Hello Everyone,
I have built a few Tubeamps so far. Out of 7 Amps 6 worked right away and one i had to Bugfix a minor mistake i made. Now i would dare to say a have a small Idea how TubeAmps work. But now i would like to have a bit deeper knowledge. Soooo.... I have a Amp Kit here with a Preamp stage of an EF86 and a ECC83 and the Amp Stage consists of two EL84 in PP mode. The Amp is Rectified by an EZ81 and i have a PT with sec: 300V(100mA), 8V(2A), 6.3V(3.2A) and 6,3V(2A)
So, i have 100mA to work with the Tubes. I start to calculate:
EF86 (Ia = 3,0mA)
ECC83 (Ia = 1,2mA)
EL84 (Ia = 48mA)
a Total of 100.2 mA needed.
My Question:
I assume now, a PT shall NOT have more than the 100mA ? (because it would burn the tubes?) but as close as possible?
I am aware of the Condensators and Resistors in the Schematics but they are irrelevant for Ia (not for the voltage of course)
Or is my way of thinking complete wrong?
Thanks for Helping a Newbie
Pero
I have built a few Tubeamps so far. Out of 7 Amps 6 worked right away and one i had to Bugfix a minor mistake i made. Now i would dare to say a have a small Idea how TubeAmps work. But now i would like to have a bit deeper knowledge. Soooo.... I have a Amp Kit here with a Preamp stage of an EF86 and a ECC83 and the Amp Stage consists of two EL84 in PP mode. The Amp is Rectified by an EZ81 and i have a PT with sec: 300V(100mA), 8V(2A), 6.3V(3.2A) and 6,3V(2A)
So, i have 100mA to work with the Tubes. I start to calculate:
EF86 (Ia = 3,0mA)
ECC83 (Ia = 1,2mA)
EL84 (Ia = 48mA)
a Total of 100.2 mA needed.
My Question:
I assume now, a PT shall NOT have more than the 100mA ? (because it would burn the tubes?) but as close as possible?
I am aware of the Condensators and Resistors in the Schematics but they are irrelevant for Ia (not for the voltage of course)
Or is my way of thinking complete wrong?
Thanks for Helping a Newbie
Pero
No.
The current is determined by the tubes, not the transformer.
100mA PT is not nearly enough for 100mA consuming amp.
Download PSUD2, model your PSU - that will tell you the RMS current through the PT (after the initial surge).
Make sure your PT is rated for AT LEAST that current (more is better).
Last edited:
I advise you to study Ohm's Law (I = V/R).
The current only goes up if the voltage goes up and/or if the resistance that the amplifier is to the power supply goes down. Neither of those happen when a transformer can deliver more current than it actually does in a certain circuit.
Edit: You shoul however count on some more voltage with a PT when the PT is (for example) rated for 300V at 200 mA but you are using it in a circuit that only draws 100 mA. This is caused by the resistance of the PT itselfs (with Ohm's Law: The voltage drop in the OPT itself is halved when only half of the maximum current is being used).
Edit 2: At full power two EL84's at 300 V draw 114 mA. With most audio this will hardly happen, but if you are thinking of an amplifier for music instruments, I advise you to use a PT with some more current capability.
Last edited:
Current capability of power transformers is a quite vague subject.
The main parameters you're looking at are the ones leading to temperature increase. The major reasons for heating are copper losses.
Operation at high flux density increases the primary idle current, hence the importance of the primary copper losses.
The second parameter is transformer cooling. Mounting, chassis shape and material and transformer location are important.
When I wind my transformers, I use a certain of my own rule of thumb of watts per transformer surface area. One of my last project was a 225VA 5V 3*15A heater transformer with hand wound decafiliar heater windings.
After 5 hours at full load, the transformer didn't reach 40 degrees Celsius.
The main parameters you're looking at are the ones leading to temperature increase. The major reasons for heating are copper losses.
Operation at high flux density increases the primary idle current, hence the importance of the primary copper losses.
The second parameter is transformer cooling. Mounting, chassis shape and material and transformer location are important.
When I wind my transformers, I use a certain of my own rule of thumb of watts per transformer surface area. One of my last project was a 225VA 5V 3*15A heater transformer with hand wound decafiliar heater windings.
After 5 hours at full load, the transformer didn't reach 40 degrees Celsius.
But how does your contribution translate to the parameters of PT's which are mostly mentioned (operating voltages and maximum currents)?
for me temperature rise is number one criteria,
RDH4 chapter 5 has all information you will ever need to design and make your own...
heating in power transformers are mostly due to core hysteresis and eddy current loss due to:
one, quality of cores, M6 GOSS will have lower losses than say an M50 NOSS steel, also a 0.35 mm laminate will have lower eddy currents than a 0.5 mm laminate..
second, the lower flux density you run your steel in, the lower the loses, i run my trafos at lower than 1T...
third, 500 circular mills per ampere is typical design point for copper sizes in a traffo, the ARRL wire table says you can use anywhere from 700 to 300 cm/ampere for EI trafos..
Last edited: