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    VTA ST-70, VTA ST-120 and VTA M-125 amplifier information

    Bob Latino
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    VTA ST-70, VTA ST-120 and VTA M-125 amplifier information Empty VTA ST-70, VTA ST-120 and VTA M-125 amplifier information

    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 side 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).



    Complete series of eight VTA M-125 build videos on Youtube > VTA M-125 build videos on Youtube

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

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

    Another VTA M-125 review > VTA M-125 review from 2015

    VTA Tube Amp Kits Facebook page > Facebook > VTA Tube Amp Kits

    VTA ST-70 amp wiring photo below

    VTA ST-70, VTA ST-120 and VTA M-125 amplifier information ST-70-photograph-new-K42-Y

    VTA ST-120 amp wiring photo below

    VTA ST-70, VTA ST-120 and VTA M-125 amplifier information ST-120-color-photo-K42-Y

    VTA M-125 amp wiring below

    VTA ST-70, VTA ST-120 and VTA M-125 amplifier information M-125-interior-wiring-2

    ADDED on 3/5/20 > In certain parts of the USA and Canada line voltages can be higher than normal. While 120 VAC is supposed to be the norm, customers have reported to me voltages in the 122 to 125 VAC range. These higher than normal line voltages can cause shorter than normal tube life in your tube amp > especially the rectifier. In order to counteract this issue, all the VTA ST-120 and VTA M-125 monoblock amps will now come with DUAL PRIMARY connections. If your line voltage is normal (below 121 VAC), you use the standard connection. If your line voltage is 121 VAC or higher, you use the alternate connection which will drop the voltages on the secondary windings by about 2 1/2 %. Example > If your line voltage is 124 VAC, using the alternate connection will cause everything on the secondary side of the transformer to give out AC voltages as if your line voltage was about 120.28 VAC.

    Added on 5/19/20
    > How to bias your VTA ST-70 or VTA ST-120 amp - see below (ST-70 bias is .400 VDC per each output tube and ST-120 bias is .500 VDC)

    For the VTA ST-120 for the year 2012 to present day ...

    1. Set you multimeter to the 0 to 2 volt DC scale

    2. Place the black probe on the chassis and the red probe in the front LEFT tube socket pin that says "Biaset 1.56 V". Adjust the bias pot on the driver board next to the front left tube so that the meter reads .500 volts DC (that is POINT 5 volts DC or 1/2 volt DC). Next place the red probe in the front tube socket pin that is OPPOSITE the pin that you just used. Use the rear bias pot on the left channel to adjust the rear left output tube to .500 volts DC. If you have a VTA ST-70, then the bias will be .400 volts DC instead of .500 volts DC.

    3. Do the same as #2 above for the right channel

    Facts about bias below ..

    Since all tubes share the bias current > as you adjust one tube UP, the other tubes will go DOWN very slightly. Go back and forth between the four output tubes until all are between .490 and .510. That is plenty close enough. You will never all four output tubes to stay at .500 exactly.

    As the amp warms up from a cold start, the bias will rise very slightly over the first half hour. After 1/2 hour recheck the bias and reset the output tubes so that all are between .490 and .510 VDC.

    If all your tubes have a wandering bias UP or DOWN over time, then probably your line voltage is wandering UP and DOWN.

    If just ONE tube just cannot be biased at .500, then that tube is probably bad or going bad OR possibly the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground is bad and does not measure 10 ohms anymore.

    If ALL tubes cannot be biased, then probably your rectifier has a problem.

    Check your bias about once a month. As the output tubes wear in, the bias will change slightly.

    Bob



    Last edited by Bob Latino on Wed May 20, 2020 1:11 pm; edited 15 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
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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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    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.
    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?
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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
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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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    Post by Hops on Sun Nov 03, 2019 9:34 pm

    Question - How close to 0.5 volts is close enough? On a St-120, how close should the individual Bias values be to each other? Right now I am increasing Bias if it is less than. 0.5 volts, and reducing it if it is over 0.52 volts. Is this reasonable? Should I either tighten up or open up the window?. I use a Variac to target 118 volts, and set it to 118 volts when I check Bias. My VTA-120 probably has 30 hours on it. Even using a variac and a killawat, my voltage probably ranges from 116 to 120 during any given hour.
    Bob Latino
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    Post by Bob Latino on Sun Nov 03, 2019 9:57 pm

    Hops wrote:Question -  How close to 0.5 volts  is close enough?   On a St-120,  how close should the individual Bias values be to each other?   Right now I am increasing Bias if it is less than. 0.5 volts,  and reducing it if it is over 0.52 volts.   Is this reasonable?  Should I either tighten up or open up the window?.    I use a Variac to  target 118 volts,  and set it to 118 volts when I check Bias.   My VTA-120  probably has 30 hours on it.   Even using a variac and a killawat, my voltage probably ranges from 116 to 120 during any given hour.

    If your line voltage is all over the place, then don't even try to keep your amp at a "perfect" bias. If all your output tubes are biased between .48 and .52 VDC, that is close enough.

    Bob
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    Post by dakchi on Sun May 03, 2020 10:02 am

    Hi Bob,
    It will help new customers a lot to have an assembly video of ST-120. I am interested in buying ST-120 kit, but I find building it based on text instructions very difficult for a newbie. The extra 485$ you charge to build it for us is half of the cost of the amp. I really don't want to buy Chinese amp just because I don't have a video that shows how to build ST-120. I am sure you will sell much more kits if you make this video. A lot of people buy Chinese amps instead of your amps because they have worries that they would not be able to assemble them
    Thanks
    Bob Latino
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    Post by Bob Latino on Sun May 03, 2020 10:24 am

    We do have a video of the VTA ST-70 amp kit being built. The assembly of the VTA ST-120 is about 95% the same as the VTA ST-70. The video does not show all steps but does give an idea on how the amp goes together. You get a pictorial and a full color photograph which makes it easy to check what you did on each step. The color photograph and the video for the VTA ST-70 is seen in the first post above on this thread.

    Chinese amps > Many of them look good but some use cheap parts inside where you can't see ... Also ... when you need service on one, where do you go ? Send it back to China ? Need just a replacement part. Are you sure you can get another power or output transformer ? Below is a link to a fairly expensive Sophia tube amp made in China. This gives you an idea of what you can expect from some Chinese amps.

    What a Chinese Sophia tube amp looks like inside

    Bob
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    Post by dakchi on Sun May 03, 2020 1:13 pm

    Hi Bob,
    I agree with you that Chinese amps usually use bad quality parts and I do want to avoid buying one, but if I don't build correctly the ST-120, I might lose all the money I put in the amp. I think that it is worth to make a new video for ST-120 when you have the opportunity to build one. The video that you shared is for an old version of ST-70, does not have all the steps and does not include the optional parts. I am sure there are a lot of newbies who will not buy kits just because they do not have such video. This is the only reason why I am hesitating to buy the ST-120 kit. The photo is great but not enough to give me confidence
    I hope you understand
    Thank you Bob
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    Post by pedrocols on Thu May 07, 2020 10:07 pm

    dakchi wrote:Hi Bob,
    I agree with you that Chinese amps usually use bad quality parts and I do want to avoid buying one, but if I don't build correctly the ST-120, I might lose all the money I put in the amp. I think that it is worth to make a new video for ST-120 when you have the opportunity to build one. The video that you shared is for an old version of ST-70, does not have all the steps and does not include the optional parts. I am sure there are a lot of newbies who will not buy kits just because they do not have such video. This is the only reason why I am hesitating to buy the ST-120 kit. The photo is great but not enough to give me confidence
    I hope you understand
    Thank you Bob
    Build the amps little by little and you will be very glad you did! There is plenty of support here.

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