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    VTA ST120 Bias Adjustment

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    guentherj2

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    VTA ST120 Bias Adjustment

    Post by guentherj2 on Thu Nov 14, 2013 12:09 am

    Greetings all.

    I've had my ST120 for a few months now. I notice that if I adjust the bias to 0.550 shortly after start up, lets say about 10 minutes after playing music, and check it an hour or more after playing, the bias has dropped some - in the neighborhood of 0.225. If I adjust the bias up to 0.550 after it is good and warm, and I turn the amp off and come back to it hours later, the bias is high at about 0.680.

    My question is - should that range of voltage readings be a concern or should I go for the warmed up bias srttings and not worry about the higher voltages when it is cool (i.e., give it time to warm up and stabilize)?

    FYI - I run the amp on a variac set at about 117VAC.

    Thanks,

    John

    Bob Latino
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    Re: VTA ST120 Bias Adjustment

    Post by Bob Latino on Thu Nov 14, 2013 12:34 am

    Hi John,

    Try a different rectifier tube in the amp. If your line voltage remains at a steady 117 VAC then the bias should remain fairly steady unless the output of the rectifier changes or there is a problem (capacitor leakage and a subsequent voltage drop) with the high voltage storage system.

    If your bias level should be .550 VDC, then when the amp is cold, you should set the bias at maybe .500 VDC per each output tube. What should happen is that the bias should RISE very slightly as the amp warms up. After 1/2 hour or so, you can now reset the bias to .550 VDC per each output tube. The bias should not DROP as the amp warms up. A bias drop as the amp warms up usually indicates either a bad rectifier tube OR some issue with the high voltage storage system (quad cap, SCM or one of the capacitors on the driver board).

    The most common cause of the issue you are having is (as mentioned above) a weak or bad rectifier tube.

    Bob

    Dave_in_Va

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    Re: VTA ST120 Bias Adjustment

    Post by Dave_in_Va on Thu Nov 14, 2013 10:57 am

    I'm not a tech.
    I was running my VTA ST 70 on a Variac thinking it would constantly put out 120. The Variac voltage output went up or down depending on what was coming into it from the wall.
    Just my opinion, but unless your wall voltage is dead steady, I don't think the Variac is very helpful.
    Now I use a GFCI ($20) to protect the amp and just check my bias from time to time. My wall voltage seems to vary from about 120 to 127 (measured with a cheap voltmeter).

    sKiZo

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    Re: VTA ST120 Bias Adjustment

    Post by sKiZo on Thu Nov 14, 2013 4:22 pm

    127? Holy Crap, Batman! That's HIGH!

    I'm lucky here in that my voltage may be high at 122vac, but at least it's stable. I was able to bring it down a couple points just installing a CL90 thermister in the power line ... those never go completely open.



    Not sure what good that would do with your voltage, but the soft start is still a nice feature.

    And ya ... a variac assUmes steady current and will swing based on the incoming. I'd think in your case it's still a good thing as long as you set it so it doesn't swing below around 115vac ... another concern with variacs is that it takes some time for them to settle down. Mine will swing quite a bit on power up and it takes a good hour to stabilize. If I were to use it, I'd probably just leave it on as power draw is minimal with no load.

    +1 on the GFCI, but worth repeating ... you want a "non-latching" variety that will trip on a blip and require a manual reset.

    Dave_in_Va

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    Re: VTA ST120 Bias Adjustment

    Post by Dave_in_Va on Thu Nov 14, 2013 9:41 pm

    It's usually 123 and 119-120 in the summer. That 127 might have been a cheap voltmeter aberration.

    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Thu Nov 14, 2013 10:02 pm

    OK - I replaced my NOS Svetlana 5AR4 that I have had in the amp for about two months with a used but tested OK International Mullard 5AR4. The original Sovtek 5AR4 that the amp came with went out in a light show following a blown bias resistor a couple of months ago. I set the bias to 0.510 at start up and will check it again in about 30 minutes.

    I've monitored the variac (TEMCO TVT001) output voltage and it stays pretty steady from 117.1 to 117.4 VAC.

    More to come...

    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Sun Nov 17, 2013 3:18 pm

    The bias' may be a bit more stable with the new rectifier tube? Can the bias be too low? If so, how low is too low? Is 0.530 VDC OK?

    Bob Latino
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    Re: VTA ST120 Bias Adjustment

    Post by Bob Latino on Sun Nov 17, 2013 3:25 pm

    guentherj2 wrote:The bias' may be a bit more stable with the new rectifier tube? Can the bias be too low? If so, how low is too low? Is 0.530 VDC OK?
    .530 vdc is fine .. Those output tubes on the VTA ST-120 could actually be biased anywhere between .500 VDC and .550 VDC

    Bob

    baddog1946

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    Re: VTA ST120 Bias Adjustment

    Post by baddog1946 on Sun Nov 17, 2013 5:02 pm

    What kind of output tubes are you using? I am using KT120's if I bias them at .500- .550 they work fine but I find with .600-.650 the KT120's are brighter on the hi end and have cleaner bass too.
    I am putting them through some efficient speakers (100db) and these drivers are very responsive to the difference.
    I guess its all a matter of taste when it comes to the sound but I'm thinking the KT120's have no problem with .550 - .650 at the voltages we have in an ST-120.
    Baddog

    sKiZo

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    Re: VTA ST120 Bias Adjustment

    Post by sKiZo on Sun Nov 17, 2013 6:19 pm

    I got the tip from KWard ... he's been running his KT120's at 60mA for months now with real good results. I played around with that a bit and prefer the "punch" I get with mine at that setting, but have backed down to 55mA again till they break in properly. Kinda like a horse ... Maybe I should break in at the levels I plan to eventually keep them? I've only got around 20 hours on the tubes so far, if that.

    Speaking of bias settings ... is it normal for tubes to bias up at different rates? One of my KT120's is a bit on the slow side taking another minute or two to get up to snuff ... it follows the tube if I swap sockets. Supposed to be a matched quad, but we all know some suppliers play fast and loose on the standard for matching a set ...


    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Sat Dec 28, 2013 8:52 pm

    I've been running the ST120 for a couple of months now with a variac set to 118VAC and the bias' at around 0.510 VDC with good results. I've been tracking the bias' and they fluctuate, probably due to fluctuating incoming AC, between 0.505 to 0.515 VDC. I'm a little gun shy with the bias' since I've experienced blowing bias resistors (three times).

    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Sat Apr 05, 2014 9:45 pm

    Dang it - another blown bias resistor! This happened with a known good GZ37 in the ST120 and this is the fourth time I have had to replace a bias resistor (different ones each time). I have two sets of KT88s, they all test fine and this has happened with both sets of tubes. I'm wondering if the fact that I have such a difficult time getting the bias adjustments to hold steady in the first place is part of an underlying problem??? I typically run it at about 0.520 VDC. I will say that when this happens I am usually playing the amp pretty loud but I don't think this should be happening.

    Any ideas what is causing this recurring problem or how to troubleshoot it?

    Thanks

    Bob Latino
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    Re: VTA ST120 Bias Adjustment

    Post by Bob Latino on Sat Apr 05, 2014 10:15 pm

    Hi John,

    Very few people have blown the 10 ohm 2 watt bias resistor on these amps .. The cause is always that the tube in that socket drew too much current .. Why ? The tube may have lost bias? The tube may have experienced a transient internal short? Voltage spikes on your AC line that you are unaware of ? The reasons why this happens are sometimes unkown ..

    I will send you out a couple of 10 ohm 2 watt resistors on Monday ..

    Note - I have had the original prototype VTA ST-120 amp here since early 2008 .. about 6 years ... I play it a lot and have replaced the output tubes once sometime in 2011. I have never blown a bias resistor and sometimes I play the amp pretty loud (usually when the wife goes to visit her mother - lol)


    Bob

    deepee99

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    Re: VTA ST120 Bias Adjustment

    Post by deepee99 on Sun Apr 06, 2014 12:21 am

    Guess I would refer back to Bob L's advice in the M-125 manual, which is don't go chasing the bias. We have monster voltage fluctuations here; I've got a Furman that will clamp the line juice down to 121 or so but nothing helps on the low side.
    I run mine at about .55 (1.10v per pair) and except for operator input error have never hurt a tube or my ears. I run the Russkie KT-120s most times. When I'm entertaining royalty and put the antique TS 6550s, I run those about .50. Andy Bowman suggests with old 6550 types you start at 45 and slowly, over days, work your way up. Less can be more.

    Elrick

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    Re: VTA ST120 Bias Adjustment

    Post by Elrick on Sun Apr 06, 2014 11:19 am

    Just to be sure, are you currently running a GZ37 recitifer as stated in your post no.12, or was that a typo? Perhaps your first set of those resistors was suspect since it sounds like now each one the originals have failed.

    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Sun Apr 06, 2014 5:55 pm

    Bob - Since I am now out of 10 ohm 2 watt bias resistors and they are not readily available I will appreciate having a couple extras on hand.

    One thing I notice when biasing my amp. It is typical that the screw slots on the bias pot adjusters are not all at similar places. That is, maybe three will be between 11 and 12 o-clock and one might be at about 1 or even 2. I understand that the bias voltage reading is what counts but should they generally be at about the same place? Since this is a recurring problem for me and I have changed out KT88s and the GZ34 rectifier is it possible that I have a faulty bias pot? Thinking more about that, I have blown bias resistors at different tube locations so thats probably not the underlying problem.

    I just set my variac down to 117. 5 from 118.5 to better buffer any voltage surges.

    You've been supportive and helpful throughout this problem and I appreciate that. I will continue monitoring and let you know how things turn out.

    Thanks again - John

    deepee99

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    Re: VTA ST120 Bias Adjustment

    Post by deepee99 on Sun Apr 06, 2014 6:06 pm

    I'd check and re-check the solder connections to the bias pots. Just a thought. I had the same problem on initial fire-up. Those pins are very, very skinny.

    sKiZo

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    Re: VTA ST120 Bias Adjustment

    Post by sKiZo on Sun Apr 06, 2014 6:35 pm

    Could even be a bad pot. I got me a red plate a while back - checked everything and found a short in one of the el cheepo Chinese pots I got offa da bay. Replaced them all with some good Fender units, and no problems since.

    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Sun Apr 06, 2014 11:27 pm

    Thanks depee99 and sKiZo for the suggestions. I put the Svetlana KT88s back in and the bias adjustments are stable. I noticed that one of the Sovtek KT88s that I removed had a red label as opposed to the other three being black labels. That was the tube that was off on the bias pot setting. I guess that really shouldn't matter since that is what the bias pot adjustment is for. I'll check the pot solder connections but hope to be up and running for a while again now.

    Thanks

    peterh

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    Re: VTA ST120 Bias Adjustment

    Post by peterh on Mon Apr 07, 2014 1:47 am

    guentherj2 wrote:Bob - Since I am now out of 10 ohm 2 watt bias resistors and they are not readily available I will appreciate having a couple extras on hand.  

    One thing I notice when biasing my amp. It is typical that the screw slots on the bias pot adjusters are not all at similar places. That is, maybe three will be between 11 and 12 o-clock and one might be at about 1 or even 2. I understand that the bias voltage reading is what counts but should they generally be at about the same place? Since this is a recurring problem for me and I have changed out KT88s and the GZ34 rectifier is it possible that I have a faulty bias pot? Thinking more about that, I have blown bias resistors at different tube locations so thats probably not the underlying problem.

    I just set my variac down to 117. 5 from 118.5 to better buffer any voltage surges.

    You've been supportive and helpful throughout this problem and I appreciate that. I will continue monitoring and let you know how things turn out.

    Thanks again - John
    What is importent during bias adjustment is tube current.
    Differences in tubes and component variatiations might cause the pot's to point in different ways.

    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Mon Apr 07, 2014 12:57 pm

    peterh - that sounds right but could you please elaborate? I noticed that three of my Sovtek KT88s have black lebels and one has a red label. It is that red labeled KT88 that results in the bias pot being at a different setting than the other three. I would think it doesn't matter since I am adjusting the bias pots to get same readings (voltages) on each of the tubes but might there be an issue with having an "odd man out" tube in there?

    Thanks

    peterh

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    Re: VTA ST120 Bias Adjustment

    Post by peterh on Mon Apr 07, 2014 2:08 pm

    guentherj2 wrote:peterh - that sounds right but could you please elaborate? I noticed that three of my Sovtek KT88s have black lebels and one has a red label. It is that red labeled KT88 that results in the bias pot being at a different setting than the other three. I would think it doesn't matter since I am adjusting the bias pots to get same readings (voltages) on each of the tubes but might there be an issue with having an "odd man out" tube in there?

    Thanks
    It seems to me that you have tubes that is not matched. That is ok, but you will most likely have different bias on them.
    When you speak about voltages, is it the voltage measured across the cathode resistor ( 10 ohm in vta amps) that you mean ? In that case you should measure equal voltage for each of the two tubes on each side.
    Voltage measured on the grid is not interesting, that is more an issue of the tubes properties. The mere fact that tubes are differing ( unless one has selected tubes to create matching set's) is the reason to have individual bias pots.

    There is one more "property" of the tubes that needs matching, it's the amplification aka transconduktance. bias puts will do nothing here, if the tubes differ you will have to resort to your feedback to reduce it's impact. Normally the differences are small and the feedback will reduce the unbalance to low levels. ( some amps has an ac-balance that might compensate )

    In an vta you need 2 matched pairs, although a matched quad is fine since you among the route might combine them to be at least one pair even if 2 of them goes bust.





    deepee99

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    Re: VTA ST120 Bias Adjustment

    Post by deepee99 on Mon Apr 07, 2014 4:45 pm

    peterh wrote:
    guentherj2 wrote:peterh - that sounds right but could you please elaborate? I noticed that three of my Sovtek KT88s have black lebels and one has a red label. It is that red labeled KT88 that results in the bias pot being at a different setting than the other three. I would think it doesn't matter since I am adjusting the bias pots to get same readings (voltages) on each of the tubes but might there be an issue with having an "odd man out" tube in there?

    Thanks
    It seems to me that you have tubes that is not matched. That is ok, but you will most likely have different bias on them.
    When you speak about voltages, is it the voltage measured across the cathode resistor ( 10 ohm in vta amps) that you mean ? In that case you should measure equal voltage for each of the two tubes on each side.
    Voltage measured on the grid is not interesting, that is more an issue of the tubes properties. The mere fact that tubes are differing ( unless one has selected tubes to create matching set's) is the reason to have individual bias pots.

    There is one more "property" of the tubes that needs matching, it's the amplification aka transconduktance. bias puts will do nothing here, if the tubes differ you will have to resort to your feedback to reduce it's impact. Normally the differences are small and the feedback will reduce the unbalance to low levels. ( some amps has an ac-balance that might compensate )

    In an vta you need 2 matched pairs, although a matched quad is fine since you among the route might combine them to be at least one pair even if 2 of them goes bust.

    That would be my guess, too, that maybe you've got an un-matched tube. Jim McShane's your guy for ensuring you get matched pairs or quads. They work in push-pull fashion in VTA amps (lotsa research there on that topic elsewhere ) but basically if they're not well-matched, one tube works harder than the other in the push-pull pair and regardless of bias setting, you're compromising sound quality and tube life. To over-simplify, imagine a teeter-totter with a 200 lb person on one end and a 50-pound person on the other. Average weight is 125-lbs a side,which is what your bias pot is telling you across two tubes, and you can still go up and down but one side's working a lot harder and the other. And you won't get an even swing.





    guentherj2

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    Re: VTA ST120 Bias Adjustment

    Post by guentherj2 on Mon Apr 07, 2014 9:39 pm

    Good analogy. I busted my front teeth out on a teeter-totter when I was a kid!

    deepee99

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    Re: VTA ST120 Bias Adjustment

    Post by deepee99 on Wed Apr 09, 2014 8:21 pm

    guentherj2 wrote:Good analogy. I busted my front teeth out on a teeter-totter when I was a kid!

    Yeah, but watching the girls was fun.

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