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    ST-70 & Swapping in 12AU7 driver tubes

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    pwsnyder

    Posts: 11
    Join date: 2010-08-10
    Age: 66
    Location: Flossmoor, Illinois USA

    ST-70 & Swapping in 12AU7 driver tubes

    Post by pwsnyder on Fri Jan 14, 2011 2:26 am

    I purchased the 12AT7 version of the VTA ST-70 kit in mid-2009. I alternate preamps - but prefer my tube based unit. The problem I'm having is that there's just too much drive into the amp. Essentially, I can't move the preamp volume control (of any of my preamps) much past the 9 o'clock position - about 1/4 rotation. And that's with a turntable as the input device. You can imagine how little vol. control rotation I have when the input device is a CD player! I want to use the preamp volume pot. in a more linear region - closer to the 12 o'clock position.

    My Question: Can I experiment with using 12AU7 tubes on the front end of the ST-70 - but WITHOUT altering the 8 resistors on the circuit board?? I mean, will I harm anything by doing this? (By the way I have Bob's input attenuator control for the ST-70. For now, though, I prefer to work with the input tube configuration.)

    Appreciate any feedback.

    Bob Latino
    Admin

    Posts: 1961
    Join date: 2008-11-26
    Location: Massachusetts

    Re: ST-70 & Swapping in 12AU7 driver tubes

    Post by Bob Latino on Fri Jan 14, 2011 8:58 am

    Hi Peter,

    If you are not getting enough range out of your preamp's volume control just run the attenuator at a lower level like 12 o'clock. It will now take a higher drive level from your preamp to achieve output.

    Bob

    tubes4hifi
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    Re: ST-70 & Swapping in 12AU7 driver tubes

    Post by tubes4hifi on Fri Jan 14, 2011 3:06 pm

    it won't hurt anything to sub the 12AU7 in there, it just won't be running at optimum parameters.
    If you like the sound and/or gain situation better after trying it, then you can make some resistor changes.

    pwsnyder

    Posts: 11
    Join date: 2010-08-10
    Age: 66
    Location: Flossmoor, Illinois USA

    Swapped in 12AU7s and little change

    Post by pwsnyder on Mon Jan 17, 2011 2:16 am

    I took your advice and swapped in 3-12AU7s for the stock 12AT7s. There is a small reduction in apparent volume with the preamp Vol. control at the same setting as with the 12AT7s on the ST70 board. To be honest, though, the change is fairly minimal. The preamp Vol. pot might now be at a point a little nearer the 10 o'clock position versus 9 o'clock before the swap.

    My associated equipment is a Sound Valves VTP-100 preamp (all 12AX7s) and EPOS ELS-3 mini-monitors and a 10 inch powered subwoofer. This is all set up in an 11X14 foot room with an 8 foot ceiling. (The listed sensitivity for the ELS-3s is 87 db/1W/1M.)

    I did NOT change the 8 resistors Bob Latino originally identified as necessary to give optimum results with the 12AU7s. Does the resistor swap give a further gain reduction at the ST70 input?


    Bob Latino
    Admin

    Posts: 1961
    Join date: 2008-11-26
    Location: Massachusetts

    Re: ST-70 & Swapping in 12AU7 driver tubes

    Post by Bob Latino on Mon Jan 17, 2011 8:30 am

    Hi Peter,

    Although the VTA standard gain board does have slightly more gain than the average tube amp, part of the problem is with your Sound Valves preamp which allows too much gain in the first 1/4 turn of rotation. With most preamps this won't happen but with your preamp it evidentally is an issue. If the use of 12AU7 tubes won't drop the gain enough AND you still want to continue using the Sound Valves preamp then you should probably convert to the lower gain version of the VTA driver board. Also, the change is not 8 resistors as you mentioned in your post > it is 16 resistors - 8 per channel. The changes you need to make are below ...

    replace R9 & 10 (which are now 2.2K 1/2 watt resistors) with 1K - 1/2 watt resistors
    replace R11 & 12 (which are now 68K 1/2 watt resistors) with 36K - 2 watt resistors
    replace R13 & 14 (which are now 150K 1/2 watt resistors) with 47K - 1/2 watt resistors
    replace R15 & 16 (which are now 7.5K 1/2 watt resistors) with 6.8K - 1/2 watt resistors
    replace R17 & 18 (which are now 12K 1/2 watt resistors) with 12K - 1 watt resistors
    replace R19-22 (which are now 51K 1/2 watt resistors) with 33K - 2 watt resistors
    replace R27 & 28 (which are now 1K 1/2 watt resistors) with 470 ohm - 1/2 watt

    Bob

    pwsnyder

    Posts: 11
    Join date: 2010-08-10
    Age: 66
    Location: Flossmoor, Illinois USA

    Additional drop in gain With 12AU7 resistor Change?

    Post by pwsnyder on Mon Jan 17, 2011 2:12 pm

    Thanks for the information. I still wonder whether the resistor change brings with it additional drop in gain beyond just swapping in the 12AU7 tubes. What are your thoughts?

    Thanks Bob.


    Bob Latino
    Admin

    Posts: 1961
    Join date: 2008-11-26
    Location: Massachusetts

    Re: ST-70 & Swapping in 12AU7 driver tubes

    Post by Bob Latino on Mon Jan 17, 2011 2:46 pm

    pwsnyder wrote:Thanks for the information. I still wonder whether the resistor change brings with it additional drop in gain beyond just swapping in the 12AU7 tubes. What are your thoughts?

    Thanks Bob.


    Peter,

    You have the stepped attenuator kit. Has it been installed yet ? It would be easier to use the stepped attenuator as a master gain control to acquire more preamp range than going through the bother of replaxcing 16 resistors.

    The resistor change makes a more substantial reduction in gain than just changing out the 12AT7's to 12AU7's on the standard gain VTA driver board.

    Bob

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