Tubes for VTL ST-150?

On the used market, I just get a new tube amplifier, VTL model ST-150 (75 w/c in triode and 140 w/c in tetrode). This unit has eight Electro Harmonix tubes KT88EH. Also, there are two 12AT7 tubes and two 12BH7 tubes. In the user manual , the manufacturer refers to eight 6550C tubes. I’m not sure to understand the subtelties of these different desings between KT88 and 6550C tubes and sonic results too. Are they some members of this forum would be able to help me to clarify that?

Does the expert members of this forum would kind enough to provide advice on the choice of tubes preferred for the VTL ST-150? I would appreciate very much receiving your help. If there are some actual or past VTL ST-150 owners on this forum, I woul appreciate very much if you could share your experiences.


I have a VTL ST-150, and my understanding, from reading some posts on the subject, is that the 6550 and KT88 are considered largely interchangeable, but that the KT88 was designed to be a bit more stable in amplifiers. For myself, I have always figured that VTL knows its business, and I have stuck with the recommended tubes. But many tube rollers swear by the various KT88s. I think that the KT88EH tubes you currently have installed are considered by many to be one of the best "alternative" tubes for the ST-150.
I am using Winged C 6550s in my ST-150. I can't say how the KT-88s would compare. One thing is for sure, this is a sweet amp. You will surely love it.

I spoke with Bea at VTL and she recommends the Gold Lion KT-88 as an upgrade over the 6550. She states it is a direct replacement with no issues and it sounds better with more dynamics,etc... Also, contact Upscale Audio, they have a great selection of NOS small signal tubes for the 12AT7. The NOS tubes with Cryo treatment made a huge improvement.
I used the Gold Lion KT 88, SED winged "C" and Tung Sol 6550s in my ST-150. The Gold Lion's sounded wonderful but I had some reliability issues with them. Two of them went bad with somewhat under 1000 hours on them. The SEDs had great upper end extension but withthe speakers I was usning at the time, Vienna Acoustics Beethovens, they were a litlle light and loose in the bass. The Tung Sols were a little less extended and sparkly at the top end but had better bass authority the mids were about equal for the SED and Tung Sols. Both the SEDs and Tung Sols lasted two years or more without problems.

Right before I sold the amp last year SEDs were getting hard to find and Gold Lions were too expensive for my budget especially if they didn't have the reliability of the others. I defaulted into getting the Tung Sols and I was happy with them.

I found the Sylvania Gold Band 6201 in the 12AT7 spot was a great improvment over the stock tubes. Mullard CV 4024 were nice as well.