Amplifier modifications

[DavidRees]

I have an old Peavey Classic 50 combo and am wondering if there is a way of making the normal channel more clean than crunchy - are there people, in the London area preferably, who can take a 'host' amp and tweak it - I've always imagined that most mods are for people seeking something more whereas I'm in a sense looking for something less [unless you count headroom of course] - i'd much rather do this than complicate my life by trying to decide what it is I really want
any comments/suggestions much appreciated
David

[indysmith]

As an easy mod you could try replacing the preamp tubes with lower gain ones. a 12AY7 or 12AT7 in the place of a 12AX7 should reduce the gain and clean it up.

[Elliot]

Go 12AY7 in V1 rather 12AT7 - The 12AT7 is a high-current tube meant to work as a driver in the Phase Inverter spot, not normally a good choice for V1.  Electroharmonix 12AY7s are quite nice sounding.

[Denim n Leather]

Or use a 5751 instead of the 12AX7.

[Elliot]

shhhh Don't tell everyone that NOS 5751 are readily available, still quite cheap and can make an over gained amp behave more like a vintage one.  :D

[DavidRees]

Thanks for all the advice one and all - changing the pre amp tube for a 12 AY7 it is
David

[HTH AMPS]

a 12AY7 or 5751 will definitely help, but you might try lowering the plate resistors - they've used 150k on all the preamp plate loads, so thats not going to help with clean headroom.

Replace the following with 100k resistors... R1, R2, R3, R5.  This will lower the gain of the OD channel too, but it'll make it less fizzy too. 

[nilsampmaker]

For lower gain you can also disconnect the cathode resistors, using a switch for example

[jpfamps]

For lower gain you can also disconnect the cathode resistors, using a switch for example

For lower gain!! i.e. no gain!!

If you disconnect the cathode resistors you will get no current passing through your valves.

I think you meant cathode bypass capacitors, which will indeed lower gain.

Unless you have plenty of experience of working with PCB-based amps, I would avoid trying to do any modification on the board, as removing the PCB from Peavey amp is quite an undertaking (as is obviously getting it back in). What I really don't like about these Peavey amps is that the PCB is in three sections with solid core jumper wires between each PCB section. When you "unfold" the PCB to work on it you stress all the solder joints along the sides of the PCB sections. This also happens when you "refold" the PCB to get it back in the chassis. I've found dry joints on these jumper wires.

Also using some lower gain valves (eg 12AU7) can cause plate load resistor to dissipate more power than they are rated for, so I would be cautious about using very low gain valves.

The 5751 is probably your best option here.

[DavidRees]

again thank you everyone for your comments and advice
best wishes
David

[nilsampmaker]

Yes off course I meant the cathode bypass capacitor, thanks.

[m0jo]

I can vouch for the Electro Harmonix 12AY7, it's a great tube for beeeeautifully clear clean in my Laney VC100, who's clean channel tends to dirty up quite quickly :(

And yeah, the build on a Peavey is a bit of a one time job .. I've heard of people having to resolder entire ribbons that run between the pcb's because they just ripped the solder loose taking it out (it makes me think they weren't that gentle dismantling it ...  but still).