The Dynaco Tube Audio Forum

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


    VTA ST70 spare tubes

    Share

    bluerondo

    Posts : 27
    Join date : 2014-09-21
    Location : Irvine, CA

    VTA ST70 spare tubes

    Post by bluerondo on Sun Jan 11, 2015 4:39 pm

    Hello,
    I'm looking for recommendations for a spare set of the three small driver tubes. I currently have two RCA 5963's on the outside and the center tube is a Tung Sol 5814A. I'm looking for equal or better than these.

    Thanks, Bill

    MontanaWay

    Posts : 781
    Join date : 2014-02-06
    Age : 58
    Location : Brookings, Oregon

    Re: VTA ST70 spare tubes

    Post by MontanaWay on Sun Jan 11, 2015 5:09 pm

    for my ears, really hard to beat NOS RCA 5963 in all positions.
    Even NOS RCA 12AU7's are very nice indeed!

    10-E-C

    Posts : 87
    Join date : 2014-02-12
    Age : 63
    Location : upper east tn

    Re: VTA ST70 spare tubes

    Post by 10-E-C on Sun Jan 11, 2015 7:39 pm

    I have a mill spec rca 5963 in the center and 2 nos westinghouse 12au7s flanking the 5963. That combination works well in my st 70.

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: VTA ST70 spare tubes

    Post by sKiZo on Sun Jan 11, 2015 9:47 pm

    Depends on what you're looking to hear really. I wanted to darken my ST120 up a bit and put a Sylvania 12BH7 in the center. That also opened up the stage quite a bit. A nice pair of Westinghouse Star 5963's fill the sides ...

    If you're looking to brighten up the sound some, the RCA clear top 12AU7's are hard to beat.

    bluerondo

    Posts : 27
    Join date : 2014-09-21
    Location : Irvine, CA

    Thanks

    Post by bluerondo on Mon Jan 12, 2015 10:09 pm

    Thanks everyone for chiming in. I tend to like bright, first thing I notice is missing highs. I really don't know about stage etc.
    I think I'm going to shop around for either a RCA 5963 or RCA 12au7 for the center and Electro-Harmonix 12au7's for the other two.

    Thanks

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA ST70 spare tubes

    Post by peterh on Tue Jan 13, 2015 6:42 am

    bluerondo wrote:Thanks everyone for chiming in. I tend to like bright, first thing I notice is missing highs. I really don't know about stage etc.
    I think I'm going to shop around for either a RCA 5963 or RCA 12au7 for the center and Electro-Harmonix 12au7's for the other two.

    Thanks
    The outer ones ( the drivers) are in a long-tail-pair config and needs to be reasonably
    similar if the ac balance is to be kept. Most vendors has an option of "match" or "balance".


    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: VTA ST70 spare tubes

    Post by sKiZo on Tue Jan 13, 2015 2:27 pm

    Center tube should be a fairly close "triode match" as well.

    Brightest tube in my collection is a Marconi 12AU7.



    Shopping for tubes for my DAC and stumbled across it. Never heard of it, little in the way of reviews, but at $16 for NOS it seemed a nice price, so I added it to the order. Best I can find is it seems Marconi was a re-brander and didn't make their own tubes. White box special, it's labeled "Made in Canada", so that leaves General Electric or Westinghouse. Trace some history, and Marconi-Osram was a British company wholly owned by GE. Then again, rumor has it that Mullard made 12AU7's for GE back then, so ... Black plate with a square top getter and a date code of "C5" if that helps any.

    PS ... half the fun of finding some strange tube is trying to figure out it's pedigree ... still researching the sKiZo OU812 seen in my avatar ... tongue

    Gregg R.

    Posts : 78
    Join date : 2014-02-20
    Age : 72
    Location : Land of 11,842 Lakes

    Re: VTA ST70 spare tubes

    Post by Gregg R. on Wed Jan 14, 2015 10:31 pm

    sKiZo:

    I finally get it:

    OU812

    Oh, you ate one too!

    Gregg R.

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: VTA ST70 spare tubes

    Post by sKiZo on Wed Jan 14, 2015 10:57 pm

    ... would you like fries with that?  clown


    Gregg R.

    Posts : 78
    Join date : 2014-02-20
    Age : 72
    Location : Land of 11,842 Lakes

    Re: VTA ST70 spare tubes

    Post by Gregg R. on Thu Jan 15, 2015 10:27 pm

    sKiZo:

    I was not aware that OU812 was the title of a Van Halen album. Thanks for filling me in.

    I lost interest in rock music somewhere in the mid-'70s with advent of stadium and glam rock. I recall a member of Bachman-Turner Overdrive stating that they were no different than a guy going to work with a briefcase: BTO carried guitar cases. With MTV becoming dominant in the 1980s, rock music is simply the soundtrack to videos. Grunge bands pumped some life in the genre in the '90s, but its hard to convince me that white suburbanites got that much of a beef.

    Of course, 90% of almost anything is pure crap. Classical musicologists are scurrying to find obscure composers that would best remain forgotten. Privately, I have even questioned the value of some of the posts on this website, but the good stuff is worth it. I just got a nice PM from MontanaWay about the RCA 5693 triodes. Of course, Bob Latino's posts are all first-class, as are yours!!

    Keep up the good humour,

    Gregg R.

    Note: I am not completely negative about rock music: Rock musicians widely use vacuum tube amplifiers for their guitars; this requires factories for replacement tubes; these factories also produce tubes for hi-fi gear; in fact, I am currently trying 6L6GCs in my VTA ST-70; 6L6s are extensively used in guitar amps; I like what I'm hearing!! ROCK ON!!


    Last edited by Gregg R. on Fri Jan 16, 2015 3:35 pm; edited 3 times in total

    bluerondo

    Posts : 27
    Join date : 2014-09-21
    Location : Irvine, CA

    Re: VTA ST70 spare tubes

    Post by bluerondo on Sun Jan 18, 2015 2:06 pm

    I received my order of RCA clear tops and electro-harmonix 12au7's yesterday and I did a quick try on the harmonix, they sound good to me. Unfortunately one of the RCA clear tops had a crack so I need to return it. I plan to do some listening tests with these and do a frequency sweep with resistor dummy loads maybe by end of next week end. I'll try to post some of the scope shots.

    bluerondo

    Posts : 27
    Join date : 2014-09-21
    Location : Irvine, CA

    Re: VTA ST70 spare tubes

    Post by bluerondo on Sun Feb 22, 2015 3:44 pm

    I mainly did this for the fun of it and I should point out I’m not an audio expert so I am sharing this for general interest only.  I performed some frequency measurements on three sets of driver tubes (3 center ones on VTA ST-70) to see if I could see any difference on an oscilloscope. All used electro-harmonix KT88’s for the output tubes. All three sets seemed pretty much the same what I could see in the frequency response and a pulse step response; the fall times were the same as the rise times. There’s a very slight gain difference overall but it was small. For each tube set to get 14 watts out, the input voltage was org= 1.968 p-p, harmonix=2.135 p-p, RCA/CT = 1.925. So the harmonix had slightly less gain than the other two. The one 20-30KHz sweep, pushed near 35W on the RCA/CT’s were pretty much the same on the other two sets. The images are labeled on the top similar to the three descriptions below:

    Setup: Function generator terminated with 47 ohms (didn’t have 50) connected to both RCA inputs of the ST-70. Outputs connected to 8 ohm load resistors. The top trace is the output of the FG. The output on the bottom, sweep rate=1.6sec starting at the up arrow shown on the bottom of the plot.

    http://1drv.ms/1zxGY36

    Set 1 (ORG): Outside: RCA 5963’s, Center: Tung Sol 5814A.

    Set 2 (electro-harmonix):12AU7/ECC82 (new)

    Set 3 RCA clear-tops: NOS

    I can’t hear any difference between them myself, maybe because I don’t have a trained ear or because the time lag between swapping out the tubes makes it more difficult. Anyway, if I had to say these all seem pretty much the same as far as these measurements reveal.

    I used four 2 ohm 10W (40W total) resistors in series for each 8 ohm load. Yes, they got very HOT!
    I measure the maximum power output and it started to clip on both positive and negative peaks right at the same point at 34-35 watts. The three similar 14 watt plots sweep from 20Hz - 25KHz. The other one I sweep from 20-30KHz. The -3db frequency cutoff varied from 50,51,55KHz(RCA’s a smidgen higher) on the three sets. On the low end frequency, the sine wave started to distort funny around 13Hz.

    This was a fun project and I’m really enjoying my VTA ST-70 immensely!

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST70 spare tubes

    Post by Bob Latino on Sun Feb 22, 2015 5:32 pm

    Hi Bill,

    Your measurements of the VTA ST-70 coincide with measurements done for me a few years ago ... Your minus 3 dB bandwidth of the VTA ST-70 at 13 Hz and about 50 KHz is about right .. Ar rated power (35 WPC) harmonic distortion is below .5 % across the audio band. Square waves also show no overshoot ..

    Bob

    Tube Nube

    Posts : 641
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: VTA ST70 spare tubes

    Post by Tube Nube on Wed Feb 25, 2015 9:00 pm

    :-/

    (Just another crap post from me)

    Sponsored content

    Re: VTA ST70 spare tubes

    Post by Sponsored content Today at 10:28 pm


      Current date/time is Sun Dec 04, 2016 10:28 pm