Max grid values

[Illinest]

I'm not sure how to phrase this.

Is there a limit to the size of the signal that the grid can handle?

I had an idea to replace two 12ax7 gain stages with three 12au7 gain stages.

I figured 100x100 equals 10,000
And
20x20x20= 8,000

Which is about what I want - I think - but I'm not sure.

I'm trying to clean up a 5f1 circuit but I don't want to make it wimpy. I have an extra triode available (another story) and it seems like a good theory. But when the signal hits the tone stack it will be 20x20=400x, as opposed to the 100x of the typical Fender circuit. I'm afraid that hitting the grid of the last triodes with a big signal will create distortion.

If I had to guess - I'd guess that the B+ value has something to do with this?

 

[Enzo]

So your plan is a 20% reduction in signal gain through the stages? if your signal is too hot, why not just add a bit of signal attenuation between stages. A simple two resistor voltage divider.

Or try a pair of 12AX7 circuit, but replace one of them with a 5751.

 

[Illinest]

I was afraid that adding those resistors would create an RC filter. I didn't really think it through too much though.

I'm trying to do a stereo amp so the signal has to be split. Is it better to split before the first valve, or after?

 

[Printer2]

50V on a 12AX7 from my memory.

 

[DF96]

The max signal you can put on a triode grid before significant distortion sets in depends on the bias and the circuit, but for a very rough estimate take the supply rail voltage (B+/HT) and divide by twice the valve mu. The max signal before damaging the valve will be higher, maybe a few tens of volts.

 

[Illinest]

So a 12ax7 at 200v starts to distort at 1v?
And a12au7 at 200v starts to distort at 5v?

 

[DF96]

Very roughly, yes. Take these figures as an estimate which will probably be within a factor of 2 either way from the actual figure. Note that distortion does not start immediately you cross a threshold, but grows increasingly rapidly as you approach the threshold.

 

[Illinest]

Rough estimates work great for me. This is awesome.

 

[Printer2]

Sorry, read your question differently. If you want more headroom run at a higher plate voltage, plate resistor ans cathode resistor. Nothing magical about 100k and 1.5k.