grid leak bias ?

[Toxophilite]

My Roberts 1630 conversion would seem to have grid leak bias on one of the triodes in the preamp.
2M resistor on the grid, cathode straight to ground.
I was trying to research it and other than it is one way of biasing a tube and apparently not commonly used in guitar amps
I couldn't determine why it would be used and what benefits if any it would provide. Primarily in a tape deck format?

I was inputting into that triode of the preamp 12ax7 and then into he triode of a 6BM8
With a scooped EQ pedal in front of the amp to compensate for an EQ that was not really for guitar the amp sounded good

Is it not as effective or just a different way to accomplish the same thing?
Is it better to change it out for a more standard fender sort of setup?

[sluckey]

Grid leak bias is old technology. I can't say why Roberts chose to use it. I can't think of any advantages as compared to cathode bias. If you decide to keep it you will need a cap (rather than a resistor) between the mic jack and the grid. The cap is there on the schematic you posted of that model 1710. The cap is an integral part of grid leak bias. I would change to cathode bias and not look back.

I've seen a few cheap guitar amps that use grid leak bias. And it's common to see grid leak bias used in a tremolo oscillator. Fender used grid leak bias in most of their early models. My Magnatone M10A has a grid leak bias stage in channel 2 preamp. I guess they just wanted a slightly different sound in that channel. 
 

[DummyLoad]

probably tone. also, you can theoretically attain near maximum AF of the tube?

is that schematic is missing two vital components for shunt grid leak to work properly?

http://www.rfcafe.com/references/electrical/NEETS-Modules/NEETS-Module-06-1-31-1-40.htm

read sections 1-33 & 1-34.

perhaps a relevant excerpt from pg. 1-34:
Distortion occurs during the positive alternation when the grid draws current. Current drawn from the electron stream by the grid never reaches the plate; therefore the negative-going output is not a faithful reproduction of the input, while the positive-going output (during the negative input cycle) will be a faithful reproduction of the input.

--pete

[PRR]

Grid-leak is for SMALL signals.

It can tend to find the highest-gain part of the tube. It avoids the need for a (then-)troublesome electrolytic cathode cap.

It makes considerable sense in the second stage of a NFB tape or phono preamp, where signal is small and cost-reduction is wanted.

It is seen in the oldest guitar amps, generally pre-1955. Improved hotter pickups overloaded the grid forcing cathode bias.