Corona wrote:Hi. First post after lurking here for a while. I've been reading some of the recent posts about power transformer temperature and it's got me wondering just how hot is too hot. I have a new st70 with KT-88's that I built from the kit a few months ago. After extended use in a warm room the power transformer gets pretty hot. Not burning hot or anything. Just uncomfortably hot where you can't hold your finger on it for longer than a few seconds. Is this normal? I've put about 250 hours on the amp with no ill effects. Basically I'm just wondering if this thing is like a neon sign transformer and it's designed to run hot in normal operation.

Btw it's only the power transformer that gets significantly hot. The two output transformers just get slightly warm to the touch.

The tube setup and bias is the following
4 sovtek KT88's biased at around .510-.520 each (it won't go any lower)
3x 12AT7

The original ruby 5ar4 arced over internally after like 5 hours of use and kept blowing the fuse after that( them darn chinese). Since then it's been using an old copper plate GE 5ar4 without any issues.

Bob Latino wrote:

Corona wrote:Hi. First post after lurking here for a while. I've been reading some of the recent posts about power transformer temperature and it's got me wondering just how hot is too hot. I have a new st70 with KT-88's that I built from the kit a few months ago. After extended use in a warm room the power transformer gets pretty hot. Not burning hot or anything. Just uncomfortably hot where you can't hold your finger on it for longer than a few seconds. Is this normal? I've put about 250 hours on the amp with no ill effects. Basically I'm just wondering if this thing is like a neon sign transformer and it's designed to run hot in normal operation.

Btw it's only the power transformer that gets significantly hot. The two output transformers just get slightly warm to the touch.

The tube setup and bias is the following
4 sovtek KT88's biased at around .510-.520 each (it won't go any lower)
3x 12AT7

The original ruby 5ar4 arced over internally after like 5 hours of use and kept blowing the fuse after that( them darn chinese). Since then it's been using an old copper plate GE 5ar4 without any issues.

Hi,

KT88's use about 35% more current than EL34's and will tax a power transformer more. If you built your ST-70 from a new kit then you have an upgraded transformer. The power transformer will get hot to the touch but don't worry about it.

If you have a new kit with a VTA driver board you can easily solve the fact that your KT88's will only bias with the bias control set way over to one side. All you have to do is to alter to a LOWER value R39. This is the resistor right between the two small 100 uF 100 volt caps near the back edge on the BOTTOM of the driver board. Right now R39 is a 10K resistor. If this is one of my kits I always have every builder solder that resistor on the bottom of the board because it makes it easier to remove. You can either remove the resistor and install a 5K resistor in there OR just solder on another 10K resistor in parallel with the resistor that is already there effectively making a 5K resistor. If you do this your KT88's will bias more towards the center of the bias control. Don't go any lower than 5K or you may have trouble biasing EL34 tubes.

Bob