U73 calibration and external metering
- Thread starter B-A-R
- Start date
Help Support GroupDIY Audio Forum:
Mark Burnley
Well-known member
Hi b,
I've had a U73 on the bench and wired it as:
Input:
1a = +Ve
1b = Ve
Output:
3a = +Ve
3b = -Ve
...all fine on scope testing. The usual convention is that the top of the winding of a transformer in a schematic is of +Ve polarity w.r.t the secondary. Obviously, depending on the schematic and equipment, this doesn't always hold true. But it appears correct with the U73 when you trace this pinout to the transformers, plus, as you've deduced from the continuity test, keeps correct signal polarity in bypass.
The best V76 schematic including pinout is in the Braunbuch link recently posted here:
http://www.irt.de/IRT/publikationen/braunbuch/V76.PDF
...last page of PDF.
:thumb:
Mark
I've had a U73 on the bench and wired it as:
Input:
1a = +Ve
1b = Ve
Output:
3a = +Ve
3b = -Ve
...all fine on scope testing. The usual convention is that the top of the winding of a transformer in a schematic is of +Ve polarity w.r.t the secondary. Obviously, depending on the schematic and equipment, this doesn't always hold true. But it appears correct with the U73 when you trace this pinout to the transformers, plus, as you've deduced from the continuity test, keeps correct signal polarity in bypass.
The best V76 schematic including pinout is in the Braunbuch link recently posted here:
http://www.irt.de/IRT/publikationen/braunbuch/V76.PDF
...last page of PDF.
:thumb:
Mark
Mark Burnley
Well-known member
Hi,
I racked one up recently and gave it a run through. Sounded great! Fantastic on bass guitar. Like you, I'm going to bring some of the controls out to a panel on the side to make it more useful. I wired in a meter too. Running off a 240>220V transformer at the moment. I'm going to mod the PSU to allow it to run off 240V directly. You've got to be careful with some of the 220V U and V modules that you don't overvoltage them- a lot of people think you can just plug them directly into the 240 or 250V UK mains supply! With the U73 the bias seems very dependent on operating supply.
Now if only there was a second one and a Varicom!
:green:
Mark
I racked one up recently and gave it a run through. Sounded great! Fantastic on bass guitar. Like you, I'm going to bring some of the controls out to a panel on the side to make it more useful. I wired in a meter too. Running off a 240>220V transformer at the moment. I'm going to mod the PSU to allow it to run off 240V directly. You've got to be careful with some of the 220V U and V modules that you don't overvoltage them- a lot of people think you can just plug them directly into the 240 or 250V UK mains supply! With the U73 the bias seems very dependent on operating supply.
Now if only there was a second one and a Varicom!
:green:
Mark
Hey Mark...what did you use for external metering on the U73? I have a few ernest turner 701 meters @ 500mv....would i need to make a circuit to use these meter? What pins did you use? I'd like to re-tube my U73...any tips on calibration or Biasing? ...any info appreciated!
B
B
Michael Tibes
Well-known member
The U73 calibration is described in the manual which is most likely on kubis site http://audio.kubarth.com/. This might be in german only though, if you don't get along I can try to help you with a translation. It's not terribly difficult if I remember right.
Michael
Michael
Thanks for the reply Michael,
Yup, i have that PDF, but dont speak or read German, so..... ;-)
Also, does anyone here know If its possible to make these Ernest Turner 701's read correctly for external metering on my U73? They are 500ma, and do show small movements when the unit goes into GR...i realize most GR's are 100ma? Is that correct? Any way to drive these little guys for GR work??
Yup, i have that PDF, but dont speak or read German, so..... ;-)
Also, does anyone here know If its possible to make these Ernest Turner 701's read correctly for external metering on my U73? They are 500ma, and do show small movements when the unit goes into GR...i realize most GR's are 100ma? Is that correct? Any way to drive these little guys for GR work??
Michael Tibes
Well-known member
Just quickly what I remember (take this as unverified hints, I have to check things with the manual later):
the meter shows basically the input level, so it doesn't show gain reduction directly. The meters had a marked red area which indicated the gain reduction area (I only know this from pictures, never had any myself).
The most important calibration is the symmetry. You need something like a pulsed 10kHz sine, loud enough to push the unit into limiting. Then you simply have to adjust for minimum lowend noise (which is caused by control voltage feedthrough due to misaligned symmetry).
Michael
the meter shows basically the input level, so it doesn't show gain reduction directly. The meters had a marked red area which indicated the gain reduction area (I only know this from pictures, never had any myself).
The most important calibration is the symmetry. You need something like a pulsed 10kHz sine, loud enough to push the unit into limiting. Then you simply have to adjust for minimum lowend noise (which is caused by control voltage feedthrough due to misaligned symmetry).
Michael
Michael Tibes
Well-known member
Ok, a quick translation of the alignment procedure:
New tubes should have a burn-in period of 48 hours.
The test signal for the symmetry test should be a 50 ms long 15 kHz (or slightly higher) signal with pauses of 1 to 2 secs and a level of +22dB (wondering what scale that was, I guess +22dBu). The test signal shouldn't click, so start and end with 'zero crossing'. Adjust for minimum lowend thumb with trimpot 61.
Further adjustments after the symmetry is adjusted:
-Set mode to Kompr. (compressor), input signal 1kHz, -20 dB(u?), adjust output with trimpot 62 to -10 dB(u?). I believe we'd call that makeup nowadays.
-With an input signal of 1kHz, +6db(u?) rising to +16 dB(u?) adjust trimpot 64 so that the output signal doesn't rise higher than +8dB(u?).
I believe that should be the threshold.
-Set mode to Begr. (limiter). Input signal 1kHz, -10 dB(u?), adjust output level to -10 dB(u?) with trimpot 67.
-Set trimpot 63 to center position, input signal 1kHz +16 dB(u?), adjust trimpot 79 for an output level of 7dB(u?).
Now adjust trimpot 63 for an output of +6 dB(u?) (+0,5/-1,0 dB) with an input signal of +6 dB(u?). Check that the output doesn't rise over +6 dB(u?) (-0,5/+1,5 dB) if you raise the input signal.
Repeat this procedure with trimpot 63 and 79 until it's right, because both influence each other.
That should be it.
The alignment following the symmetry adjustment are level adjustments for the broadcast stations, the unit was planned as a final limiter before the transmitter (therefore the fixed threshold and time constants). I believe that you can just use it without these adjustments and rock on if you don't wanna squeeze the maximum out of your radio station with it ;-)
I hope this helps (and that I didn't make mistakes in the translation).
Michael
New tubes should have a burn-in period of 48 hours.
The test signal for the symmetry test should be a 50 ms long 15 kHz (or slightly higher) signal with pauses of 1 to 2 secs and a level of +22dB (wondering what scale that was, I guess +22dBu). The test signal shouldn't click, so start and end with 'zero crossing'. Adjust for minimum lowend thumb with trimpot 61.
Further adjustments after the symmetry is adjusted:
-Set mode to Kompr. (compressor), input signal 1kHz, -20 dB(u?), adjust output with trimpot 62 to -10 dB(u?). I believe we'd call that makeup nowadays.
-With an input signal of 1kHz, +6db(u?) rising to +16 dB(u?) adjust trimpot 64 so that the output signal doesn't rise higher than +8dB(u?).
I believe that should be the threshold.
-Set mode to Begr. (limiter). Input signal 1kHz, -10 dB(u?), adjust output level to -10 dB(u?) with trimpot 67.
-Set trimpot 63 to center position, input signal 1kHz +16 dB(u?), adjust trimpot 79 for an output level of 7dB(u?).
Now adjust trimpot 63 for an output of +6 dB(u?) (+0,5/-1,0 dB) with an input signal of +6 dB(u?). Check that the output doesn't rise over +6 dB(u?) (-0,5/+1,5 dB) if you raise the input signal.
Repeat this procedure with trimpot 63 and 79 until it's right, because both influence each other.
That should be it.
The alignment following the symmetry adjustment are level adjustments for the broadcast stations, the unit was planned as a final limiter before the transmitter (therefore the fixed threshold and time constants). I believe that you can just use it without these adjustments and rock on if you don't wanna squeeze the maximum out of your radio station with it ;-)
I hope this helps (and that I didn't make mistakes in the translation).
Michael
Anyone still messing with the u73? I've completed my Drip Electronics version and followed Greg's procedure for calibrating but when I switch to limit from compression I get a huge reduction in output gain and no visible gain reduction on the meter. Anyone have any ideas or experience with this? Thanks!
