1. ## Question on 6V6GTA

I am studying the data sheet of 6V6GTA, what is the maximum plate current? I see for Class A1

1) Zero signal plate current is 34mA.
2)Max signal plate current is 35mA.
3)Max in triode mode is 40mA.
4) Max power=14W.

Is the max current equal to max power point(VpXIp=14W) on the plate curve for G1=0V?

In the RCA manual, for triode, it said Imax=2XI_0 where I_0 is the zero signal plate current for Class A1 which implies Imax=2X34mA=68mA.

2. What you are reading is in the section titled Typical Operation. The maximum current is for that circuit with the listed load, screen voltage and B+. It doesn't really say it anywhere, but most people assume that is at the maximum undistorted output signal. There is no current specified under Maximum Ratings.

3. That's why I am asking, because it does not provide the absolute maximum plate current, it is hard to design.

For single end Class A, the book said to design the DC operation point at near the maximum power dissipation of the tube for a given screen voltage. Do I use the plate voltage available ( governed by the power transformer), then calculate the quiescent current in such a way that IqXVp=W=14W for one 6V6GTA? So if plate voltage is 300V, then quiescent current = 14W/300V=46.66=45mA. With this, forget the maximum current and call it a day?

4. The absolute max current is going to be at a very low plate voltage to stay at the 14w... I myself wouldn't bias it near max dissapation at idle even though it is single ended because when the input signal swings positive the current through the tube will increase above the max watts. If you know how many volts your preamp is swinging with your volt meter on ac volts with a signal generator you could center bias it to get the max headroom. So say if you swing 20 volts peak to peak then you would bias at -10 volts or more. If the preamp tries to swing the grid higher than the cathode because you biased too hot it will just get clamped and introduce a bunch of second order harmonic by cutting off half the wave, unless you are able to get class a2 with an interstage transformer or something that can give current like a mosfet. Also biasing too hot runs more current in the primary of the output transformer eating up the inductance and reducing the bass response. Hope this helps.

5. Originally Posted by Austin
The absolute max current is going to be at a very low plate voltage to stay at the 14w... I myself wouldn't bias it near max dissapation at idle even though it is single ended because when the input signal swings positive the current through the tube will increase above the max watts. If you know how many volts your preamp is swinging with your volt meter on ac volts with a signal generator you could center bias it to get the max headroom. So say if you swing 20 volts peak to peak then you would bias at -10 volts or more. If the preamp tries to swing the grid higher than the cathode because you biased too hot it will just get clamped and introduce a bunch of second order harmonic by cutting off half the wave, unless you are able to get class a2 with an interstage transformer or something that can give current like a mosfet. Also biasing too hot runs more current in the primary of the output transformer eating up the inductance and reducing the bass response. Hope this helps.
...he should also consider reading/studying about "signal rectification" and its resulting affect on the bias point.

6. Originally Posted by Austin
... when the input signal swings positive the current through the tube will increase above the max watts.
This is not true for the classic class A amplifier. Because the plate voltage goes down when the current increases, instantaneous plate dissipation actually goes down as power is delivered to the load. Even if the load line is not tangent to the 14W curve, the average plate dissipation will stay around 14W or less.

7. Originally Posted by loudthud
This is not true for the classic class A amplifier. Because the plate voltage goes down when the current increases, instantaneous plate dissipation actually goes down as power is delivered to the load. Even if the load line is not tangent to the 14W curve, the average plate dissipation will stay around 14W or less.
Thanks that's a good way to put it and makes sense. For every up swing of the sine wave it must also swing down by the same amount and the average of the two ends up being almost the same as no signal right?

8. It's true for a resistive load. For a reactive load the swings above the 14W line go up faster than the swings below it decrease because of the "power is proportional to voltage squared" effect, but tubes seem to handle it ok. If this worries you, use a circuit modeling program.

9. The peak current capability of tubes was never explicitly stated in those old datasheets. The figure they give for max signal plate current is an average, as you would see on a moving coil analog meter. They specified it that way because that was the instrument most labs would have in the 50s.

Knowing that you can work out what the peak current would be. It works out about 400mA in a 50 watt tube amp.

10. The peak current capability of tubes was never explicitly stated in those old datasheets. The figure they give for max signal plate current is an average, as you would see on a moving coil analog meter. They specified it that way because that was the instrument most labs would have in the 50s.

Knowing that you can work out what the peak current would be. It works out about 400mA in a 50 watt tube amp.

11. What about some of the tubes we use that have numbers for "pulse". I don't know what applications this would be for but there are some pretty wild numbers. I will see if I can find one, but have seen pulse ratings on pretty common tubes because maybe that would be closer to the absolute max peak current capability. Useless as that might be for audio, still interesting imo.

12. Pulse applications would be exactly that - pulses. Like a radar transmitter sending out a steady stream of pulses to bounce off whatever is out there. The horizontal output tube of an old tube TV set would pulse 15,000 times a second - little narrow but tall spike - to make the beam sweep on the picture tube. A tube might be rated for 1200v and some current, but was not designed to take that continuously.

Can you use tubes for other than their intended purpose and outside their published specs? Of course. Just don;t ask RCA or whoever how the tube will react.

13. Originally Posted by Enzo
Pulse applications would be exactly that - pulses. Like a radar transmitter sending out a steady stream of pulses to bounce off whatever is out there. The horizontal output tube of an old tube TV set would pulse 15,000 times a second - little narrow but tall spike - to make the beam sweep on the picture tube. A tube might be rated for 1200v and some current, but was not designed to take that continuously.

Can you use tubes for other than their intended purpose and outside their published specs? Of course. Just don;t ask RCA or whoever how the tube will react.
Ask this guy. I bet he knows how much it takes to blow up a 6v6.

Tube Challenge

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