Help Modifying a Fender Bassman 20

[Tubbytone]

Was looking for a little help with making some changes to my 80s Fender Bassman 20. Nice little amp, but always wished it broke up more. I came across a post on an old forum and the following was suggested:

1. Replace the .001 cap across the 820k resistor after the treble pot with a jumper
2. If this amp is for bass use, remove the 100pF bright cap on the volume pot
3. Change the 47k plate resistor to 100k
4. Replace the 22k resistor between the PI input and ground with 100k or higher, or just remove it altogether.
5. Replace the 1k / 1W resistor in the power supply rail with a choke (this would involve drilling a few holes in the chassis. Don't know if you want to go that far or not)

I have attached the schematic and am wondering if I have the correct componentswondering if I have the correct components and procedure. So for:

1. So I just jump out the cap correct? My electronics is very rusty - wouldn't that negate the resistor?
2. For guitar so N/A
3. Do I have the correct resistor circled in the schematic?
4. Do I have the correct resistor circled in the schematic? In your opinion would you replace it with a 100K resistor or remove it all together?
5. Is installing the choke, in your opinion, worth it?

This will be my first amp modification adventure. Really this is just for fun. Any help would be greatly appreciated!

Thanks,

Paul

[martin manning]

You've got it. I would make mods 1-4 and hold off on the choke. You will be pretty close to a Deluxe (no)Reverb.

[Stevem]

I think a good move on your part would be to get a feel for how preamp distortion sounds and how output stage distortion sounds.

In peticular I am talking about how the output stage can be made to pass a different ratio of even or odd order harmonics.
To play with this simply make the PI section plate load resistors more matched in value or less.

The Human ear is very sensitive to harmonic content.

The harder you drive the PI section not only will it clip sooner ( which effects the ratio of preamp to output stage distortion) but it will spit out harmonics sooner.

The PI balance set by these two resistors will also affect the harmonic content during clean output times, but harmonic content seems to get very pronounced when the output stage is clipping.

In short play around with this a lot.

Also play around with that 820 ohm feedback resistor off of the output transformer.

The voltage that this allows to get back to the PI section controls how fast the amp transitions from playing clean to going over into clipping.
You can make this transition smooth or drastic with the value of this resistor.

It also has a big effects on how much gain that stage produces.

[Tubbytone]

Thanks for the responses. I made the resistor changes tonight and man what an eye opener. It went from only slight breakup at high volume to almost immediate breakup. I am now officially been bitten by the mod bug. Have lots of reading to do and experimenting with resistor values to get a feel for how all this works together. Your feedback is so appreciated, cheers.