marshall mercury pre amp tone circuit

[makaze808]

Hi.

Schematic here. 

The tone control has to be set at 0 else the highs are just ear biting pick axe in y face horrors.  Any value changes  or small rewires I could do to get it a bit more useful? c7 does not exist on my board, it's just c8

The pre-amp section's I'm sure are not push the output valve enough, the amp can take any pedal well and they can all be set at 100 level and as much gain as I want and the amp starts to sing, problem being when I switch off there is a massive volume drop so i'd like to get the pre amp pushing it more. the amp is mine to keep so I'm not bothered about breaking into the board  or adding a sub board etc.

Thanks.

[makaze808]

I also don't mind disconnecting the vib circuit and using the traces for any improvements.

[andymac1962]

C7 is a standard "bright" capacitor,  looks like C8 is an extra bright capacitor.
Try removing these (desolder one leg temporarily of each cap to see the effect removing each one has). Remove C8 first.
You can also try increasing the values of C6, C10, C13,    to get more bass through the amp.  start off with (say) 47n for C6 & C10,   then (say) 100n for all three.  This may give too much bass, but its what your own ears tell you is good, as to where you end up.

Before playing with any valve amp,  unplug the power cord from the wall, and discharge the power supply capacitor C18. Use a 220k 1W resistor with insulated aligator clip leads. Dont electrocute yourself.  Be careful

Hope this helps.....
Cheers Andy

[PRR]

Also try changing C5 to 47uFd.