DIY Telefunken V76 Build attempt
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Landins
Well-known member
Almost there and Im very happy today. I hung the pwr-trx off the back, hooked up the well shielded lundahl and waited for the 50hx curve to appear on scope. Which i didnt! Flatline! The unit conducts on all gainstages up to max and its freakin' dead silent! Max magnification on scope gives just a few tiny mV of noise. I cried for 15 minutes. And this is with tubes, trafos, and inductors within the chassie.
I wonder what this symbol means, it appears in many places on the schematic. The first 3 tubes for example. Should it be connected to the chassi(ground)?
Or is the internal ground not connected to the chassi?
Is it only a symbol to show that they are attached to ground-line
Or is the internal ground not connected to the chassi?
Is it only a symbol to show that they are attached to ground-line
i believe that means chassis ground,
tube shields pick up stray fields, we want to dump these into the earth where they will not have the potential to hurt the signal, so the tube shields on the first 3 tubes are routed to chassis ground, but not the signal ground where they could mix with the good stuff,
now that 4th tube only has a 150 ohm cathode resistor which i guess is close enough to ground as not to cause any problems, so for simplicity of wiring,, they just use the audio gnd for that shield ground, the signal is less vulnerable by the time it gets amplified by the first 3 stages,
that last stage is almost a power amp with that 11.7 ma of screen current @ 200 volts,
11 mils times 200 = 2.2 watts, so you could wire an 8 ohm OPT to this thing and drive a speaker,
so a low Z amp is going to have less amp factor which means noise is not as much of an issue as those hi Z preamp tubes,
tube shields pick up stray fields, we want to dump these into the earth where they will not have the potential to hurt the signal, so the tube shields on the first 3 tubes are routed to chassis ground, but not the signal ground where they could mix with the good stuff,
now that 4th tube only has a 150 ohm cathode resistor which i guess is close enough to ground as not to cause any problems, so for simplicity of wiring,, they just use the audio gnd for that shield ground, the signal is less vulnerable by the time it gets amplified by the first 3 stages,
that last stage is almost a power amp with that 11.7 ma of screen current @ 200 volts,
11 mils times 200 = 2.2 watts, so you could wire an 8 ohm OPT to this thing and drive a speaker,
so a low Z amp is going to have less amp factor which means noise is not as much of an issue as those hi Z preamp tubes,
Landins
Well-known member
Introducin Pearl, my partner i crime up here in the grim north. We discussed this issue yesterday on the phone. So the shield catches stray electrons and should be grounded = more work for me as I havent done this. Noobish.
We also discussed grounding issues (issues I personally have issues with). In this build, which ground sholuld be taken into consideration? If we take the T-symbol for chassieground all grounds will be tied together. Ive done this and get good noise-readings, but is it correct? As Pearl mentioned over the phone, why are there different symbols for ground on the schem: T-symbol and the chassie-blob-like-thing for the ac-lug.
We also discussed grounding issues (issues I personally have issues with). In this build, which ground sholuld be taken into consideration? If we take the T-symbol for chassieground all grounds will be tied together. Ive done this and get good noise-readings, but is it correct? As Pearl mentioned over the phone, why are there different symbols for ground on the schem: T-symbol and the chassie-blob-like-thing for the ac-lug.
This is the chassi ground symbol from schem
Was just a little bit confused about the 2 different symbols for the same thing.
Started to think that maybe the signal ground should not be connected to chassi ground to prevent ground loops.
The signal ground would instead be connected to ground-protection later in the chain(mixing console or compressor)
But its only wild guesses :-\
Was just a little bit confused about the 2 different symbols for the same thing.
Started to think that maybe the signal ground should not be connected to chassi ground to prevent ground loops.
The signal ground would instead be connected to ground-protection later in the chain(mixing console or compressor)
But its only wild guesses :-\
Pearl, welcome here.
It looks like you are right, signal ground isn't connected to chasis and simbol you show is chasis connection. What makes me think are in/out shields connected directly to signal ground inside cassette. I would treat them as a part of chasis and ground at one side only. Other shields don't seem to have common connectiont (except chasis, but not at one point), or connection to signal ground. By testing i found out transformer coupled circuits can be very forgiving regarding grounding mistakes. There are still some of my preamps in studios with some not so good, but safe grounding practices.
I think they done it this way because V76 didn't have phantom and grounding can be a little different in this case. Maybe also because it made part of the mixer, i'm not sure, should see original preamp to be sure. If it was my mixer or standalone preamp with phantom i would ground it differently, that's for sure.
I won't go into details about preamp grounding, our member Ian Thompson Bell wrote some very good replies about it, here is just one of them:
The terms ground and earth are so ambiguous in the context of audio that I try to avoid them unless qualified in some way. The important thing to remember is the function that these connections perform. There are three to consider.
1. Safety earth. It's purpose is to save your life in the event of a fault. All exposed metalwork that a user can touch must be connected to it. It must be connected to the mains earth input connection as close to the mains input connector as practical.
2. Signal screen. It's purpose is to conduct interference currents separately from any signal currents and thus keep interference out of the signal chain. Signal screen usually includes all exposed and non exposed metalwork and cable screens.
3. Signal 0V. It's purpose is to act as a reference equipotential for all internal signals. It should carry only internal signal currents.
Ideally, these three should only be connected together at a single point in the equipment. Since the Safety Earth has to be connected to the exposed metalwork close to the mains input connector, it makes sense to make this the common point.
Problems occur in the real world because the above three are rarely if ever separate. It is almost inevitable that Signal screen and Safety Earth have elements in common. This is normally not a problem because both are unwanted currents and need to end up at the mains earth input.
Nearly all problems occur when there is overlap between Signal Screen and Signal 0V.
In a fully balanced floating equipment (one with input and output transformers), Signal Screen and Signal 0V can be connected at one point only, because the input and output signals are not referenced to Signal 0V. Screens of incoming and outgoing cables simply connect directly to the local metalwork so interference currents flow separately from the signal. This is why the AES recommends connecting pin 1 of XLRs directly to chassis (and not 0V) at the connector. Fully balanced floating systems do not suffer from hum loops because there is no connection of the input and output signal reference between equipment.
In equipment with differential inputs and outputs (often erroneously called balanced) the input and outputs are referenced to Signal 0V so although you get the benefits if common mode interference suppression, you are not immune to hum loops.
In equipment with unbalanced inputs or outputs the Signal Screen and Signal 0V are one and the same. We therefore have interference currents flowing in the Signal 0V and the likelihood of noise and hum is increased.
Hope that helps.
Cheers
Ian
It looks like you are right, signal ground isn't connected to chasis and simbol you show is chasis connection. What makes me think are in/out shields connected directly to signal ground inside cassette. I would treat them as a part of chasis and ground at one side only. Other shields don't seem to have common connectiont (except chasis, but not at one point), or connection to signal ground. By testing i found out transformer coupled circuits can be very forgiving regarding grounding mistakes. There are still some of my preamps in studios with some not so good, but safe grounding practices.
I think they done it this way because V76 didn't have phantom and grounding can be a little different in this case. Maybe also because it made part of the mixer, i'm not sure, should see original preamp to be sure. If it was my mixer or standalone preamp with phantom i would ground it differently, that's for sure.
I won't go into details about preamp grounding, our member Ian Thompson Bell wrote some very good replies about it, here is just one of them:
The terms ground and earth are so ambiguous in the context of audio that I try to avoid them unless qualified in some way. The important thing to remember is the function that these connections perform. There are three to consider.
1. Safety earth. It's purpose is to save your life in the event of a fault. All exposed metalwork that a user can touch must be connected to it. It must be connected to the mains earth input connection as close to the mains input connector as practical.
2. Signal screen. It's purpose is to conduct interference currents separately from any signal currents and thus keep interference out of the signal chain. Signal screen usually includes all exposed and non exposed metalwork and cable screens.
3. Signal 0V. It's purpose is to act as a reference equipotential for all internal signals. It should carry only internal signal currents.
Ideally, these three should only be connected together at a single point in the equipment. Since the Safety Earth has to be connected to the exposed metalwork close to the mains input connector, it makes sense to make this the common point.
Problems occur in the real world because the above three are rarely if ever separate. It is almost inevitable that Signal screen and Safety Earth have elements in common. This is normally not a problem because both are unwanted currents and need to end up at the mains earth input.
Nearly all problems occur when there is overlap between Signal Screen and Signal 0V.
In a fully balanced floating equipment (one with input and output transformers), Signal Screen and Signal 0V can be connected at one point only, because the input and output signals are not referenced to Signal 0V. Screens of incoming and outgoing cables simply connect directly to the local metalwork so interference currents flow separately from the signal. This is why the AES recommends connecting pin 1 of XLRs directly to chassis (and not 0V) at the connector. Fully balanced floating systems do not suffer from hum loops because there is no connection of the input and output signal reference between equipment.
In equipment with differential inputs and outputs (often erroneously called balanced) the input and outputs are referenced to Signal 0V so although you get the benefits if common mode interference suppression, you are not immune to hum loops.
In equipment with unbalanced inputs or outputs the Signal Screen and Signal 0V are one and the same. We therefore have interference currents flowing in the Signal 0V and the likelihood of noise and hum is increased.
Hope that helps.
Cheers
Ian
Landins
Well-known member
Thanks for the grounding info! Studioman Pearl has given this a lot of thought and he will install a ground lift for use in daisy-chained-ground-studio-situations. Progress has been quite slow the last weeks due to a hevy day-job-burden but some progress has been made. I tried the unit in studio environment the other week and results was good. Good amplifiication, the 58 was very big sounding and bassy, and extremely sensitive. And noise level was extremely low. And this is with a 1:2 lundah inputl, resistor on tube 2, and two hammond chokes in series on output tube. CJ:s chokes of divine goodness has arrived and so will a pair of vintage utc 0-16 inputs (150:50000). This will change things to the even better. And we can then compare units as Pearl is coming closer to the smoke test with his sowter-empowered beast.
Ill keep you posted and I thank you all with deep respect for all invaluble comments.
Ps. Parts for unit 2and 3 are on the way
Ill keep you posted and I thank you all with deep respect for all invaluble comments.
Ps. Parts for unit 2and 3 are on the way
Landins said:
I've been thinking about that theory; doubling inductance, doubling resistance and halving capacitance. I wonder how the laminations are stacked and whether there could be any benefit from altering it to better handle the plate current and retain enough inductance from two chokes in series. Landins could you have a look at the 156C and see if the core is exposed on one side to tell how it is stacked?
Landins
Well-known member
When I change the hammonds for the CJ-choke of goodness Ill have three 156C:s to experiment with. One thing that could be done is to rewind them with a four-chamber bobbin. Original is only two. Ill keep you posted when i dissect!
Thank you Ian for the grounding knowledge! I'm getting close to a first test of my V76.
I will put in a separate 20db pad instead of the one sitting on the gainswitch. My inputtrafo is a sowter200ohm. What type of pad would be best in this preamp? H-pad or a U-pad?
My knowledge of the impedance is poor but I found a T-Pad / H-Pad Calculator on internet. http://www.nu9n.com/tpad-calculator.html
200ohm Impedance in and out and a 20db pad, does it seems reasonable?
Best regards Daniel
I will put in a separate 20db pad instead of the one sitting on the gainswitch. My inputtrafo is a sowter200ohm. What type of pad would be best in this preamp? H-pad or a U-pad?
My knowledge of the impedance is poor but I found a T-Pad / H-Pad Calculator on internet. http://www.nu9n.com/tpad-calculator.html
200ohm Impedance in and out and a 20db pad, does it seems reasonable?
Best regards Daniel
gemini86
Well-known member
U pads are standard for modern studio usage. We don't match impedances anymore, we bridge them.
Okey, an 20db U-pad it is 
My input trafo has an impedance of 200ohm, should i choose the same impedance for the input pad?
Is there anyone who can help me with the resistor values? I cant find an calculator for u-pads
Input prim resistance 90ohm, input sek resistance 90ohm, bridge resistance 22,2ohm???
/Daniel
My input trafo has an impedance of 200ohm, should i choose the same impedance for the input pad?
Is there anyone who can help me with the resistor values? I cant find an calculator for u-pads
Input prim resistance 90ohm, input sek resistance 90ohm, bridge resistance 22,2ohm???
/Daniel
gemini86
Well-known member
Also consider the mics you'll be using. Most of them have a recommend load impedance. That will be the total of your three pad resistors.
Landins
Well-known member
To the finals. In studio today with friend/studioman pearl for some serious testing of amp. CJ:s chokes on tube 2/4 and new input trafo and grounded tubeshields (noob mistake deluxe). Great sound, distortion on high gain and totally dead silent from hum. We are happy.
Dissection of hammond choke gives a one chamber bobbin and an ei-core. Rewinding may be an option. Ill do a shootout between the hammonds in series and the CJ:s and see how the character of sound changes.
Dissection of hammond choke gives a one chamber bobbin and an ei-core. Rewinding may be an option. Ill do a shootout between the hammonds in series and the CJ:s and see how the character of sound changes.
pearl said:
Why would you take away original switch when it has more than 20dB attenuation in each of first two steps? You could make a custom pad with few positions, and separately another switch/pot for controlling nfb around first two tubes.
This can help you understand U pads:
http://www.uneeda-audio.com/pads/
There are other options too, but this one seems simplest and should work well. I need 20dB pads in front of 40dB fixed gain preamps very often, so original pad/gain switch in V76 isn't something i would want in modern studio. This is probably why it is distorting at high gain settings, it will get even worst with condenser mikes.
First 2 pos. are usual mic pad, 3rd is connected to nfb. It seems like you have too much gain, V72 would be simpler in this case as it has much lower gain and wouldn't need much complication with pads.
Your T pad seems strange to me, modern mic front end looks more like this one (1st pdf):
http://www.jensen-transformers.com/apps_sc.html
I'm not sure why you want another pad after pad. Hairball's T attenuator is meant for 600ohm, simply adding parallel resistors would probably change potentiometer law. Haven't tried it, wish it works well as i have a few.
Btw, have you or Landis tried CJ's chokes?
Your T pad seems strange to me, modern mic front end looks more like this one (1st pdf):
http://www.jensen-transformers.com/apps_sc.html
I'm not sure why you want another pad after pad. Hairball's T attenuator is meant for 600ohm, simply adding parallel resistors would probably change potentiometer law. Haven't tried it, wish it works well as i have a few.
Btw, have you or Landis tried CJ's chokes?
I took the picture from the dripelectronics. I will only go for the first pad in the picture(i think they show 2 diiferent ways of doing it) . I will later change values in the pad to get satisfied. I have replaced the original gainswitch with a 24 step pot without pad. I have not tested my amplifier, but it will be ready soon. :-\
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