124 v2 180k grid resistor

[llemtt]

That resistor has also an important function in the dynamics of interstage current/voltages: when gate of V2b approaches voltage of V2b cathode, it start to draw current, that resistor determine the current into the gate, the higher the value the lower the current.

Everything in between V2a plate and V2b gate is at the heart of the ODS overdrive.

Cheers
Teo

[martin manning]

That resistor has also an important function in the dynamics of interstage current/voltages: when gate of V2b approaches voltage of V2b cathode, it start to draw current, that resistor determine the current into the gate, the higher the value the lower the current.

This is in service of preventing blocking distortion in overdrive by limiting grid current. In so doing it limits the degree to which the coupling cap feeding the grid becomes charged, which can temporarily upset the bias point.

[llemtt]

Yess correct, when V2b is overdriven the average/bias grid voltage goes more negative each time the signal swings positive generating grid current, until a new "equilibrium" point is reached (it gets very negative! even more than a few volts). When the signal decreases and V2b is less overdriven grid voltage increases returning towards the "no signal" bias point.

This is the main source of all the wonderful dynamics of the ODS overdrive. The higher the grid resistor value the greater the effect.

Usually a 10k resistor is enough to prevent blocking distortion but is has different dynamics -> like changing attack/release time of a compressor.

A 180k value is the highest you can use without too much audible high frequency loss, a 220k is too much, so it makes perfect sense a 5%-10% fine tuning in a particular amp.

A 150k resistor gives more transparent high frequencies, but has a tighter/different dynamic response that can be felt (sometimes I prefer it).

[pottedplant]

that is very fascinating, i've often wondered about the unique dynamics of an ods, this seems to be a major factor in it and i had never considered it. thanks for sharing what you've both learned, it's super intriguing. i need to get my build done so i can finally experience this first hand!

[Reeltarded]

This is interesting for sure, and I have one data point that might point to the modification.

200k is a less likely part to find. I have a bunch of industrial switching boards I scavenged. Each had 2-3 200k 1w resistors.

While messing with a beastly 3 stage amp I tried a bunch of low values to keep it open. When I started throwing high values the amp got smooth and harmonic. 180 was too low. 220 wrecked the shimmer (knee below my 8k target rule) so I reached into the odd pile and 200k is still glued to that second stage grid.