Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pi-amp on February 27, 2020, 08:17:22 am

Title: Marshall 1987: only 2 inputs (one high, one low) wiring
Post by: pi-amp on February 27, 2020, 08:17:22 am

Hello!
i'm building a 1987 clone and i only have room for 2 inputs.
what i want :  bright and normal 2 channels permanently bridged, and one high and low input.
Plugin in the high input feeds the 2 channels and i can blend them with the 2 volumes. Same for low input.

Now i'm wondering how to wire the thing.
I tried with the normal inputs wired normaly with the 2 68k going to one grid of the triode (say pin2)
And then the other grid simply connected to the first grid (pin 2 and pin 7 tied together)
It works, but i wonder if it's electricaly the same as if the triodes add their own grid stoppers ?

Title: Re: Marshall 1987: only 2 inputs (one high, one low) wiring
Post by: turtle441 on February 27, 2020, 09:00:55 am

That'll work.  The Plexi SE uses a similar set-up with a 33k resistor instead and works great as well.  For those math-oriented types, see Merlin Blencowe's write-up.  It appears there's a range of resistors you can consider based on noise issues, etc. 

http://www.valvewizard.co.uk/gridstopper.html

Title: Re: Marshall 1987: only 2 inputs (one high, one low) wiring
Post by: pi-amp on February 27, 2020, 10:00:31 am

thanks !