Valve preamplifier, modifications in original schematic

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Nelson Paschoal

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Hello friends, I´ve recently build this preamplifier from the schematic attached, but I did some mods, I would like to know from the experienced people if my mods were for good, they are:

- A 1uF cap in the input, followed by 1k res grid stopper
- changed 2M2 to 1M grid leak res
- eliminated the 10pF cap
- changed 12ay7 hard to find to 12AT7, and used 2K2/47uF in cathode first stage and 680R/47uF in cathode second stage
- 22k plate load in first stage and 100k plate load in second stage

Everything else was stock, except for the heater elevation, to minimize hum, I´ve noticed that this preamp doesn´t have much headroom, if someone want some readings I have signal generator and a scope.
Thanks for all!!!
 

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emrr

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What are your audio transformers and ratios?

Are you using the DI input, or not?
 

ruffrecords

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Hello friends, I´ve recently build this preamplifier from the schematic attached, but I did some mods, I would like to know from the experienced people if my mods were for good, they are:

- A 1uF cap in the input, followed by 1k res grid stopper
A capacitor is a good idea to isolate dc from the grid. A grid stopper is not necessary with low mu tubes and only adds unwanted noise
- changed 2M2 to 1M grid leak res
Good. 2M2 is innecessarily high
- eliminated the 10pF cap
Good, it only messes up the HF performance
- changed 12ay7 hard to find to 12AT7, and used 2K2/47uF in cathode first stage and 680R/47uF in cathode second stage
Why? If 12AY7 is not available then 12AT7 is a reasonable near substitute but you need to recalculate the cathode bias resistors. 47uF is probably too low a value of decoupling cap for a response down to 20Hz.
- 22k plate load in first stage and 100k plate load in second stage

You want a high value resistor in the plate of the first stage so as to maximise gain. This will also improve overall noise performance.
Everything else was stock, except for the heater elevation, to minimize hum, I´ve noticed that this preamp doesn´t have much headroom, if someone want some readings I have signal generator and a scope.
Thanks for all!!!
You have 20dB gain from the transformer plus the gain of the first stage as the minimum gain available. This is what limits headroom. To get it back you need to add a switched pad at the transformer input.

Cheers

Ian
 

Nelson Paschoal

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What are your audio transformers and ratios?

Are you using the DI input, or not?
I am not using the DI input, just the balanced input, with a 1:10 transformer made by a company here in Brazil, very good piece, well tested, made upon Jensen Jt-115K, and the output transformer is a 4:1 with two paralleled outputs, one of them I´m using for a VU Meter and the other is the output per se.
 

Nelson Paschoal

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A capacitor is a good idea to isolate dc from the grid. A grid stopper is not necessary with low mu tubes and only adds unwanted noise

Good. 2M2 is innecessarily high

Good, it only messes up the HF performance

Why? If 12AY7 is not available then 12AT7 is a reasonable near substitute but you need to recalculate the cathode bias resistors. 47uF is probably too low a value of decoupling cap for a response down to 20Hz.


You want a high value resistor in the plate of the first stage so as to maximise gain. This will also improve overall noise performance.

You have 20dB gain from the transformer plus the gain of the first stage as the minimum gain available. This is what limits headroom. To get it back you need to add a switched pad at the transformer input.

Cheers

Ian
Thanks a lot Ian, I´ll remove the grid stopper. I´ll recalculate the the plate resistor of the first stage, I remember when I´m building the unit, when I used the originally 68k a heavy distortion occured, and I could use only 30% of the gain´s pot, ´cause of that I choose a low value to not distort, but maybe I try 47k.
About the cathode bias, the first and the second stage are with -2,5V, and a curious effect occured when I used 220uF cap, the input signal and the output signal were out of fase by a couple of degrees, like a delay in signal, so I change to 47uF and everything wents normal, I dunno what happen.
 

MidnightArrakis

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I am not using the DI input, just the balanced input, with a 1:10 transformer made by a company here in Brazil, very good piece, well tested, made upon Jensen Jt-115K, and the output transformer is a 4:1 with two paralleled outputs, one of them I´m using for a VU Meter and the other is the output per se.
Nelson: GREETINGS!!! I have been following your thread here about your building this 2-tube audio preamp schematic and I thought that I would jump in and possibly help you out a bit.....only if you wish to have some assistance!!! Attached to this message is a PDF file of your recently updated audio preamp design schematic using the 12AT7 and 12AU7 vacuum tubes. My schematic version is different than yours, however, because I have used a CAD design program to draw my schematic, which means that should be you interested, I am able to design a PCB using your circuit. In my version, all of the XLR connectors, transformers and volume controls are PCB-mount types. However, I need some of your expertise to help clarify some of the components because either the text in your schematic was impossible to read or not enough information was presented in your schematic to know what type of a part needs to be used. Here is what I need:
  • A list identifying which resistors are quarter-watt and which resistors are half-watt.
  • A list more clearly identifying the values of the capacitors. I cannot tell if a capacitor is 1000uF or if it is 1000pF.
  • The -- VOLTAGE -- value for the different electrolytic capacitors.
  • The -- TYPE -- of capacitors used by C1, C5, C7 (i.e., polypropylene, film, etc.).
  • I have several different VU-meter driver schematics available. If you would like to have a VU-meter included, where would it get its signal from? (U1 Pins 1 & 8, or R12)?
I am also attaching another PDF file that will show you some of the equipment that I have personally designed for others. Let me know what you think, OK???

Take care and I hope to hear back from you shortly with some information to answer my questions. THANKS!!!

/
 

Attachments

  • BRAZIL 2-TUBE AUDIO Pre-Amp.pdf
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  • JBW-Designed - RACK STUFF -- Assemblies-Chassis-DAB-Enclosures-Panels-Systems.pdf
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ruffrecords

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Nelson: GREETINGS!!! I have been following your thread here about your building this 2-tube audio preamp schematic and I thought that I would jump in and possibly help you out a bit.....only if you wish to have some assistance!!! Attached to this message is a PDF file of your recently updated audio preamp design schematic using the 12AT7 and 12AU7 vacuum tubes. My schematic version is different than yours, however, because I have used a CAD design program to draw my schematic, which means that should be you interested,
Unfortunately your schematic, and the one it is derived from, contain a crucial error. Pin 1 of both XLR connectors is shown connected to the 'earth' symbol which in this case is analogue 0V. This error is well known and is called the pin 1 error. Pin 1 carries the screen of a balanced cable and as such it is NOT a signal carrying conductor. The signal exists only between the hot and the cold pins. Pin 1 should really be connected to the chassis if the preamp. Back at the PSU, the analogue 0V is connected to the chassis at a single point.

Cheers

Ian
 

Nelson Paschoal

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Nelson: GREETINGS!!! I have been following your thread here about your building this 2-tube audio preamp schematic and I thought that I would jump in and possibly help you out a bit.....only if you wish to have some assistance!!! Attached to this message is a PDF file of your recently updated audio preamp design schematic using the 12AT7 and 12AU7 vacuum tubes. My schematic version is different than yours, however, because I have used a CAD design program to draw my schematic, which means that should be you interested, I am able to design a PCB using your circuit. In my version, all of the XLR connectors, transformers and volume controls are PCB-mount types. However, I need some of your expertise to help clarify some of the components because either the text in your schematic was impossible to read or not enough information was presented in your schematic to know what type of a part needs to be used. Here is what I need:
  • A list identifying which resistors are quarter-watt and which resistors are half-watt.
  • A list more clearly identifying the values of the capacitors. I cannot tell if a capacitor is 1000uF or if it is 1000pF.
  • The -- VOLTAGE -- value for the different electrolytic capacitors.
  • The -- TYPE -- of capacitors used by C1, C5, C7 (i.e., polypropylene, film, etc.).
  • I have several different VU-meter driver schematics available. If you would like to have a VU-meter included, where would it get its signal from? (U1 Pins 1 & 8, or R12)?
I am also attaching another PDF file that will show you some of the equipment that I have personally designed for others. Let me know what you think, OK???

Take care and I hope to hear back from you shortly with some information to answer my questions. THANKS!!!

/
Hello friend,

Thanks for your interest in this thread, I will update my schematic, I used Proteus for the drawing but something happen when upload and the image gets distorted, but I´ll fix it with all the questions answered.

  • I have several different VU-meter driver schematics available. If you would like to have a VU-meter included, where would it get its signal from? (U1 Pins 1 & 8, or R12)?
Actually I get the signal from the secondary of the output transformer, but the performance isn´t that good, the pointer barely moves or hits hard the end of the scale, even with a limiter resistor and properly rectification, so if you have a better idea, let me know.

I checked the PDF file, awesome work bro, it´s very high tech, far from my little knowledge in the subject.
 

Nelson Paschoal

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Unfortunately your schematic, and the one it is derived from, contain a crucial error. Pin 1 of both XLR connectors is shown connected to the 'earth' symbol which in this case is analogue 0V. This error is well known and is called the pin 1 error. Pin 1 carries the screen of a balanced cable and as such it is NOT a signal carrying conductor. The signal exists only between the hot and the cold pins. Pin 1 should really be connected to the chassis if the preamp. Back at the PSU, the analogue 0V is connected to the chassis at a single point.

Cheers

Ian
Hello Ian,

I´ll take some picture of the equip, I don´t get the pin 1 error, I have to get the pin 1 to the common ground or what?
 

MidnightArrakis

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Messages
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Unfortunately your schematic, and the one it is derived from, contain a crucial error. Pin 1 of both XLR connectors is shown connected to the 'earth' symbol which in this case is analogue 0V. This error is well known and is called the pin 1 error. Pin 1 carries the screen of a balanced cable and as such it is NOT a signal carrying conductor. The signal exists only between the hot and the cold pins. Pin 1 should really be connected to the chassis if the preamp. Back at the PSU, the analogue 0V is connected to the chassis at a single point.

Cheers

Ian
Like this???

/
 

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ruffrecords

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Like this???

/
That is right. Of course, this is only part of the story. Another part is the power supply and a third part is the enclosure. They may each have their own schematic and even their own PCB. The power supply often has its own schematic and PCB and then there would be a top level one showing how the PCBs connect to each other, the enclosure and the outside world.

Cheers

Ian
 

moamps

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Pin 8 of the input transformer should be connected to common ground. C1, C3, C6 and C7 have significantly too large values. C2 and C4 should be higher, IMO. The WCF is not properly designed; one resistor is missing, the output signal should be taken from pin 1 of the output tube, C6 value??
 

MidnightArrakis

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Pin 8 of the input transformer should be connected to common ground. C1, C3, C6 and C7 have significantly too large values. C2 and C4 should be higher, IMO. The WCF is not properly designed; one resistor is missing, the output signal should be taken from pin 1 of the output tube, C6 value??
I didn't design this circuit. I was only trying to help out the OP by taking their hand-drawn paper schematic and creating a CAD-generated one that a PCB could be designed from. I am -- NOT -- a circuit designer!!!

The original hand-drawn paper schematic was practically illegible in several areas and the component values shown are the best that I could decipher from the hand-drawn version. I have requested that the OP provide me with the correct component values (C6), but so far, I have not received any feedback yet.

>> DON'T SHOOT THE MESSENGER!!! <<

Since you appear to be more knowledgeable about how this circuit is supposed to be, then feel free to provide me with whatever changes and updates are necessary to make this a viable and working circuit for the OP and I will update my schematic to reflect your input and then, hopefully, the OP will be happy.

My "2-cents worth" was merely in creating a CAD version of his original hand-drawn paper schematic and that's about it!!!

/
 

MidnightArrakis

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That is right. Of course, this is only part of the story. Another part is the power supply and a third part is the enclosure. They may each have their own schematic and even their own PCB. The power supply often has its own schematic and PCB and then there would be a top level one showing how the PCBs connect to each other, the enclosure and the outside world.

Cheers

Ian
[The power supply often has its own schematic and PCB and then there would be a top-level one showing how the PCBs connect to each other, the enclosure and the outside world] -- And, I am well-familiar with all of this, with my having been doing exactly this type of documentation for only 40+ years. However, the OP has only provided this thread a hand-drawn paper schematic of a single circuit and none of the information that you have listed. Should they ever provide us with any of your suggested information, then I could create a more -- systems-level -- set of drawings to show all of these electrical interconnections. But, until then..........
 

ruffrecords

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[The power supply often has its own schematic and PCB and then there would be a top-level one showing how the PCBs connect to each other, the enclosure and the outside world] -- And, I am well-familiar with all of this, with my having been doing exactly this type of documentation for only 40+ years. However, the OP has only provided this thread a hand-drawn paper schematic of a single circuit and none of the information that you have listed. Should they ever provide us with any of your suggested information, then I could create a more -- systems-level -- set of drawings to show all of these electrical interconnections. But, until then..........
My apologies, I was not trying to teach my granny to suck eggs (where did that expression come from?). As an electronics engineer I have worked with many PCB designers over the years so I have learned to give them the complete picture before they start work. If you don't, they have to make assumptions which, as they are not electronics engineers, they should not have to.

The schematic shown in the first post is from a well known and much dissected design. I cannot rememeber its name right now but the original article does include a power supply schematic

Cheers

Ian
 

Nelson Paschoal

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I didn't design this circuit. I was only trying to help out the OP by taking their hand-drawn paper schematic and creating a CAD-generated one that a PCB could be designed from. I am -- NOT -- a circuit designer!!!

The original hand-drawn paper schematic was practically illegible in several areas and the component values shown are the best that I could decipher from the hand-drawn version. I have requested that the OP provide me with the correct component values (C6), but so far, I have not received any feedback yet.

>> DON'T SHOOT THE MESSENGER!!! <<

Since you appear to be more knowledgeable about how this circuit is supposed to be, then feel free to provide me with whatever changes and updates are necessary to make this a viable and working circuit for the OP and I will update my schematic to reflect your input and then, hopefully, the OP will be happy.

My "2-cents worth" was merely in creating a CAD version of his original hand-drawn paper schematic and that's about it!!!

/
Sorry friend, here´s the correct values, you just use the incorrect unit:
C1 - 1000 nF
C3 - 100 nF
C6 - 1000 nF
C7 - 1000 nF

The output section of your drawing have some mistakes indeed, but we are doing some nice improvements, I promise that I´ll update my schematic and my equipment with all the tips mentioned by all the users, thanks a lot everybody. I am working from 7am to 21pm because of that I did not update you.
 

Nelson Paschoal

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Location
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My apologies, I was not trying to teach my granny to suck eggs (where did that expression come from?). As an electronics engineer I have worked with many PCB designers over the years so I have learned to give them the complete picture before they start work. If you don't, they have to make assumptions which, as they are not electronics engineers, they should not have to.

The schematic shown in the first post is from a well known and much dissected design. I cannot rememeber its name right now but the original article does include a power supply schematic

Cheers

Ian
Hi Ian,

Yes, the first schematic is from Tape Op, but I can´t see the power supply since I don´t have a subscription to the magazine, but I designed my own, check the attached file, and considerer the grounding that I forgot in the drawing and a 10H choke.
 

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