ac30 OT with 2xEL34 or 6L6 ?

[phillyhudson]

Hi all - I have most of the components for new ac30 type amp , but the chassis only has space for 2 octal power valves - so no 4x EL84,s - if the OT is 4K will that work with el34's or 6L6's without changing anything [apart the sound] ?
ta

[sluckey]

Hi all - I have most of the components for new ac30 type amp , but the chassis only has space for 2 octal power valves - so no 4x EL84,s - if the OT is 4K will that work with el34's or 6L6's without changing anything [apart the sound] ?

You will need to change the output tubes cathode resistor. 250Ω is commonly used for 6L6s. 130Ω is more common for EL34s. Matchless liked to use separate 270Ω resistors, one for each tube.

[ChopSauce]

Matchless liked to use separate 270Ω resistors, one for each tube.

Would that allow for non matched tubes?

[sluckey]

Matchless liked to use separate 270Ω resistors, one for each tube.

Would that allow for non matched tubes?

Not much different from using a single shared resistor. But using separate resistors does provide a convenient way to measure current flowing through each tube for matching purposes.

[phillyhudson]

great thanks - I can get started now

[pdf64]

Matchless liked to use separate 270Ω resistors, one for each tube.

Would that allow for non matched tubes?

My experience is that a separate bias resistor per cathode does indeed generally allow valves that pass very different currents with a shared resistor to balance up pretty well.

[martin manning]

My experience is that a separate bias resistor per cathode does indeed generally allow valves that pass very different currents with a shared resistor to balance up pretty well.

This has to be the case. A common Rk forces the same Vg-k on both tubes, while splitting the resistor into two (at twice the value of the common one) lets the higher current tube seek a lower Vg-k/lower current, and the lower current tube seek a higher Vg-k/higher current, bringing them closer together. Total current draw will be approximately the same, and I estimate the miss-match will be around 1/3 of what it is with a shared Rk.

Matchless liked to use separate 270Ω resistors, one for each tube.

Would that allow for non matched tubes?

If you want to add a trimmer to the separate Rk setup, it's pretty easy. Connect a resistor from each cathode to the outer terminals of a trim pot, and connect the wiper to ground. This adds an adjustable parallel resistance to the primary Rk's, which should be increased a bit to account for the paralleled resiatance. For this EL34 case, 330Ω cathode resistors, two 3k3 0.5W fixed resistors, and a 5k 0.5W trim pot would work well.