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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all products

    Hi-Gain vs. Lo-Gain


    Posts : 10
    Join date : 2008-12-21

    Hi-Gain vs. Lo-Gain Empty Hi-Gain vs. Lo-Gain

    Post by craigtone on Sun Dec 21, 2008 8:21 pm

    Hey there ... I have the original tubes4hifi ST-70 driver board before there was a Hi and a Lo-Gain version. Mine is definitely hi-gain! I think my sensitivity is around 500mV! With 102dB sensitivity speakers, I'd like to convert to lo-gain. Thought it would make a good tech posting here on the forum for others in my shoes!

    TIA ...
    Bob Latino
    Bob Latino

    Posts : 2976
    Join date : 2008-11-26
    Location : Massachusetts

    Hi-Gain vs. Lo-Gain Empty Hi gain vs Low gain VTA board

    Post by Bob Latino on Sun Dec 21, 2008 10:21 pm

    The VTA driver board as set up with the normal components and 12AT7 medium gain tubes works well with most preamps and speaker combinations. You MAY have a little too much gain if you have very high sensitivity speakers (as you do) and/or a preamp that has high gain. You can convert a normal gain board to a low gain version if you replace the following components.

    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 - 1 watt resistors
    replace R27 & 28 (which are now 1K 1/2 watt resistors) with 470 ohm - 1/2 watt

    Pull the three 12AT7 tubes and use either 12AU7 tubes or 12BH7 tubes in their place. Check the board layout below for the location of the components.


    Hi-Gain vs. Lo-Gain VTAdriverboardNEW_sideways

    Posts : 1667
    Join date : 2008-11-30

    Hi-Gain vs. Lo-Gain Empty VTA70 hi-gain VS low-gain version

    Post by tubes4hifi on Sat Jan 03, 2009 7:56 pm

    yes, the sensitivity of the older version (now noted as hi-gain) using 12AT7 tubes is 0.50v input for full output, the original ST70 had a sensivity of 1.20v input for full output, so the gain is more than twice as much. Switching to 12AU7 tubes (along with the resistor changes) should cut the input sensivity by almost but not quite half, I haven't had a chance to actually measure the difference. Also, some people like the sound of 12AU7 tubes better (there are SO MANY difference brands to choose from !! )

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    Hi-Gain vs. Lo-Gain Empty Re: Hi-Gain vs. Lo-Gain

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