Getting Started On RCA Console I picked up

[mashaffer]

Interestingly using 6N1P the f3 point at the output simulates in the 80KHz range but the 6SN7 only barely make 20KHz unless an un-bypassed Rk is used. I suppose that is the difference in Cg-p (3.8pf v.s 2.7pf).

Is it normal for 6SN7 to have that kind of bandwidth issues or is there something wrong with my circuit or the simulation? I wouldn't thimk the SN7 would have such a good reputation if it was so limited.

 

[mashaffer]

It looks like the driving impedance of the previous stage. Even with a buffer the 500k balance pot is a problem. perhaps using a CF buffer and then a smaller (say 50k) balance pot would be a better idea or maybe putting the buffer after the balance pot.

 

[BinaryMike]

You could try simulating the final gain stage and its input divider separately. Perhaps a few pF bypass around R21 is all that's necessary.

 

[mashaffer]

Putting the 500k balance pot after the tone circuit and then a CF after that feeding the 6SN7 seems to fix it. I am not sure yet whether that balance control will hurt the tone control yet. if so another CF after the tone control may be needed.

 

[mashaffer]

I also rechecked tone control behaviour and it seems to be OK with this approach.

 

[mashaffer]

Checked out the PT it is 600V CT so plenty of voltage to work with. The unused OPTs look to be about 6K driving 4 ohms. Might be useful for a 6EM7 project. On the speaker front I think I will set the 8 inchers firing down through the bottom to make for more room to locate the FRs on the front and less trouble avoiding interfering with the preamp and tuner chassis.