DIY Telefunken V76 Build attempt

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CJ

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you might have open loop gain (zero feedback) when your switch is set on 9, so make sure you have the 40 K after the 2 uf cap that comes off the rotary sw.

i have V1 and V2 mounted on rubber grommets for shock absorbers to cut down on microphonics,

also running 250 volts DC for B+, headroom is not a problem in the first stages as input signal is only around 100 milli volts from mic, in fact, lower plate voltage on Vi will be just a bit more quiet,

you can run anywhere from 47K to 220K for V2 resistor if you are not using a choke here,

here is pic of my V76, could have made it smaller but it is still very quiet, using DC on heaters and non lytic for 4 uf output cap, using original V72 choke for V2 and a DIY choke for V4 which is mounted on the other side of the chassis along with the output cap,>

 

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CJ

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here is the schematic i used,

what were the specific errors that you found?

 

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My3gger

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We thought there is an error on one schematic where it looks like R72 (one of V3 cathode resistors) first joins circuit ground and then also HT ground. It's on the same schematic you and i posted, those two connection can't be seen clear. In V76 manual it's clear it joins both points, that also makes circuit's ground. Landis had other grounding problems which were resolved (post #24), he probably used R72 as a ground connection to psu.
So it looks like there are no differences between schematics. Pdf manual has voltages and looks cleanest.
You said 47k to 220k can be used instead of choke for V2, do you know what would be starting point for V4 plate resistor? IIRC you were one of the guys who tested V72 with resistor instead of chokes. If this is truth, what's your opinion about sound with choke vs resistor? With resistor gain is of course lower, do you maybe know how much (more or less) it would be in V72? I'm asking because it maybe gets so low it makes no sense to build V76 with resistors. When you take in account added cost for 2 tubes and complexity, one could get Sowter choke and build V72 with higher gain mod.
CJ, your posts are very valuable and respected, thanks a lot!
 

CJ

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ahh, i got it, this is a single pwr supply, so the -350 is really ground, just put a dot where that resistor touches the ground buss and you are fine.

a resistor will work but what sets these things apart from other amps are the chokes, they might sound the same with a resistor but i think there might be other things going on like phase shift and distortion which adds subtle changes to the sound,

V76 is probably the best preamp you can build, that gain control is so much better than anything out there, if you stare at it for a couple of hours you can see that as the gain goes up and down, the 50uf/25 volt cathode cap on V1 is gradually changed from being in the circuit and out of the circuit,

ever notice how you get more treble rolloff as gain is changed on most preamps?  not here, the amp runs flat no matter what the gain setting is, this is because of the gain switch.

it took me about a year to build mine, so i did a lot of tweaking on the circuit and was not able to improve on it, there are many feedback loops going on, AC and DC feedback, look at the screens and cathodes on V3 and V4, look at the cathode cap on V3, those points have been tweaked with voltage dividers to inject the right amount of feedback into the circuit,

so if you add in all the variables, chokes and low noise tubes and feedback and a cool gain switch, you have a standard to which you can compare all the other preamps you build to, and it handles all the mics equally well, from SM 57 to phantom powered condensers, and all points in between, your one stop mic pre for getting the right sound no matter what the application,

you can use an 11 pos rotary sw if you want more gain or less gain at both ends, just scale the resistors up or down depending on which end you want to add gain points to,

if you run a condenser mic into the V76 and run open loop gain by jamming a break before make sw in between settings, and stick the mic outside with a 30 foot cord to prevent feedback, you can hear ants crawling on the moon, thanks for the props, i like to see people build these as it is quite the cool project as you learn just about everything you cn do to a tube pre and you end up with a cool amp in the process,  :D

 

Landins

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Great action goin on here. Been offline up in the mountains. The fault i found was concerning no negative psu connection in circuit. As 3gger pointed i connected via r72. I also found that 50nf cap between screen and g3 shall be connected to cathode. The gwagner repro does not show this. This messed up the feedback between v3 and v4. Chokes will be great and i cannot thank cj enough for winding a couple for me. Your build looks great btw. The point to point approach looks so beautiful. I will post pics when im done.
 

CJ

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found some old links that might be useful>

http://www.wagner-microphones.com/v76.htm

from mother link>

v76 schemo

http://www.wagner-microphones.com/images/v76.gif

freq plots  mod vs un-mod>

http://www.wagner-microphones.com/images/V76plot.jpg

 

Landins

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Definitely worth cutting out the filters! Read the schemo with care thou. Thats the one that differs from the original Tele-manual.
 

My3gger

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I'm using original manual in pdf and don't have any problems. Looks pretty good when printed... It's posted in Landis's reply #33.
 

Landins

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You beat me to it 3-man!

Just for fun i put to hammond 150h chokes in series on the output tube. It gives an ok result. But it will be surpassed when cj:s chokes arrive. Some issues with noise still, mostly ground related. I replaced feedback R from v2 to v1 to try and eliminate the feedback problem on the highest gain settings. But problem is still present. I may have to increase R values in the gain switch?
 

My3gger

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Landins said:
You beat me to it 3-man!

Just for fun i put to hammond 150h chokes in series on the output tube. It gives an ok result. But it will be surpassed when cj:s chokes arrive. Some issues with noise still, mostly ground related. I replaced feedback R from v2 to v1 to try and eliminate the feedback problem on the highest gain settings. But problem is still present. I may have to increase R values in the gain switch?

Ruffrecords wrote a few very good posts about grounding, there are also Jensen's and Rane's papers about grounding. Maybe check for Ruffrecord's as it is very clear about preamps. It gives examples and explains why. If you can't find them let me know, i have two of them in my files.
 

Landins

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Great progress today on ground(hog)day. Sanding off paint from chassis, big bolt and lots of cirkular cable pinch-on-ends (dont know name in english), stacked up all grounds at one point. Hiss and noise is down to minimum. But power trafo must be moved out of chassie. 50 cycle (swedish hz) hum is present. Signal is totally clean exept for this issue. And i can go up to eleventh gain stage without feedback! From what im hearing its amplifing damn well. My 58 sounds extremely sensitive and big. Happy times!
 

Landins

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Frate illuminate: I have a bunch of lundahl 4601 (old input trafos from a slaughtered Tore Seem rail). Its structure is similar to the now produces 1540. Nice, well shielded trafo this is. What do you think of tryin this for input eventhou ratio is only 1:2? 
 

CJ

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here is a V76s print just for the heck of it,

 

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Landins

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This is great news! I cannot thank you enough. This will give the final touch to the build. Just give me payment info so i can make up for this. Otherwise i fly over personally, money in hand. 
 
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