... Seymour Duncan Model 2000 Convertible 100W amp schematic
... the amp has a Variable Damping output ...
V1 is a long-tail phase inverter. R36 and R37 are individual self-bias resistors for each half of the inverter, and the "tail resistor" R39 is connected to the bias supply.
- Pin 7 is the Signal Input for this inverter.
- Pin 2 is the Feedback Input for this inverter/opamp.
- Feedback comes from the Orange tap through C25 to R35, then is applied through grid-stopper R12 to the Feedback Input of the Inverter.
- So "Variable Damping" is just a variable feedback; it changes how R60 and R61 are/are-not connected across C25.
... Between the PI and power tube there is a 12AU7 tube that is connected a way I don't undestand how it works
in junction to this (V3) tube there is a Variable Power pot ...
How many times has someone asked about having remote or foot-pedal control of something within the amp? Maybe a master volume? And how many times have we said it is a bad idea to run signals on long wires outside the amp?
I think the Variable Power control is a wah- or volume-style pedal (with only a pot inside) whose sole task is to select a control voltage between 0v and about -110v, which then controls the master-volume setting of the amp.
In other words, it's a remote master volume control that avoids the problems of running signal outside the amp to a spot the player can control the setting.
V2 is a Differential Amplifier stage, between the phase inverter at V1 and the output tubes. Notice that "F" is a negative voltage even larger than the bias voltage "G" applied to the output tubes.
V3, the Variable Power stage, sits between V2 and the coupling cap to the output tube grids.
- V3 is also a differential stage.
- V3's cathodes are connected to ground through self-bias resistors.
- V3's plates are connected through resistors to the plates of V2.
-
V3's grids (Pin 1 of J10)
receive a voltage swept from very-negative "F" (Pin 3 of J10)
to ground (Pin 2 of J10)
Seems to me V3 is being used as a
variable resistor: the changing resistance is that of V3 internal plate resistance.
- "F" is given in the voltage chart as -110v.
- Don't know V3's plate voltage, but 300v is a reasonable guess since "B" is 450v.
- The Page 4 of the
12AU7 data sheet shows we can likely cut-off V3 with a bias of -25 to -30v.
The bottom of page 3 of that data sheet shows when the 12AU7 has 300v on its plate that "Rp" (internal plate resistance) is 15kΩ when the 12AU7 passes 3mA. If a large enough negative voltage is applied to V3's grids, the tube will cut off, and be ∞Ω to ground.
- 450v is dropped across 200kΩ (R44, R47) with a current of 2.25mA.
- V3 can't be pulling all 2.25mA, as some has to be for V2 to operate/amplify.
- So V3 is doing double-duty: it likely
varies the plate voltage of V2 and is a variable resistance-to-ground.
The output tubes are left alone. "Variable power" appears to be achieved by altering how much distortion happens at V2, due to applying a heavier loading (close to 12kΩ due to V3's Rp in parallel with R58 or R59) and also lowering the plate voltage of V2. Both of those will shrink the output voltage swing of V2.
All this can be done with the 100kΩ pot on-board the amp. Or another (probably lower-value) pot can be plugged into the jack provided, which gives the player remote control of the control-voltage (bias of V3) that determines how much clean drive signal can get to the output tubes.
Topic: Seymour Duncan Model 2000 - How do V3, Power Control and Variable Damping work ? (Read 3252 times)