Screen Voltage & Tone

[rp]

Realized my 5C8 has the power tubes' pin one to ground like most early tweeds (metal 6L6s). The output stage is 6.6K Edcor 30W, 250R cathode resistor, 375V a->k, 10K dropper btwn C1 and C2, so I’m good for drop-in EL34s and 6V6s. Btw the paralleled 6SC7 in v3 that I’ve been messing with for 2 years is gone and amp is back to the early 1953 schematic w/ the 6J5 (much sweeter) and for the EL34s test I upped the voltage a bit with a 5AR4 swapped for the 5U4.

The EL34s sound really fine, piano like and more balanced than the 5881 I had in there. As expected, the EL34s really pull the screen voltage down, from 315V with the 5881 to 278V. I tried jumping in an other 10K across the first dropping resistor (now 5K) to bring the screens up a bit to 325V - surprisingly, all the preamp voltages were unchanged. What changed a lot was the tone. The piano like tone and the warmth was gone, tone was less rich, and it’s was sharper and brighter, too detailed, like brand new strings for the first hour, and not much different than the 5881 tone. And it brought out the hifi of the Edcor which was kind of synergistic with the EL34s and the lower screen voltage - added to the piano like clang (which seems an Edcor trait) but balanced and smoother.

BTW amp has 100R stoppers to the screen grids, stoppers not droppers, I figured the 10K in the power string was doing plenty of dropping.

Beats me, I thought screen voltage had more to do with feel than tone. Any ideas why such a tone difference? Any reason not to keep the screens so much lower than the plates? Seems everyone is always striving to get the screens down.

[martin manning]

Screen voltage changes dynamically to a varying degree depending on the screen resistor, and that is how the "feel" is affected. Larger resistors cause the voltage to sag transiently with large signal peaks. With your small screen resistors (100R), you are not getting much of that effect. Large screen resistors will limit screen current, and protect the screens from excessive dissipation.

Changing screen node voltage is similar to changing OT impedance re the position of the load line, so I think you could expect tonal effects along those lines. Reducing screen voltage by 10% should be similar to decreasing the primary impedance 20%.

[Stevem]

Is that OT interleaved?

[rp]

Is that OT interleaved?

I would think so:
https://www.edcorusa.com/cxpp30-ms-6_6k

Changing screen node voltage is similar to changing OT impedance re the position of the load line, so I think you could expect tonal effects along those lines. Reducing screen voltage by 10% should be similar to decreasing the primary impedance 20%.

Good info, that could certainly do it, especially with the edcors I notice I can easily hear mismatching the output taps. One more variable to figure - great

[pdf64]

...As expected, the EL34s really pull the screen voltage down, from 315V with the 5881 to 278V. I tried jumping in an other 10K across the first dropping resistor (now 5K) to bring the screens up a bit to 325V ...

I think that the more relevant voltages (eg for the load lines) are at full load, rather than idle.