Lowering preamp gain via bias or split-load resistors?

[Gaz]

I recently came across the idea of using split-load resistors to attenuate gain (i.e. using two 50k resistors to form a typical 100k plate load, but taking the signal from between the resistors, rather than directly from the plate.) In this example the gain is approximately cut in half.

I've lowered gain in a stage by raising the cathode resistor on a typical 12AX7 stage from 1k to 10k (with a 100k plate resistor), which cut the gain in about half as well, but I was wondering if anyone has tried the former - using split-load resistors?

If so, what are the tonal differences? One obvious difference I can think of is that using split-load resistors would enable the designer to use a cathode bypass cap on the stage without worry of increasing the gain too much. Thanks for any ideas or experience with this method.

[tbryanh]

The first stage of the Fender 6G15 reverb unit uses split load resistors.

Split-load resistors affects the output impedance of the stage which could affect the bandwidth of the amp, probably the point where the highs start to roll off and the roll-off rate.

Unbypassed cathode resistors provide negative feedback which tends to make the output signal more linear (less tubey).

Unbypassed cathode resistors probably affect the output impedance too. Formulas can probably be dug up to find out.

[GibsonLover]

Raising the cathode resistor also reduces the current thru the tube, causing it to operate lower on the 'knee' of it's conduction curve, having a tonal effect. I like the split load method because it preserves your intentions on the cathode circuit. After all, a 100K plate load split into two 50K plate loads in series is still a 100K load (static load, again preserving your biasing intentions) on the tube. The difference in load impedance contributed on the dynamic side of the equation from the split load is easier to deal with 'after the fact'. I believe the effect is really no different than a voltage divider between stages, and how many time have you seen that (like in every high gain amp)?

[Gaz]

thanks for the insight!