Input resistor Question / Advice ?!?!

[evh_slash]

Hey guys, basically I took my Mesa preamp to my tech to have some mods done to it, a Bass shift and Pull deep (Push/Pull bass cuts + a boost) mods and he performed them for me based on the schematics. One of the schematics had swapped the 1Meg input resistor (grid resistor) to a 680k resistor. My tech was reluctant to change the input grid 1M resistor to 680k as he says this lowers the input impedance of the preamp. At the time I hadn't really given it much thought and just asked him to follow the mod sheet and swap the 1M for 680k anyway.

I'm wondering now if installing the 680k input resistor on the was a mistake... Pretty much every amp has a 1M resistor at the input, so I can't understand why somebody thought changing it to 680k as all it does is mess up the impedance... It definitely plays no role in the bass shift/deep mods.

Would the drop from 1M to 680k have caused any change in tone/high-end content/signal strength/gain ??

Should I change it back to 1M or just stick with the 680k now its already soldered in??

 

[zzmoore]

Re: Input resistor Question / Advice ?!?!

Ehhhhh.....I would have left the 1M also, But I would be much more concerned about the lack of grid resistor on V1a
Do YOU notice much difference.?
Unless you have the Volume (on your guitar) turned down pretty far, there "probably" is not much loss.

 

[Edgecrusher]

Re: Input resistor Question / Advice ?!?!

My question is... Why change it to 680k? Doesnt make any sense that I can see to do so.

I wonder if the lack of grid resistor was a "typo" so to speak by whoever drew the schematic. Wouldnt be the first time ive seen it.

 

[zzmoore]

Re: Input resistor Question / Advice ?!?!

My question is... Why change it to 680k? Doesnt make any sense that I can see to do so.

I wonder if the lack of grid resistor was a "typo" so to speak by whoever drew the schematic. Wouldnt be the first time ive seen it.

I guess that is the Thousand Dollar Question.....why.?
Let ME ask this then.......This schem, it is hand-drawn and says "Studio Preamp".
What is the history of this thing, and how did it come to the OP.
As far as a typo, sure.....could easily be a mistake. VOX was notorious for Wrong/Incomplete schems.
So OP.....where did you get this Schem/Idea from.?

 

[evh_slash]

Re: Input resistor Question / Advice ?!?!

I guess that is the Thousand Dollar Question.....why.?
Let ME ask this then.......This schem, it is hand-drawn and says "Studio Preamp".
What is the history of this thing, and how did it come to the OP.
As far as a typo, sure.....could easily be a mistake. VOX was notorious for Wrong/Incomplete schems.
So OP.....where did you get this Schem/Idea from.?

Its a pretty common mod schematic passed around on the Mesa Boogie forum. I mean the mods do what they're supposed to do, but the 680k thing seems really stupid. I think I'll change it back to 1M but theres no rush... I'll have it changed when I take it to get a filter cap change... its nearly 20-25 years old anyway!!

Cheers guys.

 

[evh_slash]

Re: Input resistor Question / Advice ?!?!

I guess that is the Thousand Dollar Question.....why.?
Let ME ask this then.......This schem, it is hand-drawn and says "Studio Preamp".
What is the history of this thing, and how did it come to the OP.
As far as a typo, sure.....could easily be a mistake. VOX was notorious for Wrong/Incomplete schems.
So OP.....where did you get this Schem/Idea from.?

Its a pretty common mod schematic passed around on the Mesa Boogie forum. I mean the mods do what they're supposed to do, but the 680k thing seems really stupid. I think I'll change it back to 1M but theres no rush... I'll have it changed when I take it to get a filter cap change... its nearly 20-25 years old anyway!!

Cheers guys.

 

[zzmoore]

Re: Input resistor Question / Advice ?!?!

Ah.....I see.
What about that grid resistor on V1a.? The schem says no...Edgecrusher thinks it might be a typo.........

 

[DrNewcenstein]

Re: Input resistor Question / Advice ?!?!

Then surely someone at the Mesa forum knows the reason behind it?

But, what does the input impedance address? Is this where you find options for Active and Passive options like on bass amps? Or is this so you can run your CD player/iPod into it for some stupid reason?