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    Bias voltage for ST70 with VTA driver board?

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    tygr1

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    Bias voltage for ST70 with VTA driver board?

    Post by tygr1 on Thu Dec 11, 2014 7:08 pm

    I just picked up an ST70 with the 12AU7 VTA driver board. What bias voltage am I looking for on the EL34s?

    Bob Latino
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    Re: Bias voltage for ST70 with VTA driver board?

    Post by Bob Latino on Thu Dec 11, 2014 7:20 pm

    Set each EL34 output tube's bias at .400 volts DC ..

    Bob

    tygr1

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by tygr1 on Thu Dec 11, 2014 7:26 pm

    Thank you Bob!

    MexicoMike

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by MexicoMike on Fri Dec 12, 2014 9:40 am

    Just curious - I don't understand circuitry so forgive my ignorance...

    On the stock 70, the bias for two EL34s is 1.56V so I would ASSUME (incorrectly?) that the bias for one tube would be 1/2 of that. So why would the individual bias be .4 instead of a bit under .8?

    GP49

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by GP49 on Fri Dec 12, 2014 11:31 am

    Because there is a different amount of cathode current being specified, 40mA per tube vs. 80mA per tube. Dynaco's original called for more cathode current than what's being recommended by Bob and others. Lower cathode current increases tube life. It also heats and stresses the tube less. It is known that old stock, original Mullard EL34s were more rugged and durable than current production, including the imitation "Mullard" EL34s being made now, so the slightly higher distortion (I think you'd notice it only on test equipment, not by ear) from running at 40mA is probably a good tradeoff for improved tube reliability and service life.

    MexicoMike

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by MexicoMike on Fri Dec 12, 2014 12:01 pm

    Thanks for the explanation. Any idea what the "expected" EL34 tube life difference might be between the lower bias and original bias? I was under the impression that the EL34 wasn't really stressed that much in an original ST70. At least, I read that on the internet, so I assumed it MUST be true! Smile

    GP49

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by GP49 on Fri Dec 12, 2014 1:15 pm

    I have no hard information about how much longer a tube would last at lower cathode current. The ratings for the original Mullard EL34 include a VERY high B+ of 800V; I'd expect that if you put 800V on a current-production EL34 from anybody, the chances are it would arc over. However, assuming that the B+ voltage is within reason, it will be heat from the idling cathode current that makes the difference. 40mA is HALF of 80mA and heat would be proportional to the amount of current, all else being equal. Power, therefore heat, is proportional to current.

    This only affects the tubes at IDLE; when playing music the current is changing all the time, and mostly above the idling cathode current.

    PeterCapo

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by PeterCapo on Fri Dec 12, 2014 1:46 pm

    Sorry, but I am a little confused.  If we are talking about the original Stereo 70, Dynaco's "biaset" procedure had you set 1.56 volts across the 15.6-ohm bias resistor, which was 100mA per EL34 pair, which is 50mA per EL34, not 80mA per tube, correct?

    Based upon the voltage chart in the Dynaco manual, and also allowing for a higher AC line, let's say the plate voltage is around 450 VDC.  Assuming I am applying the math correctly, 450 VDC * 50mA = 22.5W plate dissipation.  Or,  450 VDC * 40mA = 18.0W plate dissipation.

    EL34s, even today's, are usually rated for at least 25W plate dissipation, and some are supposedly tougher, for example the EH 6CA7 is supposedly very rugged, or so I have read.

    I don't think it's as much an issue of the voltage on the plate as it is the power dissipated in the plate that wears a tube out?  But, do power tubes usually die from the plates being worn-out from power dissipation or do they more often die when the cathode coating can no longer supply as much of an electron cloud as when the tube was new?

    So, you get a savings of 4.5W power dissipation in each EL34 plate by dropping the bias current by 10mA, again, if I have applied the math correctly.   Not sure if current production power tubes really wear out that much sooner than old stock tubes due to power dissipation issues vs. other failures that I seem to read about most of the time..?

    PeterCapo

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by PeterCapo on Fri Dec 12, 2014 4:47 pm

    Actually, let me adjust that math.  I think I was too high in estimating the plate voltage at 450 VDC.  Let's go with the baseline reference taken from the original Dynaco manual, which is 410 VDC with the understanding this can vary somewhat depending on which power transformer is in use, what the AC line voltage is, and potentially other factors like whether or not a thermistor is in use:

    410 VDC plate * 50mA bias = 20.5W plate dissipation per EL34

    410 VDC plate * 40mA bias = 16.4W plate dissipation per EL34

    Not sure I'd sweat the tube longevity issue, based upon this.  If I missed anything, please let me know.

    GP49

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by GP49 on Fri Dec 12, 2014 5:28 pm

    I must admit, Peter is right. I have to stop trying to do calculations in my head using parts values from memory!!!

    Forget that 80mA stuff...

    Ed Rosenberger

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by Ed Rosenberger on Fri Dec 12, 2014 10:51 pm

    Bob Latino wrote:Set each EL34 output tube's bias at .400 volts DC ..

    Bob

    This may be a dumb question but all output tube references to bias voltages (.4 vdc) that I've seen have been for EL34 tubes.

    I'm using the VTA octal driver board and the instructions also call for a .4 vdc output bias when using EL34 tubes.
    I have new EH 6CA7 output tubes.....so does the bias voltage remain the same for these four tubes?

    I am not to the power-on stage yet.....just kinda looking ahead.

    stewdan

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by stewdan on Sat Dec 13, 2014 12:41 am

    Hi Ed --- The 6CA7 is "equivalent" to the EL34, so the bias is the same.
    Stew

    Ed Rosenberger

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by Ed Rosenberger on Sat Dec 13, 2014 2:09 am

    stewdan wrote:Hi Ed --- The 6CA7 is "equivalent" to the EL34, so the bias is the same.
    Stew

    Okay Stew....thank you for taking the time to relieve one of my "don't have a clue" anxieties.

    Wish knowledge could be gained through brain-wave osmosis.....I'm almost too old for using books Wink.

    audiobill

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by audiobill on Sat Dec 13, 2014 5:47 am

    Books? What are those?

    Ed Rosenberger

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by Ed Rosenberger on Sat Dec 13, 2014 5:41 pm

    audiobill wrote:Books?  What are those?

    Yeah....kinda gives away our extended age huh Wink. Actually I haven't been to a library or opened the cover of a book since I could type my topic into a search engine like google. Rather sad statement.

    If I end up wishing for better sound and want to try different output tubes (for some reason this is called "tube rolling"), I will need to find out how to tell if a tube is ("equivalent" to the EL34), before I can do anything productive.
    I'll learn.....don't no how long I'll retain it though....Wink.

    dmtparker

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    Correct bias??

    Post by dmtparker on Tue Jan 20, 2015 6:51 pm

    Bob Latino wrote:Set each EL34 output tube's bias at .400 volts DC ..

    Bob
    I have seen several places, your dictum to set bias @ 40mA/tube, but David Gillespie in his article on "Dynaco ST-70, Baseline Testing" says the following:
    "For example, with a Biaset voltage of 1.40 vdc (45 ma total current flow per tube), 1 kHz THD climbs nearly 55%. If you reduce it further to 1.25 vdc (40 ma per tube) it climbs another 170%, for a total of 225%! So, reducing the current draw of each tube by 6% of its rated cathode current, when it is already operating at just 30% of rated cathode current and 62% of rated plate dissipation, all to gain what, 225% more distortion? That is a very poor compromise to make, versus the very well thought out operating conditions that Hafler set up for the tubes to operate at."
    That seems like an awful trade-off. What gives? Do you have different measurements of THD @ 40mA vs. 50mA? I'm sure my original ST-70 had 100's & 100's of hours on it @ 50mA bias as it was my main amp all through college and I never replaced the tubes. No

    Bob Latino
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    Re: Bias voltage for ST70 with VTA driver board?

    Post by Bob Latino on Tue Jan 20, 2015 7:36 pm

    dmtparker wrote:
    Bob Latino wrote:Set each EL34 output tube's bias at .400 volts DC ..

    Bob
    I have seen several places, your dictum to set bias @ 40mA/tube, but David Gillespie in his article on "Dynaco ST-70, Baseline Testing" says the following:
    "For example, with a Biaset voltage of 1.40 vdc (45 ma total current flow per tube), 1 kHz THD climbs nearly 55%. If you reduce it further to 1.25 vdc (40 ma per tube) it climbs another 170%, for a total of 225%! So, reducing the current draw of each tube by 6% of its rated cathode current, when it is already operating at just 30% of rated cathode current and 62% of rated plate dissipation, all to gain what, 225% more distortion? That is a very poor compromise to make, versus the very well thought out operating conditions that Hafler set up for the tubes to operate at."
    That seems like an awful trade-off. What gives? Do you have different measurements of THD @ 40mA vs. 50mA? I'm sure my original ST-70 had 100's & 100's of hours on it @ 50mA bias as it was my main amp all through college and I never replaced the tubes. No

    Yes - BUT - Mr. Gillespie is working with an ST-70 with a STOCK 7199 (or 6GH8A) driver board. All his facts and figures relate to the stock driver circuit. The VTA driver board is a different animal and has lower distorion than the stock driver board at all drive levels .. maybe biasing that 7199 driver circuit at higher bias levels (55 milliamps per each EL34) does reduce distortion but it will surely lead to shorter tube life than biasing at 40 milliamps.

    Answer this one > If the original 7199 driver circuit was as "good" as Mr. Gillespie says, how come no other manufacturer besides Dynaco ever used this 7199 "one driver tube per channel" circuit ? Dynaco used this 7199 driver circuit not because it was good, but because it saved one tube per amp. A saved 7199 driver tube @ $1.50 (1960's pricing ) X 350,000 amps = over 1/2 million (1960's dollars) saved. When the whole ST-70 kit with tube cage and all tubes in 1960 costs $99 and you save $1.50 it means something.

    Bob

    PeterCapo

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by PeterCapo on Tue Jan 20, 2015 9:20 pm

    If it is indeed true that no one but Dynaco used the 7199 in the same way, and if we try to take this as an indicator of an economics-over-sound-quality purpose, then it is similar to making an argument from silence.  Lack of specific evidence for why others did not use the 7199 in the same way Dynaco did is not necessarily evidence for or against the quality of the Stereo 70.

    FWIW, Frank Van Alstine's vaunted $2,000 Ultravalve amplifier presently uses a single pentode/triode tube per channel like the original Stereo 70 does - 6GH8A instead of 7199.  Same church, different pew.    But, what does it matter, anyway?  The 7199, even by itself in each channel on the driver board, still results in a mighty fine sounding amplifier - at least in the opinion of some, perhaps many, even today.  IMO, I don’t think the Dynaco folks would have accepted cost savings at significant expense of sound quality.  They would have never been able to stand out as the “poor man’s McIntosh,” or the poor man’s Marantz for that matter, if it was primarily about cost savings at the expense of sound quality, even in those days.  The whole point of the Dynacos was to get you most of the way to the McIntosh or Marantz – both still superb-sounding amplifiers today - for a lot less money.  The fact that they produced such a stable, good-measuring and good-sounding amplifier so efficiently (i.e., one 7199 per channel), attests to their technical insight.  I think they really knew their stuff.

    The business about the power tube biasing...  Let’s look more at plate power dissipation, bias current multiplied by plate voltage, to get a sense for how hard the tube is being driven.  For anyone concerned about the difference between biasing each tube at 40mA vs. 50mA, try it at each setting and see what you think.  In either case, I am not overly concerned about this one.

    The original stereo 7o is what it is and the various mods are what they are.  Let’s make a fair evaluation of all of them, on a level playing field.

    dmtparker

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by dmtparker on Wed Jan 21, 2015 9:31 am

    Bob


    Yes - BUT - Mr. Gillespie is working with an ST-70 with a STOCK 7199 (or 6GH8A) driver board. All his facts and figures relate to the stock driver circuit. The VTA driver board is a different animal and has lower distorion than the stock driver board at all drive levels .. maybe biasing that 7199 driver circuit at higher bias levels (55 milliamps per each EL34) does reduce distortion but it will surely lead to shorter tube life than biasing at 40 milliamps.


    Bob

    I am confused. When setting the bias - either on the original or on a VTA mod - I thought you were only setting bias for the EL34's, NOT changing the bias on the driver board as well. If that is the case, Gillespie's eval is pertinent regardless of driver board. I am sure the VTA has much lower distortion than the 7199, but if you only change the bias to the EL34's, any CHANGE in distortion is coming from the EL34's as a result of different bias, NO?

    Bob Latino
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    Re: Bias voltage for ST70 with VTA driver board?

    Post by Bob Latino on Wed Jan 21, 2015 10:25 am

    What Mr. Gillespie is saying is that the stock driver circuit "requires" that the output tubes be at a higher bias setting in order for the stock driver circuit to drive the output tubes at a lower distortion level. The VTA driver circuit does not require that the output tubes be biased that high in order to get the most out of the driver circuit.

    One of the "advantages" of biasing higher is that the amp will stay in class A (both output tubes conducting) at slightly higher volume levels before switching to class AB1 (each output tube of the pair conducting on only one half the cycle). When the amp is working in class A it WILL have somewhat lower distortion levels. This is really the only advantage of biasing at higher bias levels. The downside of higher bias levels is, of course, much shorter tube life. Dynaco knew that they had those extremely well made Mullard EL34's as their output tubes. These tubes handled the higher bias levels in stride. Today's output tubes are not as well made and do not handle higher higher biasing strategies as well as the original Mullard EL34's.

    Bob

    dmtparker

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by dmtparker on Wed Jan 21, 2015 10:56 am

    Bob Latino wrote:What Mr. Gillespie is saying is that the stock driver circuit "requires" that the output tubes be at a higher bias setting in order for the stock driver circuit to drive the output tubes at a lower distortion level. The VTA driver circuit does not require that the output tubes be biased that high in order to get the most out of the driver circuit.

    One of the "advantages" of biasing higher is that the amp will stay in class A (both output tubes conducting) at slightly higher volume levels before switching to class AB1 (each output tube of the pair conducting on only one half the cycle). When the amp is working in class A it WILL have somewhat lower distortion levels. This is really the only advantage of biasing at higher bias levels. The downside of higher bias levels is, of course, much shorter tube life. Dynaco knew that they had those extremely well made Mullard EL34's as their output tubes. These tubes handled the higher bias levels in stride. Today's output tubes are not as well made and do not handle higher higher biasing strategies as well as the original Mullard EL34's.

    Bob

    Thank-you. That makes sense to me. It was common for Dynaco to make the most of "special" parts in both their tube and (especially) ss amps - a typical Hafler 'most bang for your buck' approach. That, unfortunately, creates problems for us today not having access to such 'special' parts.

    GP49

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by GP49 on Thu Jan 22, 2015 12:50 am

    Bob Latino wrote:
    Answer this one > If the original 7199 driver circuit was as "good" as Mr. Gillespie says, how come no other manufacturer besides Dynaco ever used this 7199 "one driver tube per channel" circuit?

    Actually Dynaco was by no means the only manufacturer that used it.

    H.H. Scott did, also (for example, using the 6GH8 in integrated amplifiers, driving 7591 and EL84 output tubes); and that's just off the top
    of my head. I've no doubt that if more research were done, I could come up with others, too.


    peterh

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by peterh on Thu Jan 22, 2015 5:37 am

    What is "the right" value of bias for a vta-70 with el34 ?
    So far we have seen no figures that supports either value of bias. The fact that a lower bias would
    enhance tube lifetime has no objection, but the effect on distortion is yet to be examined.
    The effects are in two areas ( maybe more ) :
    1/ crossover distortion, here the driver circuit has no influence since it is the powertubes that creates it. This
    should be measured at low level ( 1w ?) and possibly with feedback disconnected.
    2/ non-linear behaviour due to working point not symmetrical. This is a function of the whole amp
    and also depending on output level , the higher output the more distortion. This is what D.Gillespie did
    in the mentioned article.

    What could be done here ? One could ask for Mr Latino to give Mr Gillespie to opportunity to rerun the tests with
    a VTA-70 and possibly figure out what level of bias works best. It could be as easy as sending him a driver board.
    Another possibility is that someone who has an amp and the proper instruments to do , preferably (1) above
    as this is the most likely "real life" listening level.

    I am in a possession of a hp3580A but i have no clean enough source to be able to have any meaningful figures.

    Anyone takes this glove ?



    PeterCapo

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by PeterCapo on Thu Jan 22, 2015 11:23 am

    GP49, In addition to the Van Alstine Ultravalve using a 6GH8A in the same way the Stereo 70 used the 7199, the old Scott tube integrated series 222, 233, and 299 used the 7199 http://hhscott.com/integrated_amps_stereo.htm   Also, the Heathkit AA-100, AA-111 and AA-201 amps used the 7199 http://www.heathkit-museum.com/hvmhifi.shtml   There may, of course, be others.

    Dmtparker, I don't think the original Dynacos used "special" parts to achieve their goals such that the original designs are not viable with parts available today.  Again referring back to the Gillespie article, the original Stereo 70 did not really push the power tubes unreasonably hard.  Some of today's amps, VTL and Manley come to mind, spec a quad of current production EL34s at around double the output of the Stereo 70 – look at the VTL IT-85 and the Manley Snapper, for example.

    I think the point was about tubes of long ago being more robust than today's tubes, and I do not think this was unique to the Mullards, in particular.  There's this whole mystique built up around the Mullard/Dynaco thing, but there were plenty of other robust tubes in those days, too.

    It's not a case of the original Dynacos needing "special" parts in order to be viable.  I built a new Stereo 70 to the original design but with certain enhancements that may put even more stress on the tubes, and it has been running just fine with current production EL34 types.  Now, this of course neither proves that current production EL34 types are as robust as they were years ago, nor does it settle any question about which bias point is best.  But, it does prove, at least to me, that the original design is still viable as a fine-sounding amplifier without having to jump through any "special" hoops.

    Perhaps current production power tubes, in general, are not as robust as the ones from years ago – I wouldn't be surprised.  But, I think the question has been exactly what kind of performance advantage can be gained, and if it is even audible, by biasing at either 40mA vs. 50mA and what the tradeoff might be with regard to the longevity of today's tubes.  

    It would sometimes be better to have hard data instead of a lot of guesswork and opinion.  The Gillespie article provided some hard data.  But, as I have been reading around in the various forums over time, it amazes me how much guesswork many of us engage in, in the course of this hobby.

    zx

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    Re: Bias voltage for ST70 with VTA driver board?

    Post by zx on Thu Jan 22, 2015 11:43 am

    I Well today I have only 3ea pr of Dynaco MK3s stock 6AN8s....an had 4-5 ST70..hhscott...Heath..all with 7199s .....for my old ears one tube has less,or more noise....hehe
    Most of the newer input board...I have heard.. change the stock sound to much.....but that's just me...
    But if we don't know if we don't go....right....have fun with tubes...life is short........




    Thanks for the site Bob..................

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