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    VTA High Gain

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    mantha3

    Posts : 298
    Join date : 2010-11-10

    VTA High Gain

    Post by mantha3 on Wed May 11, 2011 9:31 am

    I own an ST120 I built last year in Oct.

    I'm running the High Gain VTA. This runs to standard 3 12AT7 tubes. I found a pair of GE 5965 12AV7 12AT7 Amp Vacuum Tube MIL SPEC 2X Getters. These tubes are clear top with the getters on the side which is somewhat unique.

    These tubes a Simular to 12AT7 but with a bit lower gain. I was toying with the idea of trying one of these in the front center of the VTA... I run the ST120 with the 21 step Attenuator option I got from Bob.

    The tubes are well built millitary spec.. Big plates.. Look well built. I was toying with the idea of buying these on EBAY and the guy selling dropped the price for the pair from 19 to 15 so I did it. what the heck.

    Looking forward to trying the new tube




    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA High Gain

    Post by Bob Latino on Wed May 11, 2011 11:16 am



    A 12AT7 has a gain factor of 60 while the 12AV7 has a gain factor of 41. They are close enough so that I am pretty sure the 12AV7 wll work fine in there. The 12AV7 will draw more current than a 12AT7 but the power transformer on your ST-120 is "overbuilt" and will take the extra current draw in stride.

    If you do give them a try let us know on this thread what you find ?

    Bob




    Last edited by Bob Latino on Wed May 11, 2011 11:58 am; edited 1 time in total

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: VTA High Gain

    Post by baddog1946 on Wed May 11, 2011 11:46 am

    nice chart only missing the 12BH for a complete set.

    Bob Latino
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    Re: VTA High Gain

    Post by Bob Latino on Wed May 11, 2011 12:01 pm

    baddog1946 wrote:nice chart only missing the 12BH for a complete set.

    Baddog - I altered the photo in post #2 above to reflect the gain factor of a 12BH7. 12BH7's have a gain factor of 16.5 - even lower than a 12AU7

    Bob

    Scott
    Guest

    Re: VTA High Gain

    Post by Scott on Wed May 11, 2011 5:10 pm

    Is it correct to say that any of the tubes listed in the chart would work in the ST120 ?

    Would all of those choices also work in the ST70 ?

    naturall_mystic

    Posts : 9
    Join date : 2011-04-14

    Re: VTA High Gain

    Post by naturall_mystic on Thu May 12, 2011 1:48 pm

    A chart of all compatible tubes for all the Dynaco amps with some commentary would be a noble project. Perhaps one day when I know a bit more about all this I'll take it on or perhaps someone will beat me to it. Very Happy

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: VTA High Gain

    Post by mantha3 on Fri May 13, 2011 4:08 pm

    Bob,

    That gain factor chart is nice. I think the person who sold me these 2 tubes may have been somewhat misleading when he said they are a good sub for a 12AT7... Oh well. I'll give em a run and see what happens!


    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: VTA High Gain

    Post by mantha3 on Fri May 13, 2011 4:34 pm


    No go....

    I put one of these in the center on the VTA and got a decent hum from both speakers.. I ran the tube over to the right phase splitter on the VTA and the hum went to the right speaker. I got this same issue with both of these tubes.. Oh well. I think these are good tubes.. Look new and the heater is lighting. The sound is good at a moderate to high volume. You just get a decent hum when the volume/attenuator is at 0.

    Too bad. Cool tubes. Real beefey suckers. Big Plates. The heaters are bigger and are like mini rectifier tubes. Kinda neat with the clear top and dual side getters. Def. a neat look.

    I'll stash em away. 15 bucks.. Oh well. Perhaps if I ever convert to a low gain board they may work. Stash em for now.


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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: VTA High Gain

    Post by Pooch on Fri May 13, 2011 5:24 pm

    from audiotubes.com

    5965:
    This industrial tube is close enough to the 12AT7 that audiophiles have started buying them up and commenting on the low noise of this tube. It most likely is because it has a lower gain, a mu of 47 compared to 60 of a 12AT7. It is kind of like using a 5751 in place of a 12AX7--the lower gain being the main difference. These tubes are still plentiful, and are really well made. GE made a fine version of this tube in their "5-Star" broadcast line. A few have an extra mica spacer on top, known as "triple mica" and noted for lower microphonics. There is also a rare Telefunken version of this tube with long plates.

    A CJ PV8 has these (59650 and 5751 in it, along with 12ax7 in the phono. Have no idea why? Maybe they got a good price on the 5751,5965?
    Gordy

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