Tube Amplifier - Output Transformer always cold

[Hari Iyer]

Now my Dared VP-300B tube amplifier must have clocked over 400 Hrs of burn in. Since past 3 weeks, i have noticed that the output transformer of the tube amplifier always remain cold even after 3 to 4 hours of continuous operation at moderate sound levels. At the same time the power supply of the amplifier is very hot even after 30 minutes of operation.

This is a 6 watts+ 6 watts SET amplifier and is now 8 months old.

Do all SET amplifiers show similar behavior? Or is this something to do with the load (TL speakers)? Looking for FMs views.

 

[GeorgeO]

The mains trafos will generally heat up, not the outputs, is what I have found after owning such amps for many years.

 

[wlg]

The power transformer secondary supplies 6.3V to the valve filaments and HT voltage to the rectifier valve plates, dealing with current it will get hot, while the output is an AF transformer driving the speakers.

 

[Beast_of_burden]

The output transformers rarely get hot whereas the power tranny gets really hot. In one instance which was a SEP, got too hot to the touch. I installed heatsinks to dissipate the heat a little better. The Dareds have a separate toroidal power tranny and I had a push pull amp which got warm but never too hot.

How much did you pay for the 300b amp? Please share details of the dealer.

 

[Fantastic]

The power transformer feeds power to the SE stage and the heaters for the amplifying tubes and the rectifier. That's a fair amount of power...likely over 70 watts. Since the transformer is on an average 85% efficient , the rest is lost as heat. Toroidal transformers can do better than that.

The output transformer only handles the output audio signals. At say 85% efficiency and only 3 watts ( on peaks) the average loss will be very little. Not enough to make it warm. Considering we are in Dec , it will be even cooler as the ambient is much lower. Amps with integrated power supplies could cause the OPT to feel warmer due to proximity of the power transformer and heat conducted via the chassis.

 

[linuxguru]

The O/P trafo on a SET amp has a small DC quiescent current (basically the plate current of the output tube), so it will dissipate a small amount of power even with no signal. OTOH, push-pull tube amps often have trafos running with *zero* DC current, so they dissipate nothing when there's no signal.

Even with maximum signal, dissipation is only in the region of a few 10s to 100s of milliwatts - the O/P trafos are over 90% efficient before core saturation.