Last night, screwing around with the scope, I noticed something interesting.
I have an input stage with 220k plate load, a 1k5 Rk on a 12ax7 with a 1M gain pot after. So I assume output impedance is about 49k for this stage.
The output of this input stage is a fixed value (lets say 4Vpp) with the Gain pot rotated from 0 to 5/6. But when I turn the pot past 6 through to 10, the ouput of the first stage starts to drop to about 1/2 if I recall (~2Vpp).
Is this normal? Is the grid lead loading the input stage too much causing too much input impedance?
Vpp output with Gain Control after stage
Moderators: pompeiisneaks, Colossal
Re: Vpp output with Gain Control after stage
You can measure the actual output impedance with a couple of resistors. Simply measure the output with two different loads, prehaps 100K and 1meg. Then calculate (change in voltage) divided by (change in current).
If your volume control has a bright cap, it will really knock down the high frequencies. The Miller capacitance of the next stage can do the same thing.
I've measured the output impedance of a power amp by using 4 ohm and 8 ohm loads connected to the same tap. Use a small signal, not near full power.
If your volume control has a bright cap, it will really knock down the high frequencies. The Miller capacitance of the next stage can do the same thing.
I've measured the output impedance of a power amp by using 4 ohm and 8 ohm loads connected to the same tap. Use a small signal, not near full power.