Neumann M49 using the AC701 and fixed bias (grid bias obtained via resistance divider, taken from DC side of heater and cathode connection).  The heater voltage of the AC701 is 4 volts, the cathode is at 4 volts.  There are 200 k and 300 k resistors in series, and a 150 M resistor connected at the junction of these two resistors.  The other end of the 150 M resistor goes to the AC701 grid.  The fixed bias is -2.6 volts in this case (2/3 of -4 volts).  Do NOT attempt to measure bias at the high impedance tube grid.  Virtually NO current flows in the 150 M resistor, so the DC voltage on either side essentially is the same!  The clone mic probably contains a divider similar to this to bias the 5840 or similar tube.  Measure bias on the supply side of the 150 meg resistor... the bias will be the difference between the measurement at the resistor junction and the tube cathode.  The voltage should be negative with the plus end of the volt meter at the resistor junction, and the negative end of the voltmeter at the tube cathode.

 

[adrian]

OK Thank you!
so if I understand correctly I can check it between
the jonction of R6-R7 and R5  and  K+ (the same as H+).

I use RCA tube.
I have 1,6v between (R6 R7 R5)  and K+ : that's OK!
I replaced R10 with a pot and to obtain 45v at Anode and I need 270k !!! wich is very much regarding the BOM value (100k)

If I understand correctly the calcul for the tube running :
118v (my PSU output) - 45v (at A) = 73v
but I= U/R  so 73/270 = 0,27mA    very far from the 0,73mA  ?????? I know I must be wrong somewhere!!!

here is a quote I used to try to understand what I am doing...

it will really be only if you put a AC701K in there that you will obtain those precise results,
this is why the self bias is a no brainer but the b is much more fun and learning, 
I have 100K plate resistor, 120 - 68 = 52V so this mean 52V drop trough 100K means 0.52ma,  U= RI
52V/100K = i in ma

here is an example,
B+ = 115V
Plate = 53V
Drop across 100k (plate resistor)
= 115-53 = 62V Voltage drop across 100k
that means the tube running at .62ma
We want about 45V on the plate to get to
the specified 0.7ma then
 
the best at this point is to replace temporarly the plate resistor with a pot lets say  250K and try to tweak both bias pot and plate resistor pot until you find the nearest condition , no one uses same tube and this is a related together and the 2 parameter are related, this is the best approach once you find the sweet spot for it you can replace the plate resistor whit with a 1/4 watt resistor ,
try that and keep me posted and let me know what the balance value you find or the nearest , what tube did you used, and make sure you try the mic first.

Let me know,
Best,
dAN,

 

[Colorblind]

Hi All,  I tought I would share this with everyone , this is a Raw Picture content pictorial of the Prototype I have built for Andriejus.
While Andriejus is Polishing the Last details ,

Tip #1 always keep the lead of the HZ bridging component so you can use them to make the bridging so no need of fancy jumper and so much easier , this is build needs to be built from the top and down , except for the headbasket and capsule mount, well this is how I do it 

I am hoping you will appreciate it , this also applies to the General Build of this one ,

So here it is
https://cdn.groupbuilder.com/groupdiy/u/39511/597b45d76f1cd.zip


Best,
Dan,

Is this pictorial the same as what is posted on page 1 of this thread, or something different?  The link doesn't work for me.

Thanks,

C

 

[rmburrow]

Getting "file not found" error message...

 

[rmburrow]

OK Thank you!
so if I understand correctly I can check it between
the jonction of R6-R7 and R5  and  K+ (the same as H+).

I use RCA tube.
I have 1,6v between (R6 R7 R5)  and K+ : that's OK!
I replaced R10 with a pot and to obtain 45v at Anode and I need 270k !!! wich is very much regarding the BOM value (100k)

If I understand correctly the calcul for the tube running :
118v (my PSU output) - 45v (at A) = 73v
but I= U/R  so 73/270 = 0,27mA    very far from the 0,73mA  ?????? I know I must be wrong somewhere!!!

here is a quote I used to try to understand what I am doing...

Watch the DC plate current and voltage, drop the grid bias some to make the plate draw the desired plate current... for class A amplifier with constant plate supply voltage and plate resistor load, less grid bias voltage, tube draws more plate current and voltage at plate decreases....higher grid bias voltage, plate current goes down and voltage at plate increases.  I don't have the curves for the 5840 tube in triode connection in front of me, but my recollection is the grid voltage should run around -1 volt (measured at the resistor junction), and the plate voltage (measured at the plate) should fall in the 40 volt range, with  100k plate resistor to 120 v HT.

 

[adrian]

ok!
thank you a lot for this explanation, I think I better understand how it works!
so the bias is to adjust the current, I don't really need to have at 1,6v exactly, It's the adjusting parameter.

I go back to R10 at the initial 100k value.
I measured voltage at A and adjusted it to have 45V (120-45/100= 0,75mA).
so I'am in the good value regarding the curve .
I checked and I have -1,12 V between R6-R7 and K+

I built a second mic and to obtain 0,75mA I have -2V between R6-R7 and K+.

is this difference between these 2 mics OK? what is the correct margin of tolerance ?

 

[rmburrow]

Mic 1:  Characteristics appear good.  Try recording anything with it?  Background noise level acceptable (low)?

Mic 2:  New tube?  Try a 48 hour burn in and recheck.  Try another tube.    Sylvania has curves for the 5840 in triode connection.  -2 v on the grid seems high. 

 

[poctop]

Is this pictorial the same as what is posted on page 1 of this thread, or something different?  The link doesn't work for me.

Thanks,

C

hope this one will work ,
Best,
Dan,
https://www.dropbox.com/s/w9jqatxdi9p45wy/M49%20Build%20Pictorial.zip?dl=0

 

[Colorblind]

Thanks Dan!

 

[TillM]

I've a question about the PSU wiring.
I use the 186B20.
For 230v do i have to use pin 1 and 4 and solder pin 2 and 3 together to get 230v ?

 

[Jim50hertz]

If one is using 6s6b triode instead of the 5840 is this transformer able to cope with the extra current draw?
http://www.mouser.com/ProductDetail/Hammond-Manufacturing/186B20/?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGyR%2fQB%252b1BzrOY%3d

 

[achase4u]

Hello all.  I am grateful for this forum.  I need some direction.

I have built up my Poctop D49C to almost finished.  I am good at understanding thorough instruction type builds (Hairball 1176, CAPI BT50 etc) but less so with things like this that assume you have a little more knowledge to fill in the blanks.  I can read a basic schematic, but I am a little lost on my power supply currently as it relates to the choke/mains etc.  On the hardware side, I can build and that is all coming together quite well. 

Are there any instructional videos on basics I could watch, or articles/books that would help me in this area of power transformers?

I do realize the PSU can be deadly.

I am having trouble identifying which transformers are what on the PSU schematic on page one of this thread.  I also can't seem to identify a part of the PSU schematic that shows the IEC connection and switch connection etc.

If anyone feels like throwin' me some help, that would be cool.  I'd like to learn about this and do future projects.

I downloaded the toroidal data sheet so I see where the secondary goes to the PSU board, but I don't know where to wire the primary.  That's a start, I suppose.

 

[rmburrow]

Look at the manufacturer sheet on the power transformer.  Assuming the power transformer is single primary voltage.  Usually, the primary leads are black.  Multi voltage primaries may involve paralleling windings (for US 120 volt) or series windings (for European 220-240 volt) use.  High voltage winding (HT AC to rectifier) leads are usually red, with center tap (if present) red/white.  Heater winding leads usually green; center tap (if present) green/white.  You may be able to verify this with a suitable ohm meter before connecting power.

You ARE right, the HT off the transformer OR off the PSU output (due to capacitors) can be deadly.  Work safe.  If you are testing with power, connect the meter first, then turn on the power.

 

[achase4u]

That helped me understand.  I am missing some basic knowledge here and what I has assumed incorrectly was that one specific side of the primary had to be connected to neutral and the other to hot.  This transformer uses parallel windings and I can easily see on the data sheet which connect to one another for 120v.  So in this configuration, there is no phase reversal possibility from the primary to the secondary?

I indeed will work safe.  Nothing worth dying over here.

 

[rmburrow]

If the power transformer is split primary, follow the data sheet for paralleling the primary sections correctly.  This is important.  Hard to second guess the manufacturer on this one and get the phasing correct.