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    VTA ST-70 assembly video and new bias recommendations for all VTA amps

    Bob Latino
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    VTA ST-70 assembly video and new bias recommendations for all VTA amps Empty VTA ST-70 assembly video and new bias recommendations for all VTA amps

    Post by Bob Latino on Fri May 20, 2016 12:06 pm

    After extensive listening at various bias settings for ALL the VTA amps I am now recommending slightly lower bias settings for some of the VTA amps. These recommendations give no changes in the sound quality of the amps but will noticeably extend tube life. The listening was not only done by me but was also done by a number of VTA amp customers.

    VTA ST-70 with EL34 or 6L6 output tubes > recommended bias setting is .40 VDC per each output tube which gives a bias current of 40 milliamps per each output tube.

    VTA ST-70 with KT66 or KT77 output tubes > recommended bias setting is .45 VDC per each output tube which gives a bias current of 45 milliamps per each output tube.

    VTA ST-70 with 6550 or KT88 output tubes > recommended bias setting is .50 VDC per each output tube which gives a bias current of 50 milliamps per each output tube.
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    VTA ST-120 with 6550, KT88 or KT120 output tubes > recommended bias setting is .50 VDC per each output tube which gives a bias current of 50 milliamps per each output tube.

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    VTA M-125 with 6550, KT88 or KT120 output tubes USING TWO OUTPUT TUBES PER AMP > recommended setting is .50 VDC per each output tube which gives a bias current of 50 milliamps per each output tube.

    VTA M-125 with 6550, KT88 or KT120 output tubes USING FOUR OUTPUT TUBES PER AMP > recommended setting is 1.00 VDC per each side which gives a bias current of 50 milliamps per each output tube.

    All the user manuals will be changed to reflect the new bias recommendations ...

    Bob

    VTA ST-70 KIT assembly video below. This KIT video was done by VTA ST-70 amp builder Rick Green from FL. The video does not show all the wiring steps but does give a good idea of how the VTA ST-70 amp is assembled. Note that this amp has the older two post binding posts. All the new VTA amps (kit or wired) have THREE post binding posts and may be set up for any TWO impedances (4 and 8 ohm, 4 and 16 ohm, 8 and 16 ohm).




    Review Home Theater and Hi-Fi  > The VTA ST-120

    Review Home Theater and Hi-Fi > The VTA M-125


    Last edited by Bob Latino on Sun Feb 04, 2018 5:39 pm; edited 4 times in total
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    audiobill

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    Post by audiobill on Fri May 20, 2016 12:29 pm

    Bob, I've found no audible difference running my ST-120 with KT88s at .50vdc per tube....agree with your recommendations.
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    RockyAM

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    Post by RockyAM on Fri May 20, 2016 1:29 pm

    Bob, in my VTA M-125's I have actually run the bias in both amps at 1.0, 1.10 and 1.20 volts using 4 output tubes. I tried like hell but I honestly could hear no difference in the amps. Of course my wife keeps bitchin' at me "Why don't you just listen to the music instead of fiddling with those things?". I spent another night changing out that front driver tube on both amps and replaying a test track that I use and all I got was "How many times are you going to listen to that damn song!" LOL oh well RockyAM
    deepee99
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    Post by deepee99 on Fri May 20, 2016 3:11 pm

    I have found no audible difference between .5 and .6. Insofar as output tubes are spendy, why not let them loaf a bit?
    WntrMute2
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    Post by WntrMute2 on Sun Jul 03, 2016 8:43 pm

    Will this provide less stress on our poor 5AR4s?
    Bob Latino
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    Post by Bob Latino on Sun Jul 03, 2016 8:47 pm

    WntrMute2 wrote:Will this provide less stress on our poor 5AR4s?

    Yes - Besides longer output tube life, the rectifier will also be less stressed ...

    Bob
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    buschfsu

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    Post by buschfsu on Thu Mar 30, 2017 12:21 pm

    "VTA ST-70 with KT66 or KT77 output tubes > recommended bias setting is .45 VDC per each output tube which gives a bias current of 45 milliamps per each output tube."

    so for my non VTA stock st70 i should read on the front panel port 0.9v (0.45 each for the pair) dc instead of the 1.56v written on the panel?
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    stewdan

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    Post by stewdan on Thu Mar 30, 2017 1:09 pm

    buschfsu -- with stock Dynaco amps, I use 1.25vdc as the bias setting and don't hear anything different from using higher bias voltage settings.  My tubes seem to last longer.
    Peter W.
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    Post by Peter W. on Thu Mar 30, 2017 2:43 pm

    stewdan wrote:buschfsu -- with stock Dynaco amps, I use 1.25vdc as the bias setting and don't hear anything different from using higher bias voltage settings.  My tubes seem to last longer.

    After doing the bias mod (from this site) on my very vintage 70 using OEM (as in came-with) Siemans tubes, I am running at about 1.4V with perfect clarity. The tubes still test at least 20% over 'minimum' on the Hickok 539B.
    MechEngVic
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    Post by MechEngVic on Fri May 10, 2019 7:56 pm

    Sorry for the late post,

    It seems the issue is, will a lower than rated bias current affect sound quality. My question is: Will the quality of resistors in the bias reference current circuit or cathode bias resistor affect sound quality?
    Bob Latino
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    Post by Bob Latino on Fri May 10, 2019 8:17 pm

    MechEngVic wrote:Sorry for the late post,

    It seems the issue is, will a lower than rated bias current affect sound quality. My question is: Will the quality of resistors in the bias reference current circuit or cathode bias resistor affect sound quality?

    I have listened to the VTA ST-120 and VTA M-125 monoblock amps at 50, 55 and 60 milliamps bias current per each output tube. I have had customers here watch me alter the bias and then we sit and listen to the same tracks over again. There is no discernible audible difference between these three bias settings.

    The cathode bias resistors are 10 ohm 2% metal film resistors that nearly all measure with 1% of each other. I will measure all of these before they go out with a kit to make sure that they are closely matched. Metal film resistors will hold their value over the years much better than the carbon composition resistors found in many vintage amps.

    Bob
    tubes4hifi
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    Post by tubes4hifi on Sat May 11, 2019 2:28 pm

    I did the same exact thing 30 years ago when I first created the VTA driver board for the Dynaco ST70 amp.
    That's when I started recommending 40ma per tube for EL34s in the ST70 amp.
    No discernible difference in sound quality, and lots less heat and lots more tube life.
    No measurable difference in output (watts) either. Below 40ma sound quality and power decrease.

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