Tubes4ever wrote:sKiZo wrote:I don't imagine anything we can do to a KT120 on one of these amps can hurt them.
I do remember Bob mentioning he'd run his up to 90mV with no issues. Also, no discernible improvement in audio.
A 5U4GB would give you the needed extra current to run at 60mA. Even better would be 5V3 or better yet a 5V3A which takes 3 amps heater current. The 5V3 and 5V3A will have lower voltage drop at 250mA than a 5U4GB will have since they are rated at 350mA and are easy to find on Ebay. You can run the 5V3 which takes 3.8A easily with the uprated 5V transformer windings. I verified this with Bob.
Re: sKiZo > I have run the Tung-Sol KT120's up to 75 milliamps each tube (.750 VDC bias voltage) for hours as a test with my own VTA ST-120. The KT120 tubes will take this no problem but the power transformer will run warmer. I never went up to the .900 VDC bias voltage point for each output tube that sKiZo mentioned above nor do I recommend doing so.
Re: Tubes4ever > The VTA ST-120 has a 5 volt @ 5 amp bias line. Technically you can plug in any 5 volt octal rectifier tube in there and it will work. That is not to say that it will work "well". A 5U4 will work fine in the ST-120 but there will be a very slight power loss due to a lowered B+. I have never used a 5V3 in these amps so I can't say how well they might work. If someone does try or has tried a 5V3, let us know in this thread how it worked out ..
Re: Biasing KT88's and/or KT120's in a VTA ST-120 - The .550 VDC bias point (for each output tube) for the ST-120 is a nice compromise for good sound and good tube life. If you want to bump the bias up a little higher, you are welcome to do so but the amp won't really sound any better and tube life will go down.