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

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    Bob Latino
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    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 assembly video and VTA ST-70 in action video below ... This video was done by VTA ST-70 amp builder Rick Green. 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)







    Last edited by Bob Latino on Sat Jul 22, 2017 9:32 am; edited 2 times in total

    audiobill

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

    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.

    RockyAM

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

    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
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    deepee99

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

    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?
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    WntrMute2

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

    Post by WntrMute2 on Sun Jul 03, 2016 8:43 pm

    Will this provide less stress on our poor 5AR4s?
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    Bob Latino
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    Re: VTA ST-70 assembly video and new bias recommendations for all VTA amps

    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

    buschfsu

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

    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?

    stewdan

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

    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.
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    Peter W.

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

    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.

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