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    VTA board gain question

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    1bigpig

    Posts : 11
    Join date : 2015-03-12

    VTA board gain question

    Post by 1bigpig on Sun Mar 22, 2015 11:05 pm

    I have a question and I hope this is the right spot to ask...

    I recently purchased a "new to me" Bob built VTA ST-120 amp and have been loving it so far, but it seems to have to much gain.  I am pretty sure it has what is referred to as the "VTA Classic" board (probably 2010 vintage) that uses 12AT7 tubes.  I see that the current production uses a lower gain 12AU7 tubes.  I have also read on this board that the "Classic" board can be retrofitted with new resistors and 12AU7 tubes.  So, my question is this:

    Can I retrofit just the center tube to an 12AU7 to reduce the gain but keep the 12AT7 phase splitters?  The reason for keeping the 12AT7s is that I have a bunch of 12AT7 tubes and would like to not "waste" them--plus I have some nice Mullards.

    Is this possible?

    Thanks,
    Bruce

    audiobill

    Posts : 271
    Join date : 2014-03-13
    Location : Philadelphia

    Re: VTA board gain question

    Post by audiobill on Mon Mar 23, 2015 6:32 am

    This is done all the time with guitar amps - the 12au7 is a lower gain/better sounding preamp tube, while the 12at7s are great phase splitters.

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Re: VTA board gain question

    Post by 1bigpig on Mon Mar 23, 2015 1:55 pm

    Thanks Audiobill.

    You wouldn't know what resistors would need to be changed to use the 12AU7 tube in the center position would you?  I can work a soldering iron and desoldering wick, but I don't have a build manual nor do I know which resistors on the VTA board affect which tube...

    Thanks,
    Bruce

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA board gain question

    Post by Bob Latino on Mon Mar 23, 2015 3:09 pm

    Hi Bruce,

    If you want to change to the LOWER GAIN version of the VTA ST-120 which uses 12AU7 instead of 12AT7 driver tubes you must make the changes as listed below. The "X" on the right side shows which resistors must be changed. R 27, 28, 9, 10, 15, 16, 17, 18, 11-14, 19 - 22 must be changed to the value listed. For R17 and 18, 11 - 14 and 19 - 22 (the 2 watt resistors) I recommend using metal oxide resistors because they are smaller for their wattage rating.

    Bob

                       Low gain board parts list

    X = Resistors are different from standard gain board

    VTA ST-120 driver
    LO gain with
    12AU7 or 12BH7

    ITEM VALUE QTY
    PCB 1
    R5 & 6 100
    R27 & 28 475 X
    R9 & 10 1K  X
    R15 & 16 6.8K 1w  X
    R7 & 8 7.5K  
    R1,2 10K
    R17 & 18 12K 2w  X
    R37 & 38 27K  
    R11-14 33K 2w  X
    R19-22 33K 2w  X
    R3, 4,       270K
    R29-32      150K
    R23-26 1M  
    R40-43 10  2w  BIAS resistors not on the board
    R44 2.2K  2w   On the quad cap
    V1-3 12AU7 or 12BH7
    VS1 - 3 9p pcb  
    VR1 - 4 50K
    VR5, 6 10K  
    C11 & 12 220 pF
    C13 - 16 22uF 450v  
    C17 100uF 100v  
    D1 UF4007  
    C1 - 4 .10 400v
    C5 - 8 .22 600v

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Re: VTA board gain question

    Post by 1bigpig on Mon Mar 23, 2015 5:01 pm

    Bob, thank you for answering.

    I don't want to beat a dead horse, just want to make sure I understand.

    The changes you posted above are just to change the center (V1) tube from a 12AT7 to 12AU7?  I don't want to change all 3 tubes to 12AU7s, just the V1 tube to 12AU7 and leave V2 and V3 as 12AT7s.

    If I setup the board for all 3 12AU7s, can I still use 12AT7s in V2 and V3 or will I run into distortion problems?

    I hope this makes sense.

    Thanks,
    Bruce

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA board gain question

    Post by Bob Latino on Mon Mar 23, 2015 5:22 pm

    Bruce,

    That list is for changing ALL the driver tubes to 12AU7. The VTA board never had a resistor list for changing JUST the center driver tube.

    If you run the lower gain board with 12AT7 driver tubes in the two side phase splitter/inverter parts of the circuit, I don't know what would happen? 12AT7 tubes have much more gain than 12AU7 driver tubes and would not be the best choice in the two side tubes. I would not use them there.

    If you want to cut down on the gain of the amp with less bother, add the VTA stepped attenuator. If you set the attenuator to about the 2 o'clock position, you would then have to turn the volume control up much higher on your preamp to gain an equivalent volume level through the amp ..

    Bob

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: VTA board gain question

    Post by tomlang on Tue Mar 24, 2015 10:26 am

    Here is where I would start as predicted by LTSpice.

    The following open loop gains of the driver circuits I have simulated are as follows:

    1. All 12AU7, Gain=50 (not shown here)
    2. All 12AT7, Gain=150 (not shown here)
    3. 12AT7 first tube, 12AU7 phase splitter tubes, Gain=80
    4. 12AU7 first tube, 12AT7 phase splitter tubes, Gain=95

    I wouldn't bother with option 4, but I would try option 3 as the 12AU7's are great phase splitters and since you have so many 12AT7's you can roll away.  And the first tube changes the sound more than the phase splitter tubes.

    Option 3 is simpler, just 3 resistors and the feedback resistor not shown.  And you would have to change the feedback cap probably also and tune it with an oscilloscope.  I have not built these so YMMV.




    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: VTA board gain question

    Post by tubes4hifi on Tue Mar 24, 2015 11:41 am

    thanks Tom for those simulations, very good info!!

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Re: VTA board gain question

    Post by 1bigpig on Wed Mar 25, 2015 1:26 pm

    Thanks for taking the time tomlang!

    That is really cool.  I wish I had a better background in electronics!  I am surprised that the 12AU7s would be better phase splitters than 12AT7s but again, I am no expert.

    It turns out that I am probably going to follow Bob's suggestion of using a stepped attenuator.  The person I purchased my amp from is selling me me some of his old tubes and said that he had an attenuator that he never installed and he would include with the purchase of the tubes!

    Thanks for everybody's help!

    Bruce

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