How hot do output transformers typically run on valve amps?

Skylark

I have a little VOX AC4TVmini valve amp, tiny little thing 4W with 6.5" speaker, love it!
I've noticed the OT gets very hot after an hr or so, not enough to burn you but well above what I would consider normal, this is using the amp with the attenuator on 1/4W and volume at below 1/4 and not driving the amp hard at all. The OT it is bolted directly to the back metal chassis which means I can feel the hot spot right where it is, on previous amps the OT wasn't accessible so I've never noticed how hot they get. Its noticeably the hottest amp I've had, it has very little air flow, its in a sealed cabinet with a small vent on the top and bottom.

Is this normal, checked around on google and a very hot OT seems to be normal for some valve amps or can also be a sign of a OT that's struggling a little. The OT on the VOX AC4TVmini is apparently over engineered for the job at hand, perhaps pulled from the parts box used for a range of bigger amps.

Attila

Don't worry too much about that, there is a lot of energy passing though that OT, my single ended 8W tube amp the OT gets just as warm/hot after playing a while.

Designing and winding a audio txfmr is quite complex and is a delicate balance between frequency response, output power, impedance, total harmonic distortion (THD), loss etc, etc and and tradeoffs to get a good cost effective transformer

Skylark

Thanks Attila, did some more scratching around the internet and apparently if you can touch an OT without burning your fingers then it's fine.

Gearhead

The 'classic' tone of most early guitar amps was partly due to the OT being way undersize in hifi terms - since guitar amps see notes of about an octave above what hifi does to them, you can get away with the small iron.
However, the fact that an OT gets hot is due to the amount of DC through it, and too much heat can be caused by too hot a bias.

Skylark

The bias is fixed on this amp, can't remember what the circuit is called but it's the type that auto biases the valve, there are only 2.

beardedmoose

Skylark wrote The bias is fixed on this amp, can't remember what the circuit is called but it's the type that auto biases the valve, there are only 2.

You can re-baise a class A amp. especially when you want to change the valve type.

Gearhead

Skylark wrote The bias is fixed on this amp, can't remember what the circuit is called but it's the type that auto biases the valve, there are only 2.

The word you're looking for is probably cathode bias? Those are only as fixed as the (you guessed it) cathode resistor. If this thing is a little low for the OT/valve combination, your OT may run hotter than intended. Having said that, does the manual / designer say anything about the temperature? I would personally worry if the thing runs hotter than 60-70 degrees celcius, since higher may well affect coatings and other plastic'ies. As a rule of thumb, you can hold 60C with a flat hand for about five seconds...