![[-]](https://theultimatetone.com/images/collapse.png "[-]")
03-09-2022, 12:55 PM
Hi Guys
Strelok: Earlier on you suggested that you changed the output stage of the London Power Standard Preamp to a tweed mixer and got a huge loudness jump. The reason for that is that there is no longer attenuation at the output of the Clean and lead paths.
At the input of the LPSP 4th stage, two 470k resistors tie the Clean and Lead signals to the grid. The source end of one resistor or the other is grounded, allowing the opposite path to be active. This muting action creates a voltage divider, cutting the enabled signal in half at the grid.
When you replace the 4th stage with a Tweed Mixer, each path feeds directly into a grid (via a grid-stop, hopefully) and there is no specific attenuation. Grounding one grid or the other does not alter the signal level of the enabled path, resulting in a signal twice as high compared to the stock LPSP.
Note that if you use a relay for this selection, it is unlikely that any series resistance need be added ahead of the grid-stop. If you use jfets, then series resistance will be required as the jfet Ron is not zero.
You can use higher Rk values for the two triodes of the mixer. 1k5 is getting you extra loudness; 10k would be a very low gain of 3x through each path; values in between would achieve intermediate gains and loudnesses; likely something around 3k3 would be close to the stock signal levels. As I mentioned earlier, Rk values can be different to achieve whatever signal balance you desire.
Have fun
Strelok: Earlier on you suggested that you changed the output stage of the London Power Standard Preamp to a tweed mixer and got a huge loudness jump. The reason for that is that there is no longer attenuation at the output of the Clean and lead paths.
At the input of the LPSP 4th stage, two 470k resistors tie the Clean and Lead signals to the grid. The source end of one resistor or the other is grounded, allowing the opposite path to be active. This muting action creates a voltage divider, cutting the enabled signal in half at the grid.
When you replace the 4th stage with a Tweed Mixer, each path feeds directly into a grid (via a grid-stop, hopefully) and there is no specific attenuation. Grounding one grid or the other does not alter the signal level of the enabled path, resulting in a signal twice as high compared to the stock LPSP.
Note that if you use a relay for this selection, it is unlikely that any series resistance need be added ahead of the grid-stop. If you use jfets, then series resistance will be required as the jfet Ron is not zero.
You can use higher Rk values for the two triodes of the mixer. 1k5 is getting you extra loudness; 10k would be a very low gain of 3x through each path; values in between would achieve intermediate gains and loudnesses; likely something around 3k3 would be close to the stock signal levels. As I mentioned earlier, Rk values can be different to achieve whatever signal balance you desire.
Have fun
