power transformer getting hot

[JoshBernstein]

Hello all,
I recently finished a trainwreck rocket-ish build, and it sounds really good. Yesterday i took it to practice for the first time, and it sounded good the whole time. After i was done playing, i happened to touch the power transformer and noticed that it was hot. The amp had been on stabdby for about 30 minutes before practice, and we played for approximately 2 hours straight. The tranny is a hammond 270hx, and i know that the 270 series is notorious for its trannys getting hot, but i want to make sure i don't have a problem. There was no burning smell or smoke, but it was hot to the point that i wouldn't want to keep my hand on it for more than 5 seconds.
Thank you!
-josh

[Phil_S]

You'll get a 3rd degree burn from a 6 second exposure to 140F. That isn't even close to smoking a transformer. You'd really have to get probe or thermal imager to get an accurate temperature reading. I'm going to guess anything above 180F is too hot. I'm doubtful you are close to that. If it hurts, don't do that!

[stretch2011]

If you are in or super close to within specs you are fine. Yes it's nice to have overrated transformers.

Marshalls are rated at 150ma, el34's draw 150ma at zero signal and 198ish at max signal. Granted voltages change that.

[pdf64]

Due to the potential for cathode poisoning / sleeping sickness / cathode interface resistance, it would be better not to leave a tube amp in standby for prolonged periods.
See p33 http://www.tubebooks.org/Books/Atwood/Tomer 1960 Getting the Most Out of Vacuum Tubes.pdf
Switch it on when you need it.

[Stevem]

Get a hold of a thermal gun and check its temp , also if the filiment voltage is above 6.1 it's likely all is well!

[JoshBernstein]

Thank you for all the responses. Will get ahold of a temp gun, and the filament voltages are running at a steady 6.28 volts

[13GB5]

Paraphrasing an old Pass rule of 'thumb.' if you can touch it for 10 seconds it is 45 degree Celsius and blimey hot [112F]. Only 5 seconds it is 50 degree Celsius and crikey hot [122F]. At 2 seconds it is 55 degree bloody hot [131F]. If you have to pull away immediately it is f##kin' hot at over 60 degree Celsius [140F].

[Stevem]

With many amps you can sit them on a side or on there face in such a way that will keep output tube heat off them and check if the pT still gets as hot!

[sliberty]

The 270HX is rated at 550V CT @ 230ma. And 6A on the 6V tap.
RJ's trannies appear to be between 250-300mA
I think the HTS-9144 that we used to get from Moose was 300mA

So, maybe 230mA is a little light????

[R.G.]

Paraphrasing an old Pass rule of 'thumb.' if you can touch it for 10 seconds it is 45 degree Celsius and blimey hot [112F]. Only 5 seconds it is 50 degree Celsius and crikey hot [122F]. At 2 seconds it is 55 degree bloody hot [131F]. If you have to pull away immediately it is f##kin' hot at over 60 degree Celsius [140F].

The rule of thumb our techs used to use is that if you can keep the tip of your index finger on it for a notable period of time, it's around 130F, and by extension for "normal" transformer iron, copper and insulating schemes, the hot spot in the transformer is below 105C.

105 C is the *lowest*, most trivial and wimpiest insulation rating for transformer insulation systems. It's unlikely that you've ever encountered a manufactured transformer that doesn't meet at least that.

Iron and copper in transformers work fine up to the Curie point of the iron, something like 750C. At that point iron ceases to be magnetic and things get worse rapidly. But the insulation is long gone. The highest normal insulation class is Class 180, meaning good to about 180C inside. The old IBM giant mainframes had transformers with Class 180 insulation and they ran hot enough to actually give you burns, or to cook eggs on. They were designed to run that way.

Transformers die when the insulation gives up. So if you can keep your finger tip on it for a few seconds (and you're not Gordon Liddy - look it up) then the tranny is fine.

Even the tiniest amount of air flow through an amp drops the internal temps dramatically.

[TUBEDUDE]

Love the Liddy reference. Or Kwai Chang Kane, (carries roasting amp chassis on forearms outside and collapses in snow with new transformer tattoos burned into arms).