Tube Bias

[Delmar Evans]

I was addressing a similar situation on my 5E3 build. I pushed my cathode to a 300 ohm due to high dissipation. Still sounded good, but was still exceeding tube saturation. I wanted to have a more controllable gain that did not start at 3 on the volume control. I just tried this recently. I installed a 10 watt 150 ohm dropping resistor feeding the first filter stage. This effectively brought down the B+ voltage down 10 volts or so. I noticed that my gain structure has now moved further into the volume control to about 5/6 range versus 3 range. I may bump up to a larger dropping resistor just to see if I can get a different result. My wall voltage was running at 122.5 at the time of this test. I isolated the resistor so it does not come in contact with anything it shouldn't, and played the amp for a 3 hour rehersal. sounded great , and no overheating issues. Also no noise issues at this time. I am using all NOS RCA tubes of the correct values , and using a nos 5Y3 will also help bringing down the voltage. So based upon those readings , I am at 100.8 percent plate dissipation at 12.1 watts


Regards

Delmar

[Structo]

With cathode bias you want 90-100% dissipation.

Or what sounds good when in that ballpark.

[Andy Le Blanc]

for cathode bias its fine to run the power side to tube max dissipation, common place in older rigs.

fixed bias you hear the full gamut of opinions...

you have to remember that the static idle bias has more to do with the power supply than with the power tubes.

as long as you are with in the devices max ratings your fine, but you also need to mind the PT ma. rating.

The common operational point that comes out of old design examples
is a bias around 50% of the PT ma. .

I can tell you from experience that if a push pull circuit is set up consistently at the same bias point and screen grid voltage the rig will sound remarkably the same despite the tube type.

For example... say a rig is set up to run at the same points for 6v6 and KT88

the 6v6 will be at max plate dissipation and the KT88 will be running cold
but if the screens are set to run at the same point, relative to ther plate voltage at that bias point.... both tube types will sound very close, without moding the circuit other wise.


the real choice becomes the ma. rating of your PT... the max dissipation of your tube compliment type landing around 50% of the PT rating.

just don't forget that a tube amp is a speaker attached to a power supply,
the tubes modulate the power supply...

any ways mind the PT ma rating when your biasing.

most fixed bias rigs are actually biased very cold..... because of the the screen grids, the bias point reflecting the voltage difference between the screens and plate.

this can also be an optional bias method, set the fixed bias voltage based on the measured difference between the screens and plate.

In most common push pull circuits you want the screens to be just negative relative to the plate.

its far better to bias according to a measurable voltage condition rather than share the assumptions of others.