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I've been putting some hours on my Triode ST70 kit with 4 Svet EL34 tubes and the EF86 board with a Webber copper cap. It sounds pretty good (even without a preamp), but I've noticed that the power transformer heats up quite a bit after about 1.5-2 hours of run time (if I had to guess, it seems to get up to around 100-110 degrees F, just under the threshold of pain). I'm not sure if this is normal, as this is my first tube power amp, so should I be concerned with the temps this thing is putting out? I try to leave the celing fan on in the room where I use it, but that seems kind of cheesy to me.

I've also been considering swapping the EL34s out for KT88s. What exactly would I need to do to accomplish this (if I can do it at all), and do I really stand anything to gain enough to justify the purchase of the tubes?

Thanks in advance!

(I had to break my reply into two posts else the forum times out..sorry)

It is normal for the PT to run hot. When I had my ST70 with a new Triode PA060S, after a few hours it would be just at the point where you could keep your hand on it for maybe 5 seconds before it became uncomfortable. The power trannies of most tube amps are designed to run warm to hot. If you do the "wet fingertip" test it shouldn't sizzle but it will evaporate quickly. Its common for the chassis to also get quite warm hence the reason for using 105 degree C electrolytic caps if possible.

The problem with trying to run KT88s in an ST70 is that the output trannies are only rated for a certain wattage and idle current so you wouldn't be able to fully utilize them. If you did plop them in you might have to increase the available bias voltage but then the higher you raise the idle current the lower your B+ will drop in response and the hotter the PT will get also. You end up with the KT88s running with about 55-65 ma idle current but at a reduced B+. Now that might be what you want or not. The KT88s are certainly loafing along at that point. As to the sound, who knows?
I can tell you that people including myself have used EH6CA7s in an ST70 with good results. They should drop right in with just a bit of bias adjustment.

i say this to everyone, but if you are more of a function-over-form kinda guy, get a finned heatsink/fan combo from a computer store, mate it to the PT with thermal paste, and run it off a wall transformer at half to a quarter the intended voltage. i do that on my st-35, and the transformer went from operating at blazing hot to luke warm. even with the fan not running the aluminum fins radiate away a whole lot of heat

Thanks for the quick replies!

Will these elevated temps shorten the life of the power transformer any? I'll see what I can come up with for a transformer heat sink. They are a tad pricy to need replacement regularly.

Thanks for the tip about the EHX 6CA7s. I'll try to snag a quad off of fleabay.

Will these elevated temps shorten the life of the power transformer any?

i believe the concensus among the folks who know a thing or two is, "don't worry about it." but i liked the idea of cooling it off, and it is cheap 'n easy

Another thing to do, for reduced PT temps, is to run the amp at reduced wall voltage. I have my ST-70 on a Variac, set fro 115vac and it is only warm to the touch.

There is a paranoid feeling when it comes to transformer heat here on this forum. I have conducted numerous temperature tests with my IR thermometer. The transformers are still intact and running. Kt88's could be run as long as the 8.9A 6.3 volt rating is not being violated. The KT88's will run fine at 50mA or even 40mA for a total of 80-100 mA. I doubt that you are going to be running the amp full tilt so I wouldn't worry too much about the 35 watt rating on the transformers. I would however steer clear of full power sine wave tests.

Want something that runs hotter? I have some Wurlitzer mono block power amps that run a pair of 6L6's. They run in the neighborhood of 150 degrees and these amp have been running since the 40's.

Oh, I run a pair of 70's to power my 2 pair of Klipsch Cornwall clones. The amplifiers sit in an enclosed oak cabinet that has two glass door on the front. I run a floor fan on low to blow some air into the cabinet and to insure that I don't burn myself when powering things off.

Just my .02

I am not so much worried about the heat as I was curious. Having an unused variac at work, allowed me to borrow it and try it out.

The transformers are rated at 70 watts , just not at full fedelity ,So no problem there

i didn't worry so much about the transformer wire insulation breaking down: i just figured chronic, long term(i plan to keep this amp for life) heat inside the chassis will turn the plastics brittle, discolor everything, turn the cloth wire into powder, and maybe harm the PCB over the course of say 30 years, not to mention discoloring the wooden frame i made for it, breaking down the wood glue used on it, and discoloring the wooden shelf i have directly over the st-35.
if it was a point-to-point or turretboard amp i probably wouldn't be so inclined to cool it. as far as 150F goes, i bet my st-35 reissue rivalled that, before the heat sink....i have a temp probe, one of these days i'll verify that.
i own 10-11 all tube audio devices, at least three are 50+ years old, and none of the power trannies come even close to running as hot as the st-35

nyazzip wrote:...10-11 all tube audio devices, at least three are 50+ years old, and none of the power trannies come even close to running as hot as the st-35

I guess you don't have a pair of Mark III. : )