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    KT-150s in ST-120 !?

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    arledgsc

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    KT-150s in ST-120 !?

    Post by arledgsc on Tue Nov 15, 2016 11:49 am

    I tried KT-150s in my ST-120 amp.  I could not bias the 150s as I ran out of range on the bias pots.  There is too much voltage drop across the bias supply 150K ohm resistors to adequately bias the tubes and they were running hot.  As per the datasheet the bias resistors need to closer to 50K ohm.   The ST-120 power supply was straining hard to supply the cathode current during this brief trial. LOL... Not recommended.  The KT-120s are just perfect for this amp.

    A good friend loaned me his Audio Research Reference 75 SE with a spare set of KT-150 tubes.  This amp is monster to haul around.  Bass is very good (tight) with the Ref 75 but there is not much sparkle on top like I enjoy with the ST-120 with KT-120s.  Seems very generic sounding in comparison to the ST-120 though perhaps more accurate and less colored.  The Ref 75 is more akin to transistor amps I have tried.   I do though need a better balanced linefeed to Ref 75.  

    So the ST-120 happily lives on in my system.  It has seen a few challengers in the past 4 yrs. but has held out and stands proud.  Excellent amp Bob and great value!
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    Bob Latino
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    Re: KT-150s in ST-120 !?

    Post by Bob Latino on Tue Nov 15, 2016 1:25 pm

    arledgsc wrote:I tried KT-150s in my ST-120 amp.  I could not bias the 150s as I ran out of range on the bias pots.  There is too much voltage drop across the bias supply 150K ohm resistors to adequately bias the tubes and they were running hot.  As per the datasheet the bias resistors need to closer to 50K ohm.   The ST-120 power supply was straining hard to supply the cathode current during this brief trial. LOL... Not recommended.  The KT-120s are just perfect for this amp.

    A good friend loaned me his Audio Research Reference 75 SE with a spare set of KT-150 tubes.  This amp is monster to haul around.  Bass is very good (tight) with the Ref 75 but there is not much sparkle on top like I enjoy with the ST-120 with KT-120s.  Seems very generic sounding in comparison to the ST-120 though perhaps more accurate and less colored.  The Ref 75 is more akin to transistor amps I have tried.   I do though need a better balanced linefeed to Ref 75.  

    So the ST-120 happily lives on in my system.  It has seen a few challengers in the past 4 yrs. but has held out and stands proud.  Excellent amp Bob and great value!

    If you want to use Tung-Sol KT150 output tubes in the ST-120 you will probably have to drop R29 - R32 to 75K or even 50K in order to get them to bias. That said > the KT150's offer really nothing in the ST-120 that a KT120 won't do. The KT150 is also very expensive at $100 USD or so per each KT150. In order to get the most out of a KT150, you will probably have to get the B+ up over 600 volts and bias levels at 100+ milliamps per tube. The VTA ST-120 won't handle that. The VTA ST-120 is designed PRIMARILY for 6550 or KT88 output tubes. You can also use a Tung-Sol KT120 output tube in the ST-120 but even that tube is not being utilized to its full potential but it will work fine and bias fine in the ST-120.

    Bob
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    corndog71

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    Re: KT-150s in ST-120 !?

    Post by corndog71 on Tue Nov 15, 2016 6:54 pm

    I've looked around and have yet to see anyone make a true KT150 amp.  They all seem geared for KT88 and then manipulated to allow KT150s.   Notice most of them run in triode mode.

    I would imagine a true stereo ultralinear KT150 amp would deliver 150-200 watts per channel.  But people need to wind the right transformers for them.

    Oh, yeah and then there's Bob's point of the KT150s being expensive.  I imagine a proper amp will be equally expensive.
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    peterh

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    Re: KT-150s in ST-120 !?

    Post by peterh on Wed Nov 16, 2016 4:50 am

    corndog71 wrote:I've looked around and have yet to see anyone make a true KT150 amp.  They all seem geared for KT88 and then manipulated to allow KT150s.   Notice most of them run in triode mode.

    I would imagine a true stereo ultralinear KT150 amp would deliver 150-200 watts per channel.  But people need to wind the right transformers for them.

    Oh, yeah and then there's Bob's point of the KT150s being expensive.  I imagine a proper amp will be equally expensive.

    It's also the point of multiple sources of tubes.
    Designing and marketing an amp for a tube that is only supplied with one source ( and a source
    that may dry up any day) is a risky business.
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    tubes4hifi
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    Re: KT-150s in ST-120 !?

    Post by tubes4hifi on Wed Nov 16, 2016 3:51 pm

    in my recent experience fixing an amp that had trouble with stable bias, I found that it seems the bias supply will not supply enough CURRENT even when the bias voltage is correct,
    when you are running four KT88s or higher in an M125 or ST120 amp, because of the need to change R29-32 to 100K (KT88) or 50K (KT120 & KT150 tubes)
    KT88s will bias around -55v to get 55ma of current, but if you try to get a little more current then the bias current will start to oscillate which can be dangerous and cause the tubes to fail.
    So beware when using KT120 & KT150 tubes in an M125 or ST120.
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    sKiZo

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    Re: KT-150s in ST-120 !?

    Post by sKiZo on Thu Nov 17, 2016 1:30 am

    tubes4hifi wrote:when you are running four KT88s or higher in an M125 or ST120 amp, because of the need to change R29-32 to 100K (KT88) or 50K (KT120 & KT150 tubes)
    KT88s will bias around -55v to get 55ma of current, but if you try to get a little more current then the bias current will start to oscillate which can be dangerous and cause the tubes to fail.
    So beware when using KT120 & KT150 tubes in an M125 or ST120.

    ???

    First I'm hearing this. My ST120's been running with KT120's for quite some time now, with nary a problem using the 150k resistors spec'd and supplied with the kit. You're saying I should be changing those out for best performance? Where would that leave me if I want to swap in KT88's?

    How about going 80k as a compromise? No plans to use anything other than the KT88 and KT120's ...

    PS ... I've got the meters on my build and can see that the bias is rock steady.


    mazeeff

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    Re: KT-150s in ST-120 !?

    Post by mazeeff on Thu Nov 17, 2016 8:00 am

    tubes4hifi wrote:in my recent experience fixing an amp that had trouble with stable bias, I found that it seems the bias supply will not supply enough CURRENT even when the bias voltage is correct,
    when you are running four KT88s or higher in an M125 or ST120 amp, because of the need to change R29-32 to 100K (KT88) or 50K (KT120 & KT150 tubes)
    KT88s will bias around -55v to get 55ma of current, but if you try to get a little more current then the bias current will start to oscillate which can be dangerous and cause the tubes to fail.
    So beware when using KT120 & KT150 tubes in an M125 or ST120.

    I am equally confused. Are you saying that the ST120 power transformer, can not supply the necessary current on the bias winding to drive the KT120's (or even KT88s)? A little more detailed explanation would help!

    Mike
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    Bob Latino
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    Re: KT-150s in ST-120 !?

    Post by Bob Latino on Thu Nov 17, 2016 8:39 am

    The amps that Roy is referring to are not stock VTA M-125 amps. They are custom versions of the M-125's in which the builder used an altered circuit.

    I agree and disagree somewhat with what Roy has said. The VTA ST-120 and M-125's will supply enough bias current for the use of the Tung-Sol KT120 output tubes. You can use the Tung-Sol KT120 in either the VTA ST-120 or the M-125 monoblocks with no issues with the R29 - R32 resistor values as shipped with either amp. You should not use the Tung-Sol KT150 with either amp but some have done so. Using KT150's provide nothing in these amps that a KT120 output tube will do. The Tung-Sol KT150's are $100 USD each and the Tung-Sol KT120's are about $50 USD each. Using the $100 per tube KT150's in either of the VTA amps is really a waste of money ...

    Read above what sKiZo has said after using KT120 output tubes in his custom VTA ST-120 amp for 3 years. I have been using KT120 output tubes in my own VTA ST-120 for about 5 years with no issues using 150K resistors in R29 - R32. The bias has always been very steady in my own ST-120.

    Bob
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    arledgsc

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    Re: KT-150s in ST-120 !?

    Post by arledgsc on Thu Nov 17, 2016 11:42 am

    My only reason for trying KT-150s is that Audio Research is biasing the 150s at 27W (415V x 65mA) - same as KT-120s in their amps.  They and others claim better sound and longer tube life over KT-120s.  Yes, at double the cost.
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    sKiZo

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    Re: KT-150s in ST-120 !?

    Post by sKiZo on Thu Nov 17, 2016 9:06 pm

    I see the potential for some resistor rolling in my future ...

    must

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    resist

    .
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    .

    clown

    arledgsc wrote:My only reason for trying KT-150s is that Audio Research is biasing the 150s at 27W (415V x 65mA) - same as KT-120s in their amps.  They and others claim better sound and longer tube life over KT-120s.  Yes, at double the cost.

    So, you HAVE tried them? The extra cost may be justified if both improved sound and longevity result - people pay more for some of the super duper KT88's. That's of course IF they'll fit and your iron supports the additional demand.

    Guest
    Guest

    Re: KT-150s in ST-120 !?

    Post by Guest on Thu Nov 17, 2016 11:17 pm

    Bob Latino wrote:The amps that Roy is referring to are not stock VTA M-125 amps. They are custom versions of the M-125's in which the builder used an altered circuit.

    I agree and disagree somewhat with what Roy has said. The VTA ST-120 and M-125's will supply enough bias current for the use of the Tung-Sol KT120 output tubes. You can use the Tung-Sol KT120 in either the VTA ST-120 or the M-125 monoblocks with no issues with the R29 - R32 resistor values as shipped with either amp. You should not use the Tung-Sol KT150 with either amp but some have done so. Using KT150's provide nothing in these amps that a KT120 output tube will do. The Tung-Sol KT150's are $100 USD each and the Tung-Sol KT120's are about $50 USD each. Using the $100 per tube KT150's in either of the VTA amps is really a waste of money ...

    Read above what sKiZo has said after using KT120 output tubes in his custom VTA ST-120 amp for 3 years. I have been using KT120 output tubes in my own VTA ST-120 for about 5 years with no issues using 150K resistors in R29 - R32. The bias has always been very steady in my own ST-120.

    Bob

    I actually have to disagree here. Roy's statement is a general referral to running KT120's instead of the KT88's, and there is no mention of 'custom' M125's or altered circuits. This is where the confusion has set in.
    Also, the M125 is marketed with a recommended tube complement of KT88, 6550, KT90, KT100 or KT120, and no mention, that I can see, to change the resistor values between using KT88's or KT120's or any of the other recommended tubes.
    As has already been pointed out, probably the majority of M125's sold and built over the years are more than likely are running KT120's with the recommended resistor values and have had no issues with unstable BIAS.

    mazeeff

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    Re: KT-150s in ST-120 !?

    Post by mazeeff on Fri Nov 18, 2016 5:57 am

    While I understand the desire to protect ones business, I think Roy needs to be the one to explain this. As I understand it, Roy is the designer of the ST-120 & M125. It is startling to see three senior members of this club, say three completely different things in less than 48 hours. From what I read in Roy's post, the 150K bias resistor can not be lowered to 100K (KT88's) or 50K (kt120s), due to lack of adequate current from the bias winding of the power transformer. He seems to suggest that reducing the resistor value could result in the "Oscillation of Death (OD)", wiping out the "tubes". If you do a little reading on folks building KT120 amps from scratch, you will see that they all use a 50K bias resistor, which is what Roy stated. The KT120 spec sheet also clearly describes the 50K bias resistor. BTW, my recently acquired ST-120 also sounds perfect with KT120s and the stock 150K bias resistor!!!

    Mike
    VTA-ST120 (Recently acquired)
    VTA-70
    ST-70 (1967)
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    LeGrace

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    Re: KT-150s in ST-120 !?

    Post by LeGrace on Fri Nov 18, 2016 8:19 am

    While the focus is all around the KT120 to me what comes as an even greater surprise is that the warning extends to KT88's as well!! Does this mean that the 6550 is really the only "recommended" tube properly matching original design intent? (from a strictly technical perspective as opposed to what seems to work)
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    Bob Latino
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    Re: KT-150s in ST-120 !?

    Post by Bob Latino on Fri Nov 18, 2016 9:32 am

    MontanaWay wrote:
    Also, the M125 is marketed with a recommended tube complement of KT88, 6550, KT90, KT100 or KT120, and no mention, that I can see, to change the resistor values between using KT88's or KT120's or any of the other recommended tubes.
    As has already been pointed out, probably the majority of M125's sold and built over the years are more than likely are running KT120's with the recommended resistor values and have had no issues with unstable BIAS.

    There is no mention of of a change in resistor values for the M-125's because no change in resistor values are needed. The resistor values in question for R28 and R30 on the VTA M-125 amps are 100K. 100K resistors will work well with any of the tubes that Montana Way has mentioned above. I have used KT88, 6550 and KT120 tubes in my own M-125 amps here for over 6 years. I never had a problem. Customers have used KT90 and KT100 tubes in these amps with no problems also .. 100K will work with all tubes.

    Now, that being said, Tung-Sol recommends 50K in those positions BUT that is because Tung-Sol assumes that you are going to use your KT120 "as a KT120" with a B+ over 600 volts and bias settings in the 100+ milliamp region. On the M-125's the KT120's are being used "as a KT88" which means a B+ of about 500 VDC and bias settings of 50 milliamps per tube. Used like this, a KT120 is being used well below its design limits and a 100K resistor here will work fine.

    The bottom like on this is > You really should not use KT150 output tubes in the VTA ST-120 OR the M-125 monoblocks.

    Bob

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