Plate voltage swing of a single ended power tube.

[Alan0354]

I know in the push pull power amp with center tap output transformer, the plate voltage of each tube can swing much higher than the center tap voltage. For example, if the center tap is +400V, if the plate signal is 400V peak to peak. The plate voltage swing from +200V to +600V.

My question is about the single ended class A amp. Say if the idle voltage is +400V. If the plate has 400V peak to peak swing. Does it swing from +200V to +600V also? I believe it does, I just want to verify.

Thanks

[nickb]

Yes, it does.

[g1]

In a single ended amp? As Enzo asked in the other thread (http://music-electronics-forum.com/t35226/), where will that extra voltage above B+ come from?

[Alan0354]

In a single ended amp? As Enzo asked in the other thread (http://music-electronics-forum.com/t35226/), where will that extra voltage above B+ come from?

For one, the transformer also acts like an inductor that when you decrease current, back emf created to try to maintain the current.....by raising the voltage at the plate.

Also think of it this way, in order to have a linear sine wave at the transformer output, you have to have a linear sine wave at the plate. If the plate sit at 400V, you have to swing more positive to get a sine wave.

This is completely different if you have a resistor load ( like 100K in the preamp section), where you can only goes as high as +B. If +B is 300V and the plate is at 200V, the plate can only swing to +300V and clip. Transformer is an reactive device, it can swing up above the +B.

[nickb]

It comes from the energy stored in the magnetic field of the output transformer.

[Alan0354]

Actually this originated from the other thread about 12AT7. Fender reverb circuit use 12AT7 in single ended configuration to drive the reverb tank. +B is +400V to even +430V. this means the plate can easily swing beyond +600V when you crank up the volume of the vibrato channel because it is driven by the second stage of the preamp. The signal input is very high. +600V is way way beyond the max voltage of 350V. How can the 12AT7 survive as that circuit is very reliable.

[Jazz P Bass]

To me it is not so much the voltage on the plate as it is the current through the tube itself.

By the way, I looked up a Fender Twin Reverb RI & there is a TP 67 Vac signal on the plates.

And yes, 440 Vdc on the plates.

7.8 Vdc on the cathode.

Also, the 440Vdc source is not in isolation.

It is stuck right in the middle of the power supply circuit.

If all of these 'stray voltages' where wandering around the power supply, don't you think something else would be affected.

[Alan0354]

To me it is not so much the voltage on the plate as it is the current through the tube itself.

By the way, I looked up a Fender Twin Reverb RI & there is a TP 67 Vac signal on the plates.

And yes, 440 Vdc on the plates.

7.8 Vdc on the cathode.

Also, the 440Vdc source is not in isolation.

It is stuck right in the middle of the power supply circuit.

If all of these 'stray voltages' where wandering around the power supply, don't you think something else would be affected.

Yes, it specified 67 V ac on the plate, the is rms, that means it's 2X1.414X67=189.5V peak to peak. So the highest point is 440+94.7=534.7V.

[Enzo]

OK, I will say it one more time, then move on...

WHY is this becoming a problem NOW, after being a successful and reliable design for over 50 YEARS???

[Alan0354]

OK, I will say it one more time, then move on...

WHY is this becoming a problem NOW, after being a successful and reliable design for over 50 YEARS???

No no, I am not saying it's a problem. I am just curious about this. I know it is reliable, just want to know why. I just bought a 73 Pro Reverb waiting to arrive.............ha ha!! I just trying to bone up the schematic and spot this. This is just knowledge base.

[g1]

As far as a single ended amp swinging above V+, up to double B+ is theoretically possible.
nickb was kind enough to send me some links explaining it, thanks nick!
Electronics 26
Transformer coupled class a power amplifier - Electronic Circuits and Diagram-Electronics Projects and Design