transformer coupled tube regulated 12B4 line stage

[ceylon]

im re-working the power supply, but here is the audio amplifier.

is this build-ready? any errors? does it need correction?

 

[Palustris]

Looking at the 12B4A plate curves, it appears as if at 20mA with a plate voltage of 100V you will require about -12V on the grid.

That means you will need a bias resistor of 12 / .02 = 600r.

You don't mention what the input voltage would be.

 

[ceylon]

updated schematic



input voltage is line level.
i think the new bias resistor give it 15mA idle current, 94.5 plate voltage. thats a good operating point, right?
plate to ground voltage is 101.5V. plate voltage should be 84.36V, right?

i dont know if ill add to the front a 50k pot/stepped attenuator or slagle autoformers.

 

[ceylon]

power supply

 

[gadget73]

too early for any critical analysis but just make sure you elevate the heater supply for the EL84 to keep it within reasonable H-K voltage limits. +50v should be sufficient I'd think, and that would also be fine for the 12B4 if they share a heater winding.

being the eternal cheapwad I might use something other than an EL84 here. A 6P15P is similar enough and should handle it, just tie the suppressor pin to the cathode. Or depending on the depth of your junk box, any tube with sufficient current and plate dissipation ratings would get it done. I'd probably stick some otherwise unloved tube in this spot just so I didn't have to waste a tube that is generally more useful to me elsewhere.

 

[ceylon]

too early for any critical analysis but just make sure you elevate the heater supply for the EL84 to keep it within reasonable H-K voltage limits. +50v should be sufficient I'd think, and that would also be fine for the 12B4 if they share a heater winding.

OH-- thank you, @gadget73. ok. i had not gotten there and probably would not have remembered this.

im having trouble sourcing a non-toroid that accomodates everything, so its lookin like i may have separate PTs for the B+ and filaments.

being the eternal cheapwad I might use something other than an EL84 here.

well, i was thinkin a $10 6p14p would do fine, but if a $3 tube can do the trich no sense buyin a $10 tube!

A 6P15P is similar enough and should handle it, just tie the suppressor pin to the cathode. Or depending on the depth of your junk box, any tube with sufficient current and plate dissipation ratings would get it done.

i have a good few junk tubes.

I'd probably stick some otherwise unloved tube in this spot just so I didn't have to waste a tube that is generally more useful to me elsewhere.

i get that.

 

[gadget73]

6CM6 is another option, basically a 9 pin 6V6, but cheap since it only got used in TV sets as a vertical output tube.

 

[Palustris]

input voltage is line level.
i think the new bias resistor give it 15mA idle current, 94.5 plate voltage. thats a good operating point, right?
plate to ground voltage is 101.5V. plate voltage should be 84.36V, right?

i dont know if ill add to the front a 50k pot/stepped attenuator or slagle autoformers.

Just to make the numbers easy, we will make the voltage on the plate 101V and the plate to cathode voltage 90V with the bias -11V. Not much changes with the re-biasing to 15mA @ 90V. The bias is about -11 and with your load line almost horizontal, the operating point looks good. However, the bias resistor needs to be about 750r. 11V / .015A = 733r. The closest standard value is 750r.

 

[ceylon]

Just to make the numbers easy, we will make the voltage on the plate 101V and the plate to cathode voltage 90V with the bias -11V. Not much changes with the re-biasing to 15mA @ 90V. The bias is about -11 and with your load line almost horizontal, the operating point looks good. However, the bias resistor needs to be about 750r. 11V / .015A = 733r. The closest standard value is 750r.

gotcha. thank you, @Palustris

View attachment 3851367