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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 Dynakitparts.com products


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Jim McShane
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    • Post n°1

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    Post by C10 Wed May 12, 2021 2:28 pm

    Hi All,

    Been using a VTA-70 that I assembled for a few months now and it has been excellent for me.
    I am running it with GL KT66 and Electro Harmonix 12au7 driver tubes.
    Amp has about 250 hours on it so should be mostly stable at this point.
    What I am looking for is a recommendation for 3 driver tubes or combination of driver tubes that would soften some of the brightness of my tweeters and maybe add some bass. Any recommendations much appreciated.
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    Brap
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    • Post n°2

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    Post by Brap Thu May 13, 2021 9:46 am

    I run clear top RCA 12AU7's and love them. Look around for these. Have tried others and these are my favorite and have been using them for 7+ years.

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    Dave_in_Va
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    • Post n°3

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    Post by Dave_in_Va Thu May 13, 2021 10:37 am

    Just my opinion:
    First I'd try new speakers. Then I'd swap in EL34 tubes for the KT66.
    I believe the KT66 is the same tube as a 7581A which is a heavy duty version of the 6L6GC. I have tried the 7581A in guitar amps that use 6L6GC and what I noticed was a slight increase in brightness and maybe a touch more stiffness or lack of fullness. This was pretty subtle and needless to say comparing tubes is way more subjective than speakers.
    I once thought about trying 6L6GC in my VTA ST70 and Bob Latino mentioned that others who tried them found them a little brighter than the EL34.
    If you don't want to spend alot on experimenting, try a quad of Valve Art (Chinese) EL34. That's what came with my VTA ST70 and they sound really nice. Not as nice as the NOS Mullards that I settled on but still nicer than some other EL34 that I tried (and KT88).
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    Brap
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    • Post n°4

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    Post by Brap Thu May 13, 2021 3:17 pm

    FYI, I run KT-66's and RCA clear top driver tubes (experimented with Amperex) These have driven Klipsch Forte's, Zu Druids and now Fritz Carrera BE's. I have found through driver tube swapping, RCA's sound the best. Built a VTA ST-70 for a friend with EL34's and did a comparison with the Druids and "brightness" was about equal to KT-66's. In other amps I have built (OTL Transcendent, Joplin 2A3) I experimented with EH driver tubes. I would not recommend them but that is me.
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    Jim McShane
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    • Post n°5

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    Post by Jim McShane Thu May 13, 2021 10:27 pm

    Dave_in_Va wrote:
    I believe the KT66 is the same tube as a 7581A which is a heavy duty version of the 6L6GC.

    Actually that's not the case. The KT-66 Genalex is a different tube. It has a number of differences - the one many people are familiar with is that a KT-66 draws about .3 amps more heater current than a 6L6GC or 7581. The tubes are similar, but not identical.
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    Dave_in_Va
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    Post by Dave_in_Va Thu May 13, 2021 11:38 pm

    Thanks Jim.
    They're generally interchangeable, though, aren't they?
    So "generally" you can pop some 7581A tubes into a guitar amp that came with 6L6GC but maybe not with the KT66?
    And you can use any of the three (KT66, 7581A or 6L6GC) in a VTA ST 70?
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    • Post n°7

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    Post by peterh Fri May 14, 2021 2:14 am

    Dave_in_Va wrote:Thanks Jim.
    They're generally interchangeable, though, aren't they?
    So "generally" you can pop some 7581A tubes into a guitar amp that came with 6L6GC but maybe not with the KT66?
    And you can use any of the three (KT66, 7581A or 6L6GC) in a VTA ST 70?

    No.
    All of then needs a grid resistor to be < 100k

    It _might_ work for a while but the risk of redplating is imminent, more the older the tubes get.

    EL34 is really outstanding in this respect.
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    Bob Latino
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    • Post n°8

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    Post by Bob Latino Fri May 14, 2021 9:43 am

    peterh wrote:
    No.
    All of then needs a grid resistor to be < 100k

    It _might_ work for a while but the risk of redplating is imminent, more the older the tubes get.

    EL34 is really outstanding in this respect.

    Yes > In theory a grid resistor of a lower value (100K - 150K) may be better for KT66's BUT > I have used Gold Lion KT66's in my own VTA ST-70 with 270K grid resistors and never had an issue with any biasing problems. I also never had a Gold Lion KT66 red plate on me ...

    If you look up the specs of a KT120 output tube, they recommend using a 51K grid resistor. I have many customers using a Tung-Sol KT120 with 100K grid resistors in both the VTA ST-120 and the VTA M-125 monoblocks. No one has ever reported a biasing problem with a Tung-Sol KT120 using a 100K grid resistor ?

    Bob
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    Jim McShane
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    • Post n°9

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    Post by Jim McShane Fri May 14, 2021 10:24 am

    Dave_in_Va wrote:Thanks Jim.
    They're generally interchangeable, though, aren't they?
    So "generally" you can pop some 7581A tubes into a guitar amp that came with 6L6GC but maybe not with the KT66?
    And you can use any of the three (KT66, 7581A or 6L6GC) in a VTA ST 70?

    The 7581/7581A can interchange with a 6L6GC, it's a drop in sub. But because the KT-66 draws more heater current you may not be able to swap a KT-66 for a 6L6GC in some gear. Some amps can safely provide the extra heater current, some can't. Remember - a [b]quad[i] of KT-66s will require about 1.2 amps more heater current!

    Also, the KT-66 has a maximum grid circuit resistance value of 100K Ohms in fixed bias - so does a 6L6GC or 7581. So the grid circuit resistance is not an issue.
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    C10
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    Post by C10 Fri May 14, 2021 2:22 pm

    Thanks for the input so far. Friend is giving me some Brimar 12au7 to try out. I'll report back on my findings.

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    • Post n°11

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    Post by knotscott Sat May 15, 2021 7:37 pm

    I'm also running GL KT-66s in my two Dyna/VTA 70 (bi-amped), along with RCA 12AU7 clear tops and 12BH7 in the center. Now that the KT66s are burning in, I love the sound of that combination.
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    • Post n°12

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    Post by C10 Thu Jun 17, 2021 2:45 pm

    C10 wrote:Thanks for the input so far. Friend is giving me some Brimar 12au7 to try out. I'll report back on my findings.


    Just wanted to report back that the Brimar 12au7 are doing the trick.

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    New2Tubez
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    • Post n°13

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    Post by New2Tubez Fri Jun 18, 2021 2:41 pm

    C10 wrote:
    C10 wrote:Thanks for the input so far. Friend is giving me some Brimar 12au7 to try out. I'll report back on my findings.


    Just wanted to report back that the Brimar 12au7 are doing the trick.

    The Brimar CV4003 (12AU7) is my favorite sounding tube. I haven't tried nor will I spend for Mullard, Amperex, etc. The Brimar is pricey enough for me and I only use it in the center.
    Enjoy!

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