Direct Coupled 76/5687 linestage

[athos56]

Is the output intended to be a white cathode follower?? Because as drawn it is not and in fact there should be a resistor between the plate circuit, ac voltage divider and the supply. Right now as drawn the lower tube is basically loading the upper tube, and not contributing any drive. (I did not look at the inspiring circuit.)

If it is intended to couple supply ripple in order to cancel it I'd say you might want to fix that problem at the source, and come up with a better CCS than that tube.

Note also that the 5687 is fairly prone to VHF oscillation if the grid stopper resistors are not installed right at the socket grid pins.

Here is the ACF from the Tubecad site. The grid stoppers are at the pins. I'm thinking the noise might be PSU related because it comes from both channels.

Athos

 

[Sheldon]

For the heater lift look at post #10, the heater bias is connected to the CT of the filament winding to raise it to ~1/4th b+, I'm not sure how that works if the supply is rectified. Still sounds good.

That connection is fine (BTW, the the diode is just there to protect the BJT. Nothing to do with rectifying AC). A short exposure to 600V will not generally harm a 500V rated cap.

Sheldon

 

[athos56]

That connection is fine (BTW, the the diode is just there to protect the BJT. Nothing to do with rectifying AC). A short exposure to 600V will not generally harm a 500V rated cap.

Sheldon

Right I should have been more clear, I meant if the filament supply is rectified and then fed to a filament regulator. I've never lifted the ground of a rectified winding by attaching the CT to 1/4th B+, it was always an AC filament supply. The part shown in post #10 is my supply "regulator". I think it might be the issue because the adjustment range is way off, I'm going to replace the adjustment pot with a fixed resistor and swap out the tip50 and lr8n3 with new pieces to see if that fixes things.

 

[athos56]

Got an email from a fellow member, that along with Sheldon's comment about poor contacts lead me to clean my pins, lo and behold, no noise. Even the low level background noise is gone... Thanks!

 

[athos56]

56 DC coupled to 12BH7 ACF

I call this project officially done. I swapped out the 5687s for 12BH7s, they sound a bit better. I spent two weeks tracking down hiss, finally I just removed all of the solid state regulators and CCSs I had in there and finally found out it was the PSU regulator oscillating madly. Moreover, since I didn't have enough heater current to regulate the filaments I have to stick with the 2.5v 56 tube. Sounds Great, so I think its going to remain as is.