Siemens, TAB or Telefunken V74 modifications

[Fok]

I am new to the diy community. I am trying to modify a working pair of vintage tube V74 summing amps (wired for +15dB output). As most of you know vintage German equipments have frequency response 40Hz to 15KHz (typically -3dB beyond both end points) for broadcasting. Please refer to the partial V74 schematic below, I appreciate any members experience and advice on the following questions.

1. I like to extend the high frequency roll off to beyond 15khz, do I simply reduce the capacitance on the variable capacitor C14 on the first tube (6AK5W tube) stage?
2. What components do I need to change to extend below 40Hz?
3. What is the best level to set on the variable resistor (500Kohms) VR33 to get the best sound?

 

[rock soderstrom]

Translated from german TAB74 manual

1. The trimmer Pos.14 is used to adjust frequencies above 10kHz.
("Der Trimmer Pos.14 dient zum Abgleich von Frequenzen über 10kHz.")

minimize or reduce Pos.14 if necessary

2. A capacitor (Pos.3) placed in the center of the secondary windings of the input transformer, causes, together with the inductance of the winding, a desirable drop below 40Hz.
("Ein in der Mitte der Sekundärwicklungen des Eingangsübertrager angebrachter Kondensator (Pos.3) bewirkt zusammen mit der Induktivität der Wicklung einen erwünschten Abfall unter 40Hz.")

Increase (jumper?) capacitor Pos.3

3. The adjustment to distortion factor minimum is done with the potentiometer Pos.33 located in the screen grid circuit of the first tube.
("Der Abgleich auf Klirrfaktorminimum erfolgt mit dem im Schirmgitterkreis der ersten Röhre liegenden Potentiometer Pos.33.")

Technical optimization after tube change. "Best sound" is a subjective matter, adjust Pos.33 according to your own taste.

 

[Fok]

Rock, Thank you for your advice. I will try your suggestion.

 

[majormaestro]

i always wanted to get a vintage module, are vintage wsw siemens or telefunkens easy to repair if they ever go bad?

 

[Fok]

It depends what components go bad. Input transformer for V74 is still available from Haufe. Plenty of selection of brands of capacitors and resistors if require. Not difficult to repair. After replacing a few electrolytic caps, resistors and adjusting to lowest THD to about 0.06% the V74 sound open and dynamic.

 

[majormaestro]

It depends what components go bad. Input transformer for V74 is still available from Haufe. Plenty of selection of brands of capacitors and resistors if require. Not difficult to repair. After replacing a few electrolytic caps, resistors and adjusting to lowest THD to about 0.06% the V74 sound open and dynamic.

thanks for ur reply, i believe the module i purchased recently is siemens pm-uv.. would it be something similar on the inside? i do know it also has haufe transformers. other than that i dont know much about any electronic stuff (not a technical person). i mean in worst case scenario, this one is also easy repairable? im not sure if they all built in similar fashion

 

[Fok]

Don’t know what is a Siemens pm-uv? Please send pictures on the inside.

 

[majormaestro]

here are few pidtures

 

[rock soderstrom]

Your module is basically a universal amp but built in solid state technology. So no tubes, which is not necessarily bad. Here you will find further info: SIEMENS PM-UV MIC Pre Schematics needed.!

Here configured as mic preamp:

http://vintageaudiogearbox.com/goods/preamps/siemens/PMUV.html

 

[Fok]

Rock Soderstrom

I understand the secondary pos. 3 (middle of the secondary winding of the input transformer) should have a .15uf capacitor as a high pass filter to reduce signal below 40 Hz.

1. In my V74, there is NO capacitor in pos. 3, what does that do to the lower frequency response?

2. Should add a capacitor back?

 

[rock soderstrom]

1. As far as I know, without this cap the only limitation in the lower frequencies is the input transformer itself, but this iron will perform pretty good in this respect.

You could do a bandwith measurement of you amp, this will show how deep and high your -3dB points are. I predict a very deep bass response without Pos.3.

2. If you have no problems, I see no reason to reinstall the Pos.3 capacitor. If your measurements show that the transformer is causing excessive mischief in the lowest frequencies, then you can achieve a gentle drop in the lowest frequencies with a larger cap than is noted in the schematic. From a distance I see no real reason, measurements and your ears will tell you what is right.

 

[Fok]

Rock Soderstrom, Thank you for your comments and reply.

1. As far as I know, without this cap the only limitation in the lower frequencies is the input transformer itself, but this iron will perform pretty good in this respect.

You could do a bandwith measurement of you amp, this will show how deep and high your -3dB points are. I predict a very deep bass response without Pos.3.

2. If you have no problems, I see no reason to reinstall the Pos.3 capacitor. If your measurements show that the transformer is causing excessive mischief in the lowest frequencies, then you can achieve a gentle drop in the lowest frequencies with a larger cap than is noted in the schematic. From a distance I see no real reason, measurements and your ears will tell you what is right.

 

[Fok]

Rock Soderstrom, Thank you for your comments and reply. You mentioned adjusting trimmer Pos. 14 variable capacitor can change the RC circuit for low pass above 15kHz. I know just turning the variable capacitor will do it.

I already matched the input signal level of both V74. Do I then simply measure levels frequencies at 15kHz for both V74 to check they have the same frequencies at and above 15kHz so I get good stereo balance of both V74?

Or is there a better way to match ?

 

[rock soderstrom]

Or is there a better way to match ?

No, I think measurements are the right way to do the stereo balance of the frequency response of your two V74s.

 

[Fok]

Rock Soderstrom

Without pos 3 capacitor my listening observations are

1. The bass is less tight and not so define.
2. I tend to hear great channel separation with slightly out of phase perception. Not sure if this is due to pos 3 cap missing or differences in frequency between the two V74.

I haven’t tested the frequency response yet.

 

[sparechessroom]

It depends what components go bad. Input transformer for V74 is still available from Haufe. Plenty of selection of brands of capacitors and resistors if require. Not difficult to repair. After replacing a few electrolytic caps, resistors and adjusting to lowest THD to about 0.06% the V74 sound open and dynamic.

can you give information on where to find the Haufe Input transformer for the V74?

 

[Fok]

Please contact Haufe in Germany see below