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    12AT7 Tube Testing Importance

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    Reid

    Posts: 9
    Join date: 2009-02-05

    12AT7 Tube Testing Importance

    Post by Reid on Fri May 01, 2009 7:00 pm

    With the VTA board how important is it to have all three balanced or matched?

    Your thoughts please.

    Bugs

    Posts: 91
    Join date: 2009-04-17

    Re: 12AT7 Tube Testing Importance

    Post by Bugs on Sat May 02, 2009 10:45 am

    Reid wrote:With the VTA board how important is it to have all three balanced or matched?

    Your thoughts please.

    My understanding, gained from reading other posts on this forum is that the center tube on the VTA board has the most influence on the sound of the amp. The two outside tubes should be matched and the center tube can be rolled to find the best sound.

    Some of the other posts talk about this.

    Best,
    Bugs

    Bob Latino
    Admin

    Posts: 1956
    Join date: 2008-11-26
    Location: Massachusetts

    Re: 12AT7 Tube Testing Importance

    Post by Bob Latino on Sat May 02, 2009 5:53 pm

    Bugs is correct here. The center 12AT7 is the initial voltage amplifier for BOTH channels and is the most important of the three 12AT7 tubes on the VTA driver board. This tube determines a lot about the sound character of the amp. The left and right 12AT7 tubes are the phase splitter/phase inverter tubes for their respective channels and have less impact on the sound of the amp. One of the best tubes you can use in this center position is the Mullard CV4024 which is a 12AT7 equivalent tube. These were made by Mullard in the 1980's for the British military and had to pass stringent tests for noise, hum etc. and the two triode sections are usually fairly closely matched. Some are not labelled "Mullard" but instead are labelled "Valve Electronic" but are still "made by Mullard". Because they are relatively plentiful, they are not that expensive at $25 - $30 a tube and sometimes you can find them for $20 a tube. There is a fellow on Ebay who sells under the name "bnb tubes" who sells them for $19.50 a tube. I just picked up a pair of them from bnb tubes about a week ago. He usually has them up there all the time. Link to his Ebay store is below ...

    bnb_tubes is now up to $22 for the Mullard CV4024 tubes. At $22 they are still a good buy ... (12/8/10)

    Ebay store for bnb tubes

    Bob


    Last edited by Bob Latino on Wed Dec 08, 2010 9:37 am; edited 1 time in total

    Reid

    Posts: 9
    Join date: 2009-02-05

    Re: 12AT7 Tube Testing Importance

    Post by Reid on Sun May 03, 2009 7:00 am

    This is what I understand. The left and right tubes should be a matched pair and the center tube should have balanced triodes. The center tube can be rolled to experience sonic variances of the tubes. The most exact testing should apply only to the center tube and less expensive tubes used in the left and right position.

    I have some Mullard tubes. Some of them are "Matched" and others are "Matched and triode balanced". I should therefore keep the balanced tubes for the center and use the matched for the left and right positions.

    Great forum, thanks to all.

    Tube Nube

    Posts: 328
    Join date: 2008-12-06
    Age: 51
    Location: Calgary, AB

    Re: 12AT7 Tube Testing Importance

    Post by Tube Nube on Sun May 03, 2009 10:52 am

    I just ordered some 12au7 tubes for my pre-amp (Transcendent), but failed to opt for balance or match testing (options that are a buck or two each).

    I wonder if that was a serious lapse in judgment.

    Bob Latino
    Admin

    Posts: 1956
    Join date: 2008-11-26
    Location: Massachusetts

    Re: 12AT7 Tube Testing Importance

    Post by Bob Latino on Sun May 03, 2009 2:19 pm

    Tube Nube wrote:I just ordered some 12au7 tubes for my pre-amp (Transcendent), but failed to opt for balance or match testing (options that are a buck or two each).

    I wonder if that was a serious lapse in judgment.

    You will most likely be OK. Most dual triode tubes come through with the sections fairly close together.

    One other note on the Mullard CV4024 (12AT7 equivalent) tubes mentioned above. All Mullard small signal tubes (12AU7, 12AT7, 12AX7 etc.) will give the "Mullard flash" when you first turn the amp on when the tubes are COLD. The filament will glow brightly for about a second and then settle down. This is NORMAL for genuine Mullard small signal tubes and is actually a way of telling genuine Mullard 12A*7 from cheap copies from China that may be labelled "Mullard". The Mullard filaments are constructed in a different manner and the "flash" is not harmful to tube life. Some Amperex tubes were made by Mullard and will also flash when first turned on. When the tubes are warm they won't flash.

    Bob


    Last edited by Bob Latino on Wed Dec 08, 2010 9:38 am; edited 1 time in total

    LEVLHED

    Posts: 21
    Join date: 2009-10-26

    Re: 12AT7 Tube Testing Importance

    Post by LEVLHED on Sat Nov 21, 2009 12:52 pm

    I currently have 4 different 12AT7's and 3 different places to put them. The front of a Dared SL-2000A pre, and the voltage amplifier (first middle) and phase splitter/phase inverter position (right/left) of a VTA ST-70.

    The four types of tubes:

    (3) JJ 12AT7 (came with the ST70)
    (2) Telefunken ECC801C
    (3) Mullard 12AT7 (cyro treated)
    (2) Radiotechnique 12AT7 (came with the Dared)

    It took about two days to try every conceivable combination.
    I found the Mullards lacked a certain sparkle (too dull) to put in the ST-70.
    My preferred combination with these options:

    (2) Mullards in the Dared pre amp
    (1) Telefunken in the middle of the ST-70
    (2) JJs on the sides of the ST-70

    To my ears this gives me the right balance of definition, clarity and sparkle with out a mid-range honk or harshness. I was actually surprised that I preferred the JJ's in that position. I wanted to like any of the other ones more, but my ears told me different! I am curious what else give the same JJ sound but "better", they seem to provide that last little bit of high end shimmer without pushing it over the edge.

    If you don't know about the Dared pre and are in the market for something, I can highly recommend one of these for a kick ass budget tube pre. I do have one of the 2005 versions w/the Auricaps. I love that it has 2 sets of outputs so I can drive powered subs and has a remote control for volume. Pretty much all I need in a pre. And sounds damn good too with some NOS tubes. I put an RCA 5V4G in for the rectifier.

    My next change will be a Weber SS rectifier for the ST70. Then I will look into rolling in some different output tubes, replacing the EH EL-34's that came with my ST-70. I just have to wait until after Christmas to see what Santa brings. afro

    Bugs

    Posts: 91
    Join date: 2009-04-17

    Re: 12AT7 Tube Testing Importance

    Post by Bugs on Sat Nov 21, 2009 1:18 pm

    I have a VTA ST-70. It's the low gain version so I'm running 12AU7s instead to the 12AT7s. I run JJ ECC 82s on the outside and a gold pinned (really don't know if gold pins make a bit of difference) JJ 802s on the inside. I've tried rolling some NOS RCAs through it, but I'm happy with the JJs.

    I put in as Weber Copper Cap a few weeks ago and I'm happy with it. Like its been stated before, you need to readjust your bias (down) and I feel like it delivers a little more bass punch, however I have no way of measuring it.

    I might try some Electro Harmonix 12BH7s (cheap experiment), but that is another story.

    LEVLHED

    Posts: 21
    Join date: 2009-10-26

    Re: 12AT7 Tube Testing Importance

    Post by LEVLHED on Tue Nov 24, 2009 10:36 am

    is it possible to use either 12AU7 or 12AX7 tubes in either the voltage amplifier or phase splitter positions of the high gain board?

    If so, what considerations should be made?

    Bugs

    Posts: 91
    Join date: 2009-04-17

    Re: 12AT7 Tube Testing Importance

    Post by Bugs on Tue Nov 24, 2009 11:25 am

    To run the 12AU7s you need to have the low gain version of the VTA baord. I believe that the only difference is a few resistor values. There is recent information on it if you look through this forum.

    edgobb

    Posts: 54
    Join date: 2010-11-09
    Location: Texas

    Re: 12AT7 Tube Testing Importance

    Post by edgobb on Tue Dec 07, 2010 10:33 pm

    Bob or anyone-

    Do you have any recommendations on good tubes that have worked well for you in the outside positions of the VTA board? I've got one of the Mullard CV4024s for the center voltage amp job.

    tia
    -ed

    (i know this thread might be dead, but I thought it was worth a try.) Very Happy

    erlingt

    Posts: 13
    Join date: 2010-06-23
    Location: Denmark

    Re: 12AT7 Tube Testing Importance

    Post by erlingt on Wed Dec 08, 2010 2:33 am

    I'm using the Mullard CV4024 for the outside positions as well.

    That is, I'm using 3 CV4024, and it is -until now - the best combination I've had in my ST-70.

    mantha3

    Posts: 286
    Join date: 2010-11-10

    Re: 12AT7 Tube Testing Importance

    Post by mantha3 on Wed Dec 08, 2010 8:42 am

    I talked to Bob about tubes on one of my too many calls I made to him while I did my ST-120 build. I'm still feeling a little guilty for calling him too many times.

    Bob said the outside positions of the VTA board do well with a Phillips JAN and a Mullard in the center. I'm running that now and it rules (In my opinion). The Philips you can find pretty cheap and they are a solid NOS tube that for now are still plentiful.

    Love my amp!

    tdembsky

    Posts: 1
    Join date: 2011-01-19

    Re: 12AT7 Tube Testing Importance

    Post by tdembsky on Mon Jan 24, 2011 12:46 am

    Hello, just picked up my first Valve Amp (purchased the ST70 from Levlhed) Sounds great. As I was working with the amp, I found out how to bias the AT66's with the bias pots on the upper surface of the PCB. My question is, on the underside of the PCB there are two bias pots (blue in color) I am assuming that these are for the 12AT7's but I cannot find find out any documentation on this. How do check the BIAS of the 12AT7's and what should the bias be set at. What pins, or leads to I check with the multimeter and what is the proper bias reading for the 12AT7's

    Thanks !!!

    I am running the bias on the AT66 at .470 and they sound great --

    Bob Latino
    Admin

    Posts: 1956
    Join date: 2008-11-26
    Location: Massachusetts

    Re: 12AT7 Tube Testing Importance

    Post by Bob Latino on Mon Jan 24, 2011 7:19 am

    Those two pots on the bottom of the driver board are the AC balance controls which adjust the phase splitter section of the driver circuit. Don't touch them unless you have a signal generator or function generator and know what you are doing. The instructions on how to adjust them are at the link below.

    Adjusting AC balance controls on a VTA driver board

    Even if you readjust them it will make no discernable difference in the sound of the amp. In short, the method of setting them at their center point as outlined in the kit manual is for all intents and purposes the optimum setting.

    Bob

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