5E3 with pair of 6L6 - what bias setup?

[tubeswell]

The PT I got for my 5E3 build is 325-0-325 @180mA, 6.3 @4A and 5 @3A, so I can use a range of rectifiers. I could run a 5Y3GT for a B+ of 367V, a 5U4GB for 412V, or a 5AR4 for 422V.

The OT is about 30W, 8K primary with 4R, 8R and 16R taps, so that I could set up the output tubes to run either a pair of JJ6V6s into an 8k load, or a pair of 6L6s into a 4k load (if that is feasible).

I was thinking cathode bias, so if have a switchable cathode resistor setup that would allow me to run 6V6s or 6L6s, how hot should I set up the bias for the 6L6s? (I sort of know what to aim for in respect of the 6V6s powered by a 5Y3GT)

And would I be better supplying the screens through a choke to get the grid curves spaced further apart (if I was running into a 4k load resistance)?

[Bruce / Mission Amps]

The PT I got for my 5E3 build is 325-0-325 @180mA, 6.3 @4A and 5 @3A, so I can use a range of rectifiers. I could run a 5Y3GT for a B+ of 367V, a 5U4GB for 412V, or a 5AR4 for 422V.

The OT is about 30W, 8K primary with 4R, 8R and 16R taps, so that I could set up the output tubes to run either a pair of JJ6V6s into an 8k load, or a pair of 6L6s into a 4k load (if that is feasible).

I was thinking cathode bias, so if have a switchable cathode resistor setup that would allow me to run 6V6s or 6L6s, how hot should I set up the bias for the 6L6s? (I sort of know what to aim for in respect of the 6V6s powered by a 5Y3GT)

And would I be better supplying the screens through a choke to get the grid curves spaced further apart (if I was running into a 4k load resistance)?

Use the 5Y3GT for the 6V6s, the 5U4 or 5AR4 for 6L6s.
Install a 3-5 watt 2k7 to 5K resistor for the screen node for either the 6V6s or 6L6s.
For any power tube pairs used, stick to any cathode biasing resistor between 270ohms and 330ohms, make it 10 watts.
The two different kinds of power tubes will bias similarly with respect to the rectifier used... IE: higher plate voltage = more idle current.
Run a parallel 25uF to 47uF 100v cap across the biasing resistor.
Use the 8 ohm speaker in the 8 ohm tap for 6V6s and the 8 ohm speaker on the 16 ohm tap for the 6L6s.

[tubeswell]

Thanks Bruce. I'll do a quick schematic just to confirm this

[tubeswell]

Hi Bruce

After playing with my new schematic toy, this is what I took your advice to be. Please correct me if I'm wrong, or feel free to give me any further pointers.

I take your point about there being no need to change the cathode resistor.

Would a 50mA 4H 110R choke be better or worse as the screen node supply?

[pdf64]

Not better or worse, just different. The choke will give a few more watts power output and give a harder rockier sound with less compression. Why not make it switchable? Peter.