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Thread: DIY Tube Preamp

  1. #1

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    Default DIY Tube Preamp

    DIY Tube Audio Preamplifier

    Not sure if this is the right section to post this, but I'm building a unity gain tube buffer preamp from scratch and ran into a few roadblocks along the way. Some background on the design: A MOSFET-based current sink will eliminate the need in a high-voltage power supply. The tube will be biased with an op-amp-based servo circuit for elimination of the output DC offset. The cathode follower will be implemented for reduction of the distortions caused by excessive load to the signal source.

    The project will have three steps:

    1. Design of the device
    2. Simulation in PSPICE to verify the design
    3. Construction of a prototype of the tube preamplifier and testing.

    In addition, a method of source switching will be included to select from various audio sources such as a Turntable, CD, SACD, or DVD-Audio player. Volume control will be accomplished with a single dual channel potentiometer.

    The first choice was whether or not to use a toroidal or iron core transformer. [I did research on various transformers from Hammond Manufacturing to find an ideal match for this project.] Also, schematics were drawn in which the power supply and audio signal paths were created.

    The operating points and B+ requirements were determined to match the tube used. As the tube used was designed for mobile (car) applications, the B+ voltage does not have to be high resulting in greater safety for this project.

    My circuit simulation looks like the following:
    *source TUBE
    C_C1 2 1 0.1u
    R_R2 2 5 200k
    R_R3 14 8 100k
    R_R4 5 8 100k
    R_R8 14 0 150k
    C_C2 5 8 1u
    R_R5 2 3 1k
    R_R6 0 1 150k

    Xtube1 4 3 11 NH6dj8


    .SUBCKT TRIODENH A G K
    +PARAMS: LIP=1 LIF=3.7E-3 RAF=18E-3 RAS=1 CDO=0 RAP=4E-3
    + ERP=1.5
    + MU0=17.3 MUR=19E-3 EMC=9.6E-6 GCO=0 GCF=213E-6
    + CGA=3.9p CGK=2.4p CAK=0.7p

    Elim LI 0 VALUE {PWR(LIMIT(V(A,K),0,1E6),{LIP})*{LIF}}
    Egg GG 0 VALUE {V(G,K)-{CDO}}
    Erpf RP 0 VALUE {1-PWR(LIMIT(-V(GG)*{RAF},0,0.999),{RAS})+LIMIT(V(GG),0,1E6)*{RA P}}
    Egr GR 0 VALUE {LIMIT(V(GG),0,1E6)+LIMIT((V(GG))*(1+V(GG)*{MUR}), 0,-1E6)}
    Eem EM 0 VALUE {LIMIT(V(A,K)+V(GR)*{MU0},0,1E6)}
    Eep EP 0 VALUE {PWR(V(EM),ERP)*{EMC}*V(RP)}
    Eel EL 0 VALUE {LIMIT(V(EP),0,V(LI))}
    Eld LD 0 VALUE {LIMIT(V(EP)-V(LI),0,1E6)}
    Ga A K VALUE {V(EL)}
    Egf GF 0 VALUE {PWR(LIMIT(V(G,K)-{GCO},0,1E6),1.5)*{GCF}}
    Gg G K VALUE {(V(GF)+V(LD))}

    CM1 G K {CGK}
    CM2 A G {CGA}
    CM3 A K {CAK}
    RF1 A 0 1000MEG
    RF2 G 0 1000MEG
    RF3 K 0 1000MEG

    .ENDS

    VCC 4 0 25vDC
    VFE 13 0 25vDC

    Vsig 1 0 AC 1V


    .SUBCKT NH6DJ8 A G K
    XV1 A G K TRIODENH
    +PARAMS: LIP= 1.5 LIF= 10 RAF= 0.09 RAS= 0.2 CDO= 0
    + RAP= 0 ERP= 1.35
    + MU0= 33 MUR= 0.02 EMC= 0.0000795
    + GCO=-0.2 GCF= 0
    + CGA=1.40E-12 CGK=3.30E-12 CAK=1.80E-12
    .ENDS

    V1 0 150 1
    .dc v1 0 150 1
    *.ac dec 20 1k 1g
    .probe

    The question I have is, for some reason I cannot get a usable output from the tube. Am I doing something wrong with the circuit or can someone give me some background on simulating circuits with tube designs?

    Thanks!
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  2. #2

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    You say doesn't need to have a high voltage need for the tube? Just what is the voltage for the tube is?

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  3. #3

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    Quote Originally Posted by disneyjoe7 View Post
    You say doesn't need to have a high voltage need for the tube? Just what is the voltage for the tube is?
    The tube should be able to run off a 24V-0V-24V CT (center tapped) transformer as car audio applications have run this tube at low voltages without a problem. Or maybe I am not understanding things completely??

    In this design, the cathode follower circuit will be implemented for low distortion levels. The cathode follower design is a common buffer stage design (tube buffer) and is ideal for this project. It combines high input and low output resistance and reduces the distortion caused by excessive load to the signal source.

    Also, a low voltage power supply (LVPS) is implemented. The use of a MOSFET-based current sink will eliminate the need of a high-voltage power supply. A 48V center-tapped transformer power supply is used. Two voltage regulators mounted on heat sinks are implemented. Based on the calculations, it was decided that the power supply will utilize the following 24V-0V-24V transformer:
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  4. #4

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    The BJT, MOSFET, and 6DJ8 tube will be swapped after the final circuit is built. Testing will be performed on each of the circuits to determine whether the BJT, MOSFET, or tube produces better sound quality. Items such as distortion, including the changes in the 2nd order harmonic will be observed. As the signal levels are enough to drive the connected amplifier, a unity gain setup is ideal. The cathode follower circuit will be implemented for low distortion levels. The cathode follower design is a common buffer stage design (tube buffer) and is ideal for this project. Why? It combines high input and low output resistance and reduces the distortion caused by excessive load to the signal source. This is desired for this project and is critical in high quality sound reproduction applications. In addition, the cathode follower circuit can be improved on later if desired.

    Also, the following is the schematic being used for this project:
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  5. #5

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    I can't see that ever working. Voltage too low at tube. Your output is a input on the op amp? What is the tube doing? Your transformer needs to switch the 12volt input to get that working. The transformer is 117vac to 48v output so lets say 2 to 1 ratio, so if you switch 12v input you got what 6 voltage output.


    Sorry I don't get it.

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  6. #6

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    Quote Originally Posted by disneyjoe7 View Post
    I can't see that ever working. Voltage too low at tube. Your output is a input on the op amp? What is the tube doing? Your transformer needs to switch the 12volt input to get that working. The transformer is 117vac to 48v output so lets say 2 to 1 ratio, so if you switch 12v input you got what 6 voltage output.
    I am just passing the signal through the tube, the MOSFET current sink is used to power the tube which should work at low voltages because of its popularity in car audio applications. This is a hybrid tube/solid-state preamp, so the tube isn't really "doing anything" like it would in a traditional configuration. My goal is to get the tube sound with some silicon flavor, by combining the best of both worlds. The use of the MOSFET-based current sink will eliminate the need of a HV power supply and therefore will result in driving the tube at lower voltages.

    Here is my revised circuit diagram:
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  7. #7

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    I guess I should repost the background of this project (which I believe was discussed in another thread, if I remember correctly); let me copy/paste that here:

    My Tube Audio Design Project
    Design of a Tube Audio Preamplifier/Buffer
    By Serendipity
    December 28, 2010


    ***************************************
    Description:
    The tube buffer (cathode follower) will be implemented for reduction of the distortions caused by excessive load to the signal source. A MOSFET-based current sink will eliminate the need in a high-voltage power supply. The tube will be biased with an op-amp-based servo circuit for elimination of the output DC offset.
    ***************************************

    The project will have three steps:

    1. Design of the device;
    2. Simulation in PSPICE to verify the design;
    3. Construction of a prototype of the tube preamplifier;
    At the end of this project, a fully functional prototype of the preamplifier will be produced and the sound quality will be evaluated. The working model will be used to compare the differences in sound quality between a BJT, MOSFET, and tube-based buffer.

    My quest is to find a way to improve on the sound quality of existing audio components (Amplifier, CD/SACD Player, and Digital-to-Analog Converter) by building a quality tube buffer that is affordable but produces a high quality of sound reproduction.

    Problems

    The purpose of this Project is to determine the differences between the 6DJ8 tube, bipolar-junction transistor, and MOSFET when used in an audio preamplifier application. The BJT, MOSFET, and 6DJ8 tube will be swapped after the final circuit is built. Testing will be performed on each of the circuits to determine whether the BJT, MOSFET, or tube produces better sound quality. Items such as distortion, including the changes in the 2nd order harmonic will be observed. As the signal levels are enough to drive the connected amplifier, a unity gain setup is ideal. The cathode follower circuit will be implemented for low distortion levels. The cathode follower design is a common buffer stage design (tube buffer) and is ideal for this project. It combines high input and low output resistance and reduces the distortion caused by excessive load to the signal source. Also, a low voltage power supply (LVPS) is implemented. The use of a MOSFET-based current sink will eliminate the need of a high-voltage power supply. A 48V center-tapped transformer power supply is used. Two voltage regulators mounted on heat sinks are implemented. Based on the calculations, it was decided that the power supply will utilize the 24-0-24V transformer (discussed before).

    Design

    The tube buffer (cathode follower) will be implemented for reduction of the distortion caused by excessive load to the signal source. A MOSFET-based current sink will eliminate the need in a high-voltage power supply. The tube will be biased with an op-amp-based servo circuit for elimination of the output DC offset. In addition, a method of source switching will be included to select from various audio sources such as a Turntable, CD, SACD, or DVD-Audio player. Volume control will be accomplished with a single dual channel potentiometer. I did some research when I presented myself the following questions: What is the needed input sensitivity? What is the needed gain? What is the needed output voltage and impedance? Since most CD players have roughly a 2V output, no gain is required as 2V is enough to drive an amplifier such as the Adcom GFA-535 to full output.

    Some Thoughts

    The first choice was whether or not to use a toroidal or iron core transformer. [I did research on various transformers from Hammond Manufacturing to find an ideal match for this project.] Also, schematics were drawn in which the power supply and audio signal paths were created (Appendix A). The operating points and B+ requirements were determined to match the tube used. As the tube used was designed for mobile (car) applications, the B+ voltage does not have to be high resulting in greater safety for this project.
    The tube used is a 6DJ8 which will be biased with an op-amp-based servo circuit for elimination of the output DC offset. The 6DJ8 is readily available and commonly used in car audio applications. New old stock versions of this tube are easy to find. Furthermore, the reasoning for using an op-amp based servo circuit is to reduce the cost and complexity of the design.
    The cathode follower circuit will be implemented for low distortion levels. The cathode follower design is a common buffer stage design (tube buffer) and is ideal for this project. Why? It combines high input and low output resistance and reduces the distortion caused by excessive load to the signal source. This is desired for this project and is critical in high quality sound reproduction applications. In addition, the cathode follower circuit can be improved on later if desired.

    Source Switching

    A method of source switching must be implemented. Since most households have at least one CD player, AM/FM/Satellite Radio tuner, turntable, or DVD-Audio/SACD player, the ability to switch sources is necessary. The standard 2V output of most devices will be sufficient. For turntable use, an external phono stage can be used. Volume control will be accomplished by a stereo potentiometer attached to the front panel of the device.

    Power Supply

    As safety is critical, a low voltage power supply (LVPS) is implemented. The use of a MOSFET-based current sink will eliminate the need of a high-voltage power supply and therefore will result in a much safer design. A 48V center-tapped transformer power supply is used. Two voltage regulators mounted on heat sinks are implemented.
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  8. #8

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    Quote Originally Posted by disneyjoe7 View Post
    I can't see that ever working. Voltage too low at tube. Your output is a input on the op amp? What is the tube doing? Your transformer needs to switch the 12volt input to get that working. The transformer is 117vac to 48v output so lets say 2 to 1 ratio, so if you switch 12v input you got what 6 voltage output.

    Sorry I don't get it.
    Whoops, drawing error! The output of the tube is fed into the input of the op amp for biasing. The output should be connected to V_out, but I forgot to draw a dot right where the two lines cross. So the output is from the tube, not directly connected to the input of the op amp.

    Attached is a pic of the actual circuit that I've built so far:
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  9. #9

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    Since the purpose of this project is to determine the differences between the 6DJ8 tube, bipolar-junction transistor, and MOSFET when used in an audio preamplifier application, a CAD model will be created with common circuit components and tested in PSPICE. In reality, the BJT, MOSFET, and 6DJ8 tube will be swapped after the circuit is built and tested. The power supply can be low voltage because we are only using the tube for processing of the signal. The use of a MOSFET-based current sink will eliminate the need of a high-voltage power supply. In addition, a 48V center-tapped transformer power supply from Part Express is implemented. I also purchased a Hammond 156L filter choke from Parts Express too...

    The following pic is of the power supply that I built:
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  10. #10

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    So - I revised the circuit schematic / diagram and the latest version can be seen here below:



    What else would I need to make this work? Right now my output is 0V when you feed 2Vp-p (peak to peak) sine wave signal into the circuit in PSPICE. What sucks is that PSPICE doesn't have any tube models available, so you have to input the tube data in the Netlist yourself. And I'm not sure if Electronics Workbench would be able to do this either...
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  11. #11

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    Do you have access to a scope, to see where you went wrong?

    But then again I don't feel your tube is going to work at 12v or 24v, there's a reason why tubes need 150+ voltages or so.

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  12. #12

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    Quote Originally Posted by disneyjoe7 View Post
    Do you have access to a scope, to see where you went wrong?

    But then again I don't feel your tube is going to work at 12v or 24v, there's a reason why tubes need 150+ voltages or so.
    Nope, I don't have access to a scope at the moment. But I think this is something simple that I am overlooking (I tend to do that often) and just not sure what it is. As for the tube not working at 24V, a few people in my department have indicated that it will work and that a low-voltage cathode follower has been done in the past before. I too have doubted this at first (heck, it's a good idea to question things) but was assured that the circuit would work at 24V, and that it's been done before successfully. So here is where I stand with 2 partially completed boards and no simulation. Really scratching my head here...
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  13. #13

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    So after thinking about this I am confused. I have the following Netlist that I am trying to simulate, but have no usable output. Whatever signal I apply - sine wave, etc. doesn't result in anything - just a 0Vp-p output of the circuit. This is the Netlist I am working with:

    ***************************************
    C_C1 2 1 0.1u *Labeled (1) in the circuit
    R_R2 2 5 200k *Labeled (2) in the circuit
    R_R3 14 8 100k *Labeled (8) in the circuit
    R_R4 5 8 100k *Labeled (12) in the circuit
    C_C2 5 8 1u *Between (5) and (12) in the circuit
    R_R5 2 3 1k *Between (2) and (3) in the circuit
    R_R6 0 1 150k *Variable resistor, top left of circuit

    V1 1 0 25Vdc
    V3 2 0 -1vDC
    V2 4 0 AC 1v 0
    Rs 4 2 1k
    RG 2 0 1meg
    RK 3 0 10m
    Xtube1 4 3 11 NH6dj8


    .SUBCKT TRIODENH A G K
    +PARAMS: LIP=1 LIF=3.7E-3 RAF=18E-3 RAS=1 CDO=0 RAP=4E-3
    + ERP=1.5
    + MU0=17.3 MUR=19E-3 EMC=9.6E-6 GCO=0 GCF=213E-6
    + CGA=3.9p CGK=2.4p CAK=0.7p

    Elim LI 0 VALUE {PWR(LIMIT(V(A,K),0,1E6),{LIP})*{LIF}}
    Egg GG 0 VALUE {V(G,K)-{CDO}}
    Erpf RP 0 VALUE {1-PWR(LIMIT(-V(GG)*{RAF},0,0.999),{RAS})+LIMIT(V(GG),0,1E6)*{RA P}}
    Egr GR 0 VALUE {LIMIT(V(GG),0,1E6)+LIMIT((V(GG))*(1+V(GG)*{MUR}), 0,-1E6)}
    Eem EM 0 VALUE {LIMIT(V(A,K)+V(GR)*{MU0},0,1E6)}
    Eep EP 0 VALUE {PWR(V(EM),ERP)*{EMC}*V(RP)}
    Eel EL 0 VALUE {LIMIT(V(EP),0,V(LI))}
    Eld LD 0 VALUE {LIMIT(V(EP)-V(LI),0,1E6)}
    Ga A K VALUE {V(EL)}
    Egf GF 0 VALUE {PWR(LIMIT(V(G,K)-{GCO},0,1E6),1.5)*{GCF}}
    Gg G K VALUE {(V(GF)+V(LD))}

    CM1 G K {CGK}
    CM2 A G {CGA}
    CM3 A K {CAK}
    RF1 A 0 1000MEG
    RF2 G 0 1000MEG
    RF3 K 0 1000MEG

    .ENDS


    Q_Q1 9 9 13 Q2N2222A
    Q_Q2 11 9 13 Q2N2222A

    VCC 4 0 25vDC
    VFE 13 0 25vDC

    Vsig 1 0 AC 1V


    .SUBCKT NH6DJ8 A G K
    XV1 A G K TRIODENH
    +PARAMS: LIP= 1.5 LIF= 10 RAF= 0.09 RAS= 0.2 CDO= 0
    + RAP= 0 ERP= 1.35
    + MU0= 33 MUR= 0.02 EMC= 0.0000795
    + GCO=-0.2 GCF= 0
    + CGA=1.40E-12 CGK=3.30E-12 CAK=1.80E-12
    .ENDS

    X_V1 8 4 13 5

    .dc v1 0 150 1
    *.ac dec 20 1k 1g
    .probe
    ***************************************

    Any ideas?
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  14. #14

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    Revised Circuit:

    ***************************************
    C_C1 2 1 0.1u *Labeled (1) in the circuit
    R_R2 2 5 200k *Labeled (2) in the circuit
    R_R3 14 8 100k *Labeled (8) in the circuit
    R_R4 5 8 100k *Labeled (12) in the circuit
    C_C2 5 8 1u *Between (5) and (12) in the circuit
    R_R5 2 3 1k *Between (2) and (3) in the circuit
    R_R6 0 1 150k *Variable resistor, top left of circuit

    Xtube1 4 3 11 NH6dj8

    .SUBCKT TRIODENH A G K
    +PARAMS: LIP=1 LIF=3.7E-3 RAF=18E-3 RAS=1 CDO=0 RAP=4E-3
    + ERP=1.5
    + MU0=17.3 MUR=19E-3 EMC=9.6E-6 GCO=0 GCF=213E-6
    + CGA=3.9p CGK=2.4p CAK=0.7p

    Elim LI 0 VALUE {PWR(LIMIT(V(A,K),0,1E6),{LIP})*{LIF}}
    Egg GG 0 VALUE {V(G,K)-{CDO}}
    Erpf RP 0 VALUE {1-PWR(LIMIT(-V(GG)*{RAF},0,0.999),{RAS})+LIMIT(V(GG),0,1E6)*{RA P}}
    Egr GR 0 VALUE {LIMIT(V(GG),0,1E6)+LIMIT((V(GG))*(1+V(GG)*{MUR}), 0,-1E6)}
    Eem EM 0 VALUE {LIMIT(V(A,K)+V(GR)*{MU0},0,1E6)}
    Eep EP 0 VALUE {PWR(V(EM),ERP)*{EMC}*V(RP)}
    Eel EL 0 VALUE {LIMIT(V(EP),0,V(LI))}
    Eld LD 0 VALUE {LIMIT(V(EP)-V(LI),0,1E6)}
    Ga A K VALUE {V(EL)}
    Egf GF 0 VALUE {PWR(LIMIT(V(G,K)-{GCO},0,1E6),1.5)*{GCF}}
    Gg G K VALUE {(V(GF)+V(LD))}

    CM1 G K {CGK}
    CM2 A G {CGA}
    CM3 A K {CAK}
    RF1 A 0 1000MEG
    RF2 G 0 1000MEG
    RF3 K 0 1000MEG

    .ENDS

    Q_Q1 9 9 13 Q2N2222A
    Q_Q2 11 9 13 Q2N2222A

    VCC 4 0 25vDC
    VFE 13 0 25vDC

    V_sig 1 0 AC 1V

    .SUBCKT NH6DJ8 A G K
    XV1 A G K TRIODENH
    +PARAMS: LIP= 1.5 LIF= 10 RAF= 0.09 RAS= 0.2 CDO= 0
    + RAP= 0 ERP= 1.35
    + MU0= 33 MUR= 0.02 EMC= 0.0000795
    + GCO=-0.2 GCF= 0
    + CGA=1.40E-12 CGK=3.30E-12 CAK=1.80E-12
    .ENDS

    X_V1 8 4 13 5

    .dc v1 0 150 1
    *.ac dec 20 1k 1g
    .probe
    ***************************************

    Schematic below:
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  15. #15

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    Do you have a VOM so that you know the power supply is working given you the 24+/- voltages? If so I would start at the tube and see if your design can pass a signal with a low plate voltage.

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    Quote Originally Posted by disneyjoe7 View Post
    Do you have a VOM so that you know the power supply is working given you the 24+/- voltages? If so I would start at the tube and see if your design can pass a signal with a low plate voltage.
    Yes, I have +/-24V on the power supply so I know that it is working. I guess you are on to something though, as I believe the problem lies in the tube section and I need to make sure if the design can pass the signal with a low plate voltage. That's why I was simulating in PSPICE though, and wondering why I was not getting any results. Are you familiar with Electronics Workbench or PSPICE? I posted my simulation files in the last two posts. Maybe that can help as it gives more information about the circuit components and characteristics.

    Again, thanks for the help!!
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    I'm not aware of PSPICE but understand it's a software simulator of electronic design. I wonder if tieing the cathode of tube to ground with a resistor I start with a 1k, but would need to be lower to drive an output.

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    Steve the 6DJ8 and 6GM8 tubes run fine at 12vdc.

    I have a question. If you are running a 24-0-24 transformer why not wire it for a B+ around 70vdc? I think you went a little too complicated with the DC offset circuit. I would have just used a DC blocking cap on the output. The problem is not the theoretical plate voltage.
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
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    It seems he is running 24+/- for the op amp also.

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  20. #20

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    Quote Originally Posted by ben62670 View Post
    Steve the 6DJ8 and 6GM8 tubes run fine at 12vdc.

    I have a question. If you are running a 24-0-24 transformer why not wire it for a B+ around 70vdc? I think you went a little too complicated with the DC offset circuit. I would have just used a DC blocking cap on the output. The problem is not the theoretical plate voltage.


    Ben, are you stating filament voltage as 12vdc? I have always questioned the low 24vcd plate voltage.

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    Nope. Running the plate voltage at 12vdc. The very popular Akido pre amp runs at 24 or 48VDC. I built a low voltage tube buffer that runs a 15-0-15 tranny for the B+. After rectification it is around 44VDC. Also I built a 6GM8 pre that ran off of 15vdc B+
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
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    Quote Originally Posted by ben62670 View Post
    I have a question. If you are running a 24-0-24 transformer why not wire it for a B+ around 70vdc? I think you went a little too complicated with the DC offset circuit. I would have just used a DC blocking cap on the output. The problem is not the theoretical plate voltage.
    I could wire the transformer for a B+ around 70Vdc, but I decided to implement the DC offset circuit for "proof of concept" engineering. Yes, the DC offset circuit is complicated but I like challenges and enjoy the thinking process that goes into designing things. I'd like to make this work, otherwise I will resort to using a DC blocking cap on the output which is much easier.
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    Ben,

    In terms of the design, when I built this I wanted to make sure that the DC servo circuit is not nulling out the whole signal. Although I don't have any signal at the output I am wondering why this is happening. If you have any circuit simulation software, can you check my simulation files that I posted in #14? Again I appreciate all the help you guys have given me so far and would like to thank you personally for all your efforts!!
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    Quote Originally Posted by ben62670 View Post
    Nope. Running the plate voltage at 12vdc. The very popular Akido pre amp runs at 24 or 48VDC. I built a low voltage tube buffer that runs a 15-0-15 tranny for the B+. After rectification it is around 44VDC. Also I built a 6GM8 pre that ran off of 15vdc B+
    It was suggested that I should add a plate resistor. So I will try throwing a 10-15k resistor between the B+ rail and the tube plate and see if that results in any output. Having my fingers crossed right now as I'd really like to see this work sometime in the near future. Also Ben what do you think about the power supply section? Would the regulators have a negative impact on the sound? Maybe I could roll opamps and tubes and see the results once I get this thing working...
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    Quote Originally Posted by disneyjoe7 View Post
    Ben, are you stating filament voltage as 12vdc? I have always questioned the low 24vcd plate voltage.
    The 6DJ8 should work fine at a low plate voltage.
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    How are you powering the filament? Is that the item right after the transformer?

    And BTW your Transistor and or FET don't look like they are draw correct.

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  27. #27

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    Quote Originally Posted by disneyjoe7 View Post
    How are you powering the filament? Is that the item right after the transformer?

    And BTW your Transistor and or FET don't look like they are draw correct.
    Yes. And I'm glad you noticed the FET symbol - I've been drawing them that way since High School, just like I always mess up the brackets for Matrix equations. FML
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    Ok so comparing your circuit to other pre amp circuits, I see some troubles...

    1.) Looks like an op amp is draining the grid of the tube.

    Other pre amps which are circuit like that are grid is near the plate voltage.

    2.) You don't have a resistor to the cathode of the tube.

    Again many other circuits are resistor to the ground, then a cap to the output.


    BTW what is the square thing in a circle right next to the transformer? I see now where you're powering the filament.

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    Quote Originally Posted by disneyjoe7 View Post
    Ok so comparing your circuit to other pre amp circuits, I see some troubles...

    1.) Looks like an op amp is draining the grid of the tube.

    Other pre amps which are circuit like that are grid is near the plate voltage.

    2.) You don't have a resistor to the cathode of the tube.

    Again many other circuits are resistor to the ground, then a cap to the output.
    1.) Should I remove the second op amp to prevent draining the grid of the tube? I had thought about this before and was not sure if it was the cause of the problem.

    2.) Yes, I don't have a resistor to the cathode of the tube - some suggested adding a resistor similar to other circuits that use this method. What do you think?

    Attached is a picture of my simulation output graph; feel free to take a look at it and tell me what you think.

    Thanks, and I greatly appreciate your help with this matter.

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    I think in tube designs you need to have potential between Cathode and Plate so having the cathode float you don't have the needed potential. Also check the grid to plate voltage not sure what you need but playing with this voltage to see what the tube likes. Maybe you could get some info on specs of the tube on line.

    The graph looks like it doesn't like the design either, not working. Did I miss something?
    Last edited by disneyjoe7; 01-01-2011 at 11:50 AM.

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