TLRT

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1320 on: December 01, 2018, 01:34:59 PM »
Measure the total consumption with a multimeter, probably your trafo is overloaded.(tube consumption+bleeder resistors needs more than 300mA)


Hmm, will try the 40 Ohm resistor and a new diode.

Another question: When I ordered the parts for my U67 build, Mouser only had 68M resistors instead of the 60M ones.
I saw in one schematic that Neumann once also used this value...
How critical is it? 

EDIT: The 25 Ohm resistor definately wasn't the right one. It constantly tried to draw 320mA of current although my transformer is only rated for 300mA.
I've put in the 40 Ohm resistor of my second PSU and a new diode while I wait for the new resistors. Now the diode doesn't get that hot anymore but the hum is not really better. Maybe a little bit but it's still there...

Could the power transformer be damaged after being driven into saturation for some time?

I'll also try  to increase the filter capacitors to 47uF and 1000uF.

I'm puzzled why the hum doesn't decrease when I switch in the -10dB pad....


Murdock

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1321 on: December 01, 2018, 07:14:03 PM »
Measure the total consumption with a multimeter, probably your trafo is overloaded.(tube consumption+bleeder resistors needs more than 300mA)


How can I measure total consumption?
And which bleeder resistor do you mean? I think there is only one, in the B+ line. And that's only drawing something like 2mA.
I calculated a heater current of around 200ma.


TLRT

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1322 on: December 02, 2018, 07:55:43 AM »


How can I measure total consumption?
And which bleeder resistor do you mean? I think there is only one, in the B+ line. And that's only drawing something like 2mA.
I calculated a heater current of around 200ma.

You need to put your multimeter in Amper measurement position between pin 3-4(GND+heater) in series. For EF86 with usual 200mA consumption you need a dummy 32ohms/2w resistor,. Still I think you need a higher rating mains trafo, when I tried with 16V secondary winding I had hum also even when I raised the filter caps value.

Murdock

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1323 on: December 02, 2018, 11:57:39 AM »
You need to put your multimeter in Amper measurement position between pin 3-4(GND+heater) in series. For EF86 with usual 200mA consumption you need a dummy 32ohms/2w resistor,. Still I think you need a higher rating mains trafo, when I tried with 16V secondary winding I had hum also even when I raised the filter caps value.


Thanks!
I measured around 250mA.
The hum is not present when I have the dummy loads in place. This is an indicator for a grounding problem with the mic, right?
If it would be ripple or an overloading transformer then I would also hear the hum with the dummy loads?

EDIT: It seems to be my mic cable... If I put the dummy load resistors directly into the Tuchel at the power supply, without the mic cable attached, I get no hum.
But If I put the resistors into the Tuchel at mic cable I get the hum...
« Last Edit: December 02, 2018, 06:55:04 PM by Murdock »

Murdock

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1324 on: December 04, 2018, 12:19:35 PM »
Yep, cable was the problem.
I now have shield, 0V and chassis (pin 3) all connected to the connector shell on both ends. This made the hum almost completly dissapear.

Thanks alot TLRT for your time and help!

Still wondering about the HT shunt resistor... As I have a 10mA secondary I would have to decrease it to 27k to shunt about 8mA to ground which would leave max 2mA to the Anode. But then I had to decrease the RC resistors which in turn would decrease their effectiveness...
How important and safe do you think this "regulator" is?

TLRT

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1325 on: December 04, 2018, 05:20:14 PM »
Yep, cable was the problem.
I now have shield, 0V and chassis (pin 3) all connected to the connector shell on both ends. This made the hum almost completly dissapear.

Thanks alot TLRT for your time and help!

Still wondering about the HT shunt resistor... As I have a 10mA secondary I would have to decrease it to 27k to shunt about 8mA to ground which would leave max 2mA to the Anode. But then I had to decrease the RC resistors which in turn would decrease their effectiveness...
How important and safe do you think this "regulator" is?

You're welcome! I have 0.1A trafo, but it depends on your wall voltage and your tube's consumption. For EU with 235V avarage mains voltage I use 30-33k 2w resistors for Telefunken tubes.

Re: Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
« Reply #1326 on: December 31, 2018, 04:21:06 AM »
I've been trying to download the PSU build pdf, but I only have through page 12, then the rest are blank....anybody have the full file?  Thanks


 

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