A couple questions on preamp biasing

[zhyla]

I'm tweaking on this 5B6 Bassman (schematic, B+ @ preamp = 300 VDC) I built and I reduced the anode resistor (R4) from 250k to 150k and noticed a huge difference in the sound. Having plotted some load lines I think I've shifted the bias from 0.6mA to 0.8mA.

The spec in my RC-14 manual for this 6SC7 tube lists 2mA for the plate current. Any idea if that's a max value or a recommended value? My guess it's it's a recommended design value. That's quite a bit higher than the 0.6mA that the stock circuit would achieve. Am I missing something here?

I'm playing a guitar thru this thing and saying it sounds better with 0.8mA. Is it common for bass amps to be biased cooler?

[loudthud]

Often times the parameters listed in tube manuals are benchmarks where a particular tube is tested before it leaves the factory or what might be setup on a tube tester. You can operate the tube at different bias points but the manufacturer won't guarantee that all the tubes they ship will operate properly in your circuit because they don't test the tubes at that operating point. The factory will bias the tube at say 2ma and measure the mu and allow a tolerance of maybe +/- 10% or maybe set a minimum. It's not always easy to find out what the Acceptance Criteria for a particular tube is (or was). Each manufacturer can establish it's own test parameters unless restricted by a licensing agreement with the company that designed the tube.

[MWJB]

Smaller plate resistors increase the voltage at the tube plate (& therefore current) and increase fidelity. This also changes the character of the amp quite considerably.

A more effective way of specifically rebiasing the tube would be to increase the value of the cathode resistor, again this will increase plate voltage & current & improve fidelity.

However as you're already at 2.5K for a shared cathode, a better approach in your case - IF you want to retain more of the 5B6 character, just brighten up the tone some - might be to reduce the value of the 10K preamp dropping resistor in the power supply & tweak overall voltage feeding the preamp. Be aware that preamp tubes draw very little current, so you need quite a large difference in ohms to get a substantial difference in voltage at the preamp plate.

The earlier the amp, typically the lower voltages they ran at, so the low current is more a by product of common practice at the time, rather than being attributable to "a bass amp" practice specifically.