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    lowering gain on hi-gain VTA



    Posts : 21
    Join date : 2009-10-26

    lowering gain on hi-gain VTA

    Post by LEVLHED on Wed May 05, 2010 2:40 pm

    See quote below from a member on AK. I suppose he might be on this board too?
    Anyway as I noted below I tried the lower gain tube in the center position and I like it. Now I have questions on the other thing he mentions.

    Originally Posted by Kegger

    When you feel like doing some/a little tweaking, 12AT7's in that config have much more drive then needed, 2 quick helping mods, 1 remove the bypass cap on the driver (little electrolytic) between the center driver tube and each phase splitting tube, 2 is try some lower gain tubes like E180CC, 5965, E80CC, 6829 and such for the center driver tube.

    I've been running a VTA ST-70 for awhile now, I just today tried a nice RCA 5965 in the center spot based on the recommendation what I hear!
    I'm interested in the second quick mod you mention, but scared to death of it. (zero building experience/etc...amp bought used pre-made by Bob himself). You don't mean literally taking a pair of pliers and ripping those bypass caps of, do you? If it's not as easy/barbaric as that can you describe what needs to get done?
    I'm pretty sure I know which part you are referring to, but could you illustrate it clearly?
    What might be the negative consequences of the mod?

    You're talking about these ones, right?

    Bob Latino

    Posts : 2382
    Join date : 2008-11-26
    Location : Massachusetts

    Re: lowering gain on hi-gain VTA

    Post by Bob Latino on Wed May 05, 2010 3:20 pm


    It was found that C9 and C10, the two small electrolytic capacitors that you have circled, are not really needed in the circuit. If you remove them, the gain of the amp will drop by about .3 dB (3/10 of a dB) and the amount of distortion in the drive circuit will drop by some small fraction of a percent. You will not hear any difference in the sound of the amp if you remove them. If you want to remove them you just desolder the two solder connections on the bottom of the driver board for each capacitor and remove each capacitor.

    Note - In those kit amps, wired amps and VTA driver board upgrade sets sold in the past 6 months those two small caps are not even on the board. All VTA driver boards issued recently have a revised circuit without those two caps included.



    Posts : 21
    Join date : 2009-10-26

    Re: lowering gain on hi-gain VTA

    Post by LEVLHED on Wed May 05, 2010 3:27 pm

    I can't see .3dB being worth the effort! But maybe if I find some other reason/excuse to hit it with a soldering iron I'll do that.

    I recently ordered one of Bottlehead's Crack OTL kits (headphone amp) which is going to be my first ever electronics/tube projects. If it goes well and my confidence is inspired I'll have to revisit removing those as well as what ever other easy mods can be done to sweeten the VTA. I'll just have to be satisfied with my one tube roll for now.

    Thank you for the information!

    Is there any other tubes that might be used effectively in the middle slot to lower the gain even more?

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