The Laney GC30V I have to service has a drive channel. While waiting for parts to service it, why not have a look on this channel ?
Here is an extract of the schematic (under PSpice, but let forget that, the simulation is by far not very accurate, specially on the plate side where it does not reflects the reality observed with an oscilloscope …)
For the first triode, the bias point is at Vk = 1V (I’m not sure Rk1 is 820R, I would say 1.3k by calculation, to be checked, but this resistor is CMS on the other side of the PCB so it’s not easy to have a quick look)
Grid current limiter
We can assume that when the signal at GridU1 is near 1Vp (or 2Vpp) something unlinear will happen …
and this is when we inject 2Vcc at the first grid (so Vgk=0V) :
We can see that it’s impossible to reach positive 1V peak (negative 1V peak is OK).
If we continue to increase the amplitude of the signal at the grid, the signal is clipped on the positive side at ~1V :
When Vgk becomes 0V, electrons are attracted by the grid (rather than by the plate) and there is a grid current (from grid to cathode). It’s the grid-current limiting effect.
Here is the output signal (2 following views, second time/div rescaled to emphasize), softly and warmly clipped :
(Note : a triode inverts the output signal compared to input signal)
Our triode is biased at Vgk = -1V and Ua0 = 136V, right ?
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