50/100 watt switch?

[echo44]

Ok so I have been studying the schematic for #124
so when you switch to 50 watts tube v5 and v6 get switched from their normal position of cathodes connected in parallel to ground with V4 and V7
to running through 10k resistors and a diode that connects to ground.
How does this work since the diode is connected to ground and electrons are traveling from ground through the diode can't they keep going to create current in V5 and V6. In other words when you change to 50 watts the cathodes are still connected to ground except a diode and resister are in the path?

[martin manning]

As the electrons flow toward the cathode the voltage increases from ground, a couple of volts for the LED, and an additional amount accross the resistor, proportional to the current. This lifts the cathode above ground, making the grid (which is fixed at Vbias) more negative with respect to the cathode, which has the effect of reducing the current flow. An equilibrium point is reached where only a very small current flows, essentially biasing those two tubes into cut-off and preventing them from contributing to the output power. Most people don't find this feature terribly useful, as it doesn't reduce the aparrent volume much, and the master is quite effective if a lower volume setting is needed.

[talbany]

Ok so I have been studying the schematic for #124
so when you switch to 50 watts tube v5 and v6 get switched from their normal position of cathodes connected in parallel to ground with V4 and V7
to running through 10k resistors and a diode that connects to ground.
How does this work since the diode is connected to ground and electrons are traveling from ground through the diode can't they keep going to create current in V5 and V6. In other words when you change to 50 watts the cathodes are still connected to ground except a diode and resister are in the path?

Dumble put the LED in series with the tubes' cathodes, with a 10K resistor to short the resistor in 100 W mode, flip the switch and the 2 tubes will pass enough current to light the LED.

When you lift the 2 cathodes you still have a 2K primary but you have tubes that are wanting to see about 4K, so they end up dissipating more heat, they'll still operate, but they'll operate hotter and be biased differently. You only really lose 1/5 or so of the output power when the Load is a perfect 4 ohms,

IMO...A better way to do this would be (Gil's method) lift the ground on two of the tubes on one side of the switch, while tapping off the 6.3 VAC supply with a diode + 390 ohm resistor + LED = 100W mode.

Hope this makes some sense,,


Tony

[martin manning]

When you lift the 2 cathodes you still have a 2K primary but you have tubes that are wanting to see about 4K, so they end up dissipating more heat, they'll still operate, but they'll operate hotter and be biased differently.

But of course if you have a multi-tap OT you can for example run an 8-ohm load from the 4-ohm tap and keep things as they were in 100W mode.