Overdriving the grid of a triode

[MadDave12]

Hi, all. Quick question I haven't been able to find an answer to via Google. My training is digital, so pls forgive the naive question as I'm an analog newbie. I'd rather not destructively experiment on too many $10 tubes when I can just ask.

I'm looking at ways to get increased distortion effects from a tube guitar amp I'm slowly building.

What's the effect of sending a much larger signal to the grid of a triode tube? Does it have a different impact on the output waveform than dropping the plate voltage?

Inquisitively yours,

[Tweeker]

How is the previous stage is coupled to it?

[MadDave12]

Ummm - any way we can think of. I'd expect to be coming off one half of a 12ax7 to the second half, with any necessary coupling in between.

Am I answering the right question?

[Tweeker]

RC coupling, vs direct or transformer coupling changes the behavior. Try searching for blocking distortion for more information.

[MadDave12]

I see. Excellent discussion (aimed at my level) here, as well: http://www.aikenamps.com/BlockingDistortion.html

So, blocking distortion sounds like it's definitely not what I would want.

Sorry, then for the next question, but...

Where does "desirable" distortion come from? Current starvation?

[7N7]

Originally posted by MadDave12
I see. Excellent discussion (aimed at my level) here, as well: http://www.aikenamps.com/BlockingDistortion.html

So, blocking distortion sounds like it's definitely not what I would want.

Sorry, then for the next question, but...

Where does "desirable" distortion come from? Current starvation?

Well it depends who exactly is defining desirable distortion. For example, Eric Clapton went through a period where he used Soldano amplifiers; the resulting sound to me and some of my friends resembled a wasp in a jam jar. Soldano sells amplifiers so someone must like it. I think Eric should have listened to the sound he got in 1966!

Overloading the input means that on one half of the cycle, the valve grid may reach 0V which means that a large current will flow, limited of course by the anode load resistor. On the other half of the cycle, the valve will be turned off by the large negative signal. The result is usually copious amounts of second harmonic (with triodes) which most people consider desirable as it is musical. Inevitably as overload increases, odd order harmonics will begin to appear especially when the grid goes positive and grid current flows. This reduces the input resistance dramatically and odd_order harmonics will be generated.

Of course this is just a single stage I am describing. When an amplifier is driven into clipping lots of horrible odd-order harmonics are produced. This of course may be just what you are seeking!

Others hereabouts will be able to provide a greatly superior summary than I have, so hold on!

Best of luck in your search.

7N7

[Printer2]

Could anyone tell me how much grid current a 12AX7 can take before getting damaged. Can't seem to find any information about driving a signal tube grid positive (with a 75k grid stopper) by a low impedance source. I am trying to figure out how many volts you could put on the input.


Old thread, should I have started a new one?

[tubelab.com]

Could anyone tell me how much grid current a 12AX7 can take before getting damaged

I can only tell you that I haven't blown one up yet. With a reasonable grid stopper, and the normal 75 to 100K plate load on the previous stage, I don't think anything can be blown up. I currently have 4 stages of 12AX7 chained together and nothing blows up. There is too much gain though. I am now working on modulating the excess gain with LDR's.

[Printer2]

Normally I would say it would not harm the tube. But I am in a discussion elsewhere where the question is about feeding one amplifier 20W tube amp into the input of another. Basically the opinion was that you would blow up both amps, as silly as that sounds. While I have fed a large voltage into a triode it usually was done from a source that had a relatively high output impedance. I am not sure what an output of another amplifier would do to an input. I would try it but I am not set up for it right now.

[SemperFi]

The EAR V20 used 12AX7s in the output and I think they were driven in class-AB2. I seem to recall some older german (Telefunken?) datasheets showing curves with positive grid. If modern 12AX7s are correct clones of the old ones, they too should handle grid current.

The typical gain stages in guitar amps cannot drive the grids going positive b/c each stage presents a high impedance source, and usually there is some high ohmage and/or EQ-stuff between the stages as well. Driving grid current requires a source that can source the current. But then the question is if the distortion is any better since the result will be that the plate is pulled further down than without grid-drive, and ultimately reaches saturation just as normal, just that the amplitude is larger. I'd think driving the grids positive for the sake of tone (guitar usage) will produce more odd harmonics since it will saturate more sharply and symmetrically.

Driving a guitar amp with another will give you noise problems up your ... A high gain guitar tube amp can probably take an input signal > 20V but it will be so 5150 it wont be very useful. It'd probably be impossible to control b/c of feeding.