6V6 and 6F6 fixed bias grid No1

[VMUNIX]

I was reading the datasheet for the 6F6 tube checking whether with small adjustments could try it in place of a 6V6 in a Fender Princeton and I noticed that it "requires" for fixed bias operation a grid resistance of no more than 100K (some datasheets 50k) to prevent a runaway condition, while for auto-bias can use up to 500K.

The same applies for 6V6 tube, accordingly to the datasheet.

An externally hosted image should be here but it was not working when we last tested it.

But checking at schematics most of the times, the two resistors that connects the control grid to the bias source is about 220K, for other tubes like EL84 it's about 470K (when the datasheet states 300K) and so on. SO my question is should I stick to the datasheets in this regard ? what's the benefit (with fixed bias) to have a higher grid resistor apart from the input impedance.

 

[kevinkr]

There is no benefit in the output stage and there is some risk particularly in current production tubes that grid current in conjunction with higher grid circuit resistance will lead to run away. There is usually some margin but I'd not push resistance much beyond the suggested values.

 

[anatech]

Hi VMUNIX,
I'm going to second what Kevin just said. Besides, manufacturers didn't always follow the manufacturers recommendations. Just because they did is a poor reason to do the same thing.

-Chris

 

[gingertube]

Those max Rg1 values are for the tubes running at their max Anode + Screen dissipation.

In fixed bias, if you set idle current for 70% of max anode + screen dissipation you can safely double that max Rg1 value.

Cheers,
Ian

 

[VMUNIX]

Those max Rg1 values are for the tubes running at their max Anode + Screen dissipation.

In fixed bias, if you set idle current for 70% of max anode + screen dissipation you can safely double that max Rg1 value.

Cheers,
Ian

Thanks Ian, good tip.