KT-150s in ST-120 !?

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!

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.

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.

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.

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.

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

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.