Grid-current limiting and cut-off …

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 …)

Laney G30VC Drive channel(as always, click on pictures to get bigger view)

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 …

laney triode tube 1 Here is a 1Vpp 1kHz signal at the grid (scope at 0.5V/DIV)

and this is when we inject 2Vcc at the first grid (so Vgk=0V) :

Laney grid triode 1 tube 2Vpp2Vpp (scope at 1V/DIV)

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 :

Laney tube triode grid current limitingGrid signal clipped on positive side (scope at 1V/DIV)

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 :

laney grid current limiting 1laney grid current limiting rescaled

(Note : a triode inverts the output signal compared to input signal)

Cut-off

Our triode is biased at Vgk = -1V and Ua0 = 136V, right ?

Bias laney triodeSo, when Vgk becomes, let’s say -4Vp, the anode cannot swing above 235V (seeV1 on the schematic), and then we get cut-off. The grid is very negative and electrons cannot pass through.

Grid current Limiting And Cut OffOutput signal : note the clipping on the positive side, harder than soft clipping of the grid current limiter

Triode 2 Grid Limit And Cut-offCut-Off (positive on top) and grid current limiting (bottom, negative side) out of triode #2 (see schematic)

Laney drive channel triode 2 Cut-off Grid-current limitingTriode 2, anode and grid (inverted, scaled) waveforms

Great link (among others), great books : Valve wizard

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One Response to Grid-current limiting and cut-off …

  1. Pingback: Laney GC-30V | It-11 Audio / Tonegeek

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