Grid resistor question

[EFK]

Can someone explain to a real novice why the grid resistor at R9 in the preamp illustrated below (between V1 and V2) is such a high value? I read through the info on Aiken's site about high value grid stoppers as pertaining to high gain preamps however this is not high gain. The amp does have a HUGE amount of bass - too much, in fact, which is why I am looking at values in the preamp (although what is in there does not completely match the schematic) - and I'm wondering if this 1M resistor is one of the things which is cutting higher frequencies?

[Old Tele man]

...usually, large grid resistors on pre-amp tubes are there for "isolation" but 1Meg DOES sound too big.

...try temporarily attaching another 1Meg resistor in parallel with that one to "hear" if it affects the bass response...because 1Meg with just about ANY capacitance will surely attenuate a lot of frequencies.

[loudthud]

The 1 meg seems excessive. Don't believe everything you read on a schematic. 100K will cut a noticable amout of high end.

One thing about the tone stack, there is no isolation resistor between the treble and bass sliders. Without one, the treble control won't do much. Duncan's Tone Stack Calculator will confirm this. Install 220K or so between the sliders and take the output from the treble pot side.

[EFK]

Thanks for the help guys. I spent a few hours yesterday completely going through the circuit - as far as I can understand it - and I can say 100% that the schematic Gibson sent me is WAY off what is actually in the amp. In many ways not even close: that 1M resistor isn't there at all and in fact the schematic shows each channel using one half of V1 and one half of V2 while in reality each channel uses both halves of a single triode. Just as an example. I've got channel one figured out but I'm having problems with the tonestack (TMB) in channel 2 as it doesn't really match up with 'standard' Fender or Marshall stacks that I've been able to find and there's some kind of interaction off the treble pot with the Reverb circuit and V3 (reverb driver tube) that I haven't figured out yet.

[mooreamps]

The 1 meg seems excessive. Don't believe everything you read on a schematic. 100K will cut a noticable amout of high end.

One thing about the tone stack, there is no isolation resistor between the treble and bass sliders. Without one, the treble control won't do much. Duncan's Tone Stack Calculator will confirm this. Install 220K or so between the sliders and take the output from the treble pot side.

I was just going to say the same thing.

-g

[EFK]

Thanks for the responses - I've spent time tracing out the preamp and PI sections and will post my crude drawings in a new post regarding "Gibson GA42RVT revised schematic." It's almost as if the layout Gibson sent me is a completely different amp!!!