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    VTA Driver board mods

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    Andy Jersey

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    Join date : 2012-09-29

    VTA Driver board mods

    Post by Andy Jersey on Wed Jun 19, 2013 5:24 pm

    I was reading a custom VTA ST 120 Thread on AK and the conversation migrated to the driver board. I very respected member who has a lot of experience building , modding repairing , and building from scratch amps made these recommendations. Can anybody shed anything on the changes and the result? (these are cut and pastes of three posts)


    The "basic" story on the low gain vs high gain.

    I got in early on the VTA board, used it and liked but the gain really is just to high.
    (makes the amp noisier then it needs to be and to sensitive for most people there)

    I set out to change it, an make it more tube roll friendly, the story went from there.
    (want/need any more info then that you can PM me)

    Basically the Low Gain version is "normal gain" and the High Gain is, Super High Gain.
    The Low gain version is just fine for passive pre/attenuator as well, it's not low gain.

    I would recommend the "low gain" version for any VTA build, it is smoother, easier to
    use an it opens up quite a bit of different tubes that can be subbed/rolled on into it.


     

    Also on the "low gain" version watch out for R13 an R14, anything higher than 5k
    there and you'll get to low of voltage to the plates of the middle tubes, I say this
    because I've seen some boards, amps built with to large of resistors/voltage drop.
    (that's the RC stage resistors feeding the input tube there voltage on the boards)


     

     

    With 30k R13/R14 combined with the power supply RC stage you
    get less than 120v on the 12BH7 an less than 140v on an 12AU7.
    (you can run either in the center spot, pretty low on the 12BH7)

    Unless your running an direct coupled stage those voltages don't
    need to be that low, the tubes will perform an sound better with
    more voltage to there plates, and that has been what I've found.

    So yes the 30k R13/R14 is lower then I'd run it, at 4.7k there it'll
    get about 160v on the 12BH7 and then about 180v on the 12AU7.

    Those are "better" voltages and is what I'd run if using that setup.

    -------------------------
    Not saying "do this" in the following section here, but.....

    Now me personally I run an "Little Different" plate load and cathode
    resistors on that center tube since this low gain mod was first made.

    It's tweaked to better run a E80CC but still run the 12AU7 an 12BH7
    in there as well and perform as they should just like the other tweak.
    (all those tubes are rollable in that center spot, I prefer the E80CC)

    So on mine it is the same 4.7k at R13, then 43k at R11 an R9 at 1.2k
    (so that's, RC stage resistor to that section, plate load an cathode)


     

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

    Re: VTA Driver board mods

    Post by Andy Jersey on Wed Jun 19, 2013 8:43 pm

    As an experiment I checked voltages based on schematic of driver board. I'm reading 353v where I should have 375V (load side of 4.7k 3w from B+) 230V down stream of R13 should be 253V and then 111V downstream of R11 should be 136V . I did all readings from top side of driver board. It seems that either B+ 480v is low or I'm getting too much voltage drop across 4.7K 3 w resistor. Maybe i'll get a chance to pull bottom and see what's going on. Anybody have any thoughts?

    Bob Latino
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    Re: VTA Driver board mods

    Post by Bob Latino on Thu Jun 20, 2013 6:41 am


    If your B+ off pin #8 of the rectifier tube is 480 volts then your B+ is a little low. With 120 volts IN using a GZ34 tube rectifier, you should be at 490 - 505 volts or so. Try using another rectifier tube OR try using the Weber WZ68 solid state rectifier. With the Weber, your B+ off pin #8 should be slightly over 500 VDC ..
     
    Bob

    Andy Jersey

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    Join date : 2012-09-29

    Re: VTA Driver board mods

    Post by Andy Jersey on Thu Jun 20, 2013 9:11 am

    Thanks Bob, I'll try and get a look at it tonight. I am using a weber so I'm guessing that's not my issue. When I get the bottom off I'll know better. Any thoughts on the first part of the post?

    Bob Latino
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    Re: VTA Driver board mods

    Post by Bob Latino on Thu Jun 20, 2013 9:21 am


    What part of the post are you referring to ?
     
    Bob

    Andy Jersey

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    Join date : 2012-09-29

    Re: VTA Driver board mods

    Post by Andy Jersey on Thu Jun 20, 2013 10:44 am

    The post says R13,14 are too high and that the au/bh7s would benefit from higher plate voltages

    Bob Latino
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    Re: VTA Driver board mods

    Post by Bob Latino on Thu Jun 20, 2013 10:51 am


    Kegger on Audiokarma has his ideas about what he feels the plate voltages should be and Roy and I have our ideas. If you run these tubes a little more conservative on the plates, the tubes will last longer.
     
    Bob

    tubes4hifi
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    Re: VTA Driver board mods

    Post by tubes4hifi on Thu Jun 20, 2013 12:24 pm

    also remember that in most tube circuits, voltages can easily be 10% off or sometimes even more, due to differences in tube characteristics.
    Absolute voltages are not critical, tube circuits have a very wide area of working conditions.

    sKiZo

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    Location : Michigan USA

    Re: VTA Driver board mods

    Post by sKiZo on Thu Jun 20, 2013 2:59 pm

    Replace all the resistors with VR's and have a ball ....


    (owwww)

    (Thanx ... I needed that. rabbit

    arledgsc

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    Location : SF Bay CA

    Re: VTA Driver board mods

    Post by arledgsc on Thu Jun 20, 2013 4:46 pm

    Before everyone starts ripping up their driver boards take a look at the Philips E80CC datasheet and the tables for audio frequency triode amps that indicate lower distortion at lower and lower plate voltages.   See page 5 Table 1 and also the footnote at the bottom of the page.  Philips E80CC Datsheet  The tables stop at 200V on the plates but indicates better distortion as plate voltage goes down if you can extrapolate.   I've also seen 12AU7 datasheet charts with curves at 90V plate voltage.  The gain of the preamp is only about 10x so we aren't going to be clipping its output at even 90V plate voltage.  The 43k ohm plate resistor in your amp (with 1.2k ohm cathode) increases the preamp circuit gain about 1dB over the stock circuit.  With increased gain comes a proportional increase in noise floor as well.  The E80CCs also double the heater current requirement over 12AU7s which is something else to consider especially if you use three E80CCs.

    But the E80CCs could be loading the power supply differently and require a circuit mod for best operation and optimum sound.   After six months since building I finally have my ST-120 tweaked up beyond my wildest imagination.  So wouldn't want to jinx the mojo by any circuit changes on a whim or hunch.  The E80CCs are supposed to be very good audio tubes though.  Good luck and let us know how it turns out.

    Andy Jersey

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    Join date : 2012-09-29

    Re: VTA Driver board mods

    Post by Andy Jersey on Thu Jun 20, 2013 5:05 pm

    OK just for the record, At 125V AC I'm reading 460V at pin #8 of rectifier tube a Weber wz68.  tried putting 12k (2 6k 3w resistors in series across 4.7k 3w B+ which put me close to spot on for voltages down the line. I'm a little curious why my starting voltage is light.
    arledgsc I toyed with E80cc early on with all stock values and never found it to be neutral in my experiments (I think I tried 3 variants, a siemens , a Philips and I don't remember either a Mullard or a Brimar. I tried them single in the middle with each other and against an assortment of BH and au7s I never found anything close to what I was looking for.
    For Bob and Roy, I never intended to to make this some kind of pissing match. I was merely curious whether you had experimented at different voltages before you settled where you did or perhaps for the sake of reliability (tube life) made some kind of educated compromise.
    Lastly do you feel its worth permanently dropping the resistance on the B+ 4.7 K to more closely achieve the operating voltages (me not everyone).What I meant was I have low voltage so I would address it. I'd only suggest it to others who might have same problem

    arledgsc

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    Re: VTA Driver board mods

    Post by arledgsc on Fri Jun 21, 2013 10:28 am


    OK just for the record, At 125V AC I'm reading 460V at pin #8 of rectifier tube a Weber wz68.
    I measure 133Vdc at the center tube preamp plates with 12AU7s all the way around, WZ68, and 119Vac line voltage.   So I'm right at nominal for the ST-120 design.  For a test replace the E80CCs with 12AU7s and re-measure the B+ voltages.   The B+ supply is really high impedance so would be very sensitive to any changes in current through the tubes.

    Andy Jersey

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    Join date : 2012-09-29

    Re: VTA Driver board mods

    Post by Andy Jersey on Fri Jun 21, 2013 3:04 pm

    I think there's a little confusion. While I have run E80ccs. I've been running 12BH7s in all three positions. (I didn't write the other post I just copied and pasted it). My personal experience with E80ccs was underwhelming. My amp is sitting on  a high shelf so its a little inconvient to pull bottom again to test with au7s but in the near future I'll double check.

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Low gain only?

    Post by mantha3 on Fri Jun 21, 2013 3:48 pm

    Bob,

    What influenced the decision to go away from the high Gain VTA and now only offer the Low Gain?

    I run some 91 dB speakers and I drive the ST120 I have with a passive attenuator with a unity gain buffer after the attenuator.  I do this cause I have the ST120 and a Subwoofer from this set up.  

    I did the resistor mod a year ago to try out the low gain and I went back to hi gain.  Liked the sound better (Low gain did sound excellent too) preference wise.

    Bob Latino
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    Re: VTA Driver board mods

    Post by Bob Latino on Fri Jun 21, 2013 4:41 pm

    Hi,
     
      Roy and I found out that the gain with the 12AT7 board was a little too much gain for some preamps. Some customers reported that they couldn't get by 9 or 10 o'clock on their preamp's volume control before the sound got quite loud. The newer 12AU7 board has lower gain and works better with a wider variety of preamps. Some people like the higher gain board because the loudness does come on quicker and may give the impression of "more power" - BUT - the volume control "range" is much more limited with the higher gain board.
     
    I have used both the high and low gain boards with my own VTA ST-120 and I can't really say I prefer one over the other. The sound is the same IMHO. Sometimes the "difference" that you might hear are subtle differences between the brands of driver tubes you are using. I have the low gain CCS board now in my own VTA ST-120. I have used 12AU7, 12BH7, 5814, 5963, 6189 and the Mullard CV4003 for driver tubes in my own amp. They ALL sound good. Right now I have a Mullard CV4003 in the center position and two GE 12BH7's in the two side positions. Other tubes that have sounded really good are the Sylvania and GE 5814's, Sylvania 6189's and some RCA 12AU7's that I tried.
     
    As Roy and I have said, the CENTER (voltage amplifier) driver tube on the VTA ST-70 and VTA ST-120 has more influence on the sound of the amp than the two side (phase splitter/inverter) tubes. If you want to make subtle alterations in the sound on your VTA ST-70 or VTA ST-120 try some different center driver tubes. On the VTA M-125 monoblocks it is the FRONT tube that has more influence on the sound than the rear tube.
     
    Bob

    sKiZo

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    Location : Michigan USA

    Re: VTA Driver board mods

    Post by sKiZo on Fri Jun 21, 2013 5:15 pm

    Having one board that works well with most everything certainly makes sense to me. If you'll remember back a way when I was first contemplating the ST-120, I was thoroughly confused between high and low boards, and the tube types each supported. I got a box of 12AT7's stashed in the basement to prove it. Rolling Eyes

    Let's call the current production board an "evolution" ... I like the fact that it supports a greater selection of tubes, and I'll leave it to others to decide which are better. I know it'll probably cost me, as I'm a habitual tube roller ... woe unto me ... hee hee ...

    I ran into the same thing with my Maverick TubeMagic DAC ... Some had concerns with the volume going off the scale on the headphone side if a butterfly flapped it's wings or a mouse farted anywhere in the vicinity. Some tweakers came up with solutions, and good on them for sharing. Not something that would drive a board redesign though. 

    Anyway ... that's why they call them mods. Try something out - if it works, share ... if somebody else tries it, do so with the knowledge that it's at your own risk. 

    Speaking of board redesigns ... where are you on modifying the amp to fully support the KT-120 at full drive?

    |BIFF!|
    (owwww)

    (Thanx ... I needed that) drunken

    Bob Latino
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    Re: VTA Driver board mods

    Post by Bob Latino on Fri Jun 21, 2013 5:48 pm


    Re: KT120 output tubes. The VTA ST-120 is designed to use KT88 or 6550 output tubes and may also use the Tung-Sol KT120 at strictly KT88 or 6550 voltages. There are no plans to modify the VTA ST-120 to use KT120 output tubes at "full drive". Doing so would take away the amps ability to use KT88 and 6550 tubes. What if Tung-Sol (the only manufacturer of KT120 output tubes) decided to stop production of the KT120 ? You would be stuck with an amp without a current manufacturer of tubes ... Not a good thing .. KT88 and 6550 output tubes are made by a number of manufacturers and are more easily sourced ...
     
    Consider this ... Mike Sanders of Quicksilver Audio produced some nice monoblock amps in starting in 1984 that used 8417 output tubes. Unfortunately later in the 1980's they stopped making 8417 output tubes. Because of the difference in tube characteristics, 8417 output tubes are not replaceable with 6550 or KT88 output tubes unless you rework the circuit. There are now still many of these great amps out there that most people steer clear of because they use an "orphan" output tube. Last I heard Mike would redo the circuit to use more easily found tubes (EL34 or KT88) for $250 or so ..
     
    Bob

    sKiZo

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    Re: VTA Driver board mods

    Post by sKiZo on Fri Jun 21, 2013 6:35 pm

    Just yankin' yer chain there, Bob ... pirat

    I've been reading thru Vacuum Tube Valley (old stuff, so no rush) and just read an article about the vaunted RCA 6146 which was heralded as the new generation high power tube when it came out. Expensive as hell, so not many were produced, and what was produced ended up mostly in commercial equipment. Apparently, the only "home" amp to use it was the Altec 1530. 



    Calling that beast a "home" amp was a bit of a stretch too. 

    Here's hoping the KT-120 doesn't disappear to the land of lost tubes and left socks ... Apparently the KT-150 already has? Been rumored for production for years now - kinda like the video game Duke Nukem Forever ... people waited ... like ... forever for it to come out, and once it did, it wasn't much to speak of - certainly not up to the hype.


    Last edited by sKiZo on Fri Jun 21, 2013 6:42 pm; edited 2 times in total (Reason for editing : Added pic ...)

    mantha3

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    Re: VTA Driver board mods

    Post by mantha3 on Fri Jun 21, 2013 6:37 pm

    All good Bob and SkiZo.  I was just curious what the reason was behind the Low Gain standard.  I don't know if I could hear the difference between the Hi and Lo gain when I was screwing around with these VTA changes.  I used the 4003 Mullard center.  Good stuff and good sound!

    I was reading about that 8417..  It was an interesting tube.  


    I was reading about a EL156 today too...  Is that a KT88 sub?

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