distortion on leading edge of note

[martin manning]

What was/is your screen voltage at the tube pin? B+ still at 430V?

[pula58]

What was/is your screen voltage at the tube pin? B+ still at 430V?

Hi Martin,
B+ at 430, screens (at the sockets) are at about 420 now. Previously (before adding the extra screen resistance) the screens (at the sockets) were 1 volt or two below the B+, i.e, about 428V.

I get the sense that my thread is related-to, at least it is at the moment, the thread on screen voltage by AndyL: "voltage divider for screen grids."

[martin manning]

What was/is your screen voltage at the tube pin? B+ still at 430V?

Hi Martin,
B+ at 430, screens (at the sockets) are at about 420 now. Previously (before adding the extra screen resistance) the screens (at the sockets) were 1 volt or two below the B+, i.e, about 428V.

I get the sense that my thread is intersecting, at least it is at the moment, with the thread on screen volatge by AndyL.

Just what I was thinking re Andy's thread. 430V B+ and screen voltage a few volts below is not at all unusual, and the typical Fender set-up is to take the screen current right off the choke, with 470 ohm screen resistors.

Note that the plate curves in the data sheet are drawn for 400V Vg2, so I believe in your case the knee is really about 30 mA higher with Vg2 up at at 430V. It's pretty interesting that you can reduce or eliminate the +/- 120 Hz by adding more resistance, though. The screen voltage gets pulled down with high current, of course, but that is included in what you are seeing. I wonder what would happen if you had just increased the screen R's to 1k?

[pula58]

What was/is your screen voltage at the tube pin? B+ still at 430V?

Hi Martin,
B+ at 430, screens (at the sockets) are at about 420 now. Previously (before adding the extra screen resistance) the screens (at the sockets) were 1 volt or two below the B+, i.e, about 428V.

I get the sense that my thread is intersecting, at least it is at the moment, with the thread on screen volatge by AndyL.

Just what I was thinking re Andy's thread. 430V B+ and screen voltage a few volts below is not at all unusual, and the typical Fender set-up is to take the screen current right off the choke, with 470 ohm screen resistors.

Note that the plate curves in the data sheet are drawn for 400V Vg2, so I believe in your case the knee is really about 30 mA higher with Vg2 up at at 430V. It's pretty interesting that you can reduce or eliminate the +/- 120 Hz by adding more resistance, though. The screen voltage gets pulled down with high current, of course, but that is included in what you are seeing. I wonder what would happen if you had just increased the screen R's to 1k?

Originally I had 1K screens R's...now I have 470 ohms on each tube, plus the 2.2K of shared Resistance. It appears to act quite differently to when it just had the 1K on each screen.

[Structo]

Did that fix it?

[pula58]

Did that fix it?

Well...in the lab it looked "cured" on the oscilloscope, but playing the amp: it still sounds like it has the same problem!