split plate loads

[Nigelbrew]

excuse me if this is a basic question but if i use a split plate load to attenuate the signal between preamp stages,:

1. will i have a radically different tone then if i employed the same voltage division using a resistive divider?

2. when used in the preamp stage of a high gain amp, will i still have all the distortion generated by that stage as i would have if i used a single plate resister but just at an attenuated voltage?

thank you in advance for any help or advice

Nigel

http://www.myspace.com/nigelbrewersarisefromoblivion

[dai h.]

isn't that just the same thing as a volume (voltage divider) except that it's on the hot side? Like this: say for the 1st stage on a 5F6-A, if you used a 100kA pot in place of the plate resistor and tied the output coupling cap to the wiper. The 100kA pot's B+ end is at (AC) ground, thanks to the local de-coupling cap (the 8uF). So say if you used a fixed 82k/18k with the 82k side towards the B+ end and the output taken from the junction of the two, it'd be as if you turned down a 100k volume slightly.

[R.G.]

1. will i have a radically different tone then if i employed the same voltage division using a resistive divider?

No. Dai is correct. The tone probably won't be even perceptibly different.

2. when used in the preamp stage of a high gain amp, will i still have all the distortion generated by that stage as i would have if i used a single plate resister but just at an attenuated voltage?

Yes. However, it will not drive the next stage with as high a signal. This can be good if the next stage was being driven into grid blocking , or bad if the distortion you were wanting actually came from the next stage

isn't that just the same thing as a volume (voltage divider) except that it's on the hot side?

Yes, it is. Exactly.

[Matt T.]

Seems like there would be a difference in either the AC load on the output of the preceeding stage (because you added a divider) or a different grid load for the grid of the following stage (because you didn't use the divider there). But that's just a guess.

[Steve A.]

Seems like there would be a difference in either the AC load on the output of the preceeding stage (because you added a divider) or a different grid load for the grid of the following stage (because you didn't use the divider there). But that's just a guess.

Matt:

To my ears there is a difference. The voltage divider on the grid seems to have some sort of RC effect (or whatever).

Kinda like if you turn the volume down on your stereo, and the sound gets a little bit duller.

But then again I can hear Martians speaking to me if I listen very carefully...

Steve Ahola

[Ray Ivers]

Nigel,

1. will i have a radically different tone then if i employed the same voltage division using a resistive divider?

You mean from the stage with the split plate load? Possibly - see below (although I don't know about "radically different").

2. when used in the preamp stage of a high gain amp, will i still have all the distortion generated by that stage as i would have if i used a single plate resister but just at an attenuated voltage?

IMO, yes (but see below as well).

I don't know if you're also interested in Zout and/or loading issues, but IMO there can be a definite difference in those areas between the split-load and resistive-attenuation circuits; whether it amounts to a hill of beans to you or not I really can't say.

Suppose you're using a total plate load of 100K, with a 9K resistor to B+ and a 91K resistor from a 12AX7 plate. The max undistorted voltage swing at the resistor junction will vary from roughly B+ to a lesser voltage, representing roughly a 90% reduction in signal swing from the 12AX7's plate output (side note: it really doesn't take much signal swing at all to overdrive most 12AX7 gain stages' grid circuits).

However, any signal swing that does occur will be at a pretty low impedance (for a 12AX7) - never more than 9K ohms, and without any additional loading on the split-load stage's plate output. This situation is quite different from placing a 100K pot across the plate output and turning it down 90%, where an additional 100K of resistance to ground loads down the plate output all the time. Also, large series resistances (which can add noise and exacerbate Miller effect) are avoided. To me, it's like having your cake (attenuation) with icing (low Zout, Rseries, Cmiller) too.

Just my $.02.

Ray