![[-]](https://theultimatetone.com/images/collapse.png "[-]")
05-29-2023, 12:04 AM
Hi Guys
There is a variation of the scheme presented above that is simpler in two regards, but first I will describe the difference.
The circuit for the most part is like the above schematic. The difference is that the stock Rk + Ck remain as they are and a second R-add is inserted between them and ground. The bias pots are wired in parallel with R-add, oriented as shown with pot-0 to ground and pot-X to the junction of R-add, Rk and Ck.
The first advantage of this version over the first is that the maximum current is still the same as stock. In the first circuit the range resistors in series with the bias pots could allow for higher than stock currents if not chosen correctly based on the pot value.
The second advantage is simply the saving of the range resistors, one R for SE, two Rs for PP, one R per individually biased tube.
Ck could be changed out with a higher-voltage unit per the previous posts, but this scheme allows it to remain stock.
On the general scheme of cathode bias and the standard range of output tubes used in guitar amps, you could view Rk/tube = 500R. So, SE would use Rk=500R, PP would use Rk=250R and four tubes using a shared Rk=125R. The latter is highly NOT recommended despite every AC-30 and clone doing so. These values seem to work reasonably well for the large bottle tubes and EL-84, each adapting to the Rk value based on the tube's own transconductance.
Because the tubes idle at full dissipation, it is extremely important to have proper value screen-stops, as TUTs indicate.
There is a variation of the scheme presented above that is simpler in two regards, but first I will describe the difference.
The circuit for the most part is like the above schematic. The difference is that the stock Rk + Ck remain as they are and a second R-add is inserted between them and ground. The bias pots are wired in parallel with R-add, oriented as shown with pot-0 to ground and pot-X to the junction of R-add, Rk and Ck.
The first advantage of this version over the first is that the maximum current is still the same as stock. In the first circuit the range resistors in series with the bias pots could allow for higher than stock currents if not chosen correctly based on the pot value.
The second advantage is simply the saving of the range resistors, one R for SE, two Rs for PP, one R per individually biased tube.
Ck could be changed out with a higher-voltage unit per the previous posts, but this scheme allows it to remain stock.
On the general scheme of cathode bias and the standard range of output tubes used in guitar amps, you could view Rk/tube = 500R. So, SE would use Rk=500R, PP would use Rk=250R and four tubes using a shared Rk=125R. The latter is highly NOT recommended despite every AC-30 and clone doing so. These values seem to work reasonably well for the large bottle tubes and EL-84, each adapting to the Rk value based on the tube's own transconductance.
Because the tubes idle at full dissipation, it is extremely important to have proper value screen-stops, as TUTs indicate.
