SF Champ Mods?

[refin]

I thought I had posted this before,but couldn't find it.
I have a SF Champ,just added a Weber 8F-150 speaker.I was wondering,are there any common/easy mods (outside of tubes /speaker) that can be done to bring out a little "more"? I know,it is what it is,but I want to make sure I'm not missing any cool things.A snip here,a replaced part here,you know.
Amp gurus? Thanks in advance!

[Kap'n]

You can easily make the amp more grind-y, by increasing the value of the 6.8kohm resistor at the bottom of the tone stack for more mids.

There's also a 250pF capacitor running from grid to ground on the power tube. That can get snipped out. It sucks some of the high end away.

[refin]

Thanks,Kap'n........I wish I knew half of what you might have forgotten about amps!
Being a guitar only guy,I have trouble with even the most simple schematics (like a Champ).
Is this the 250pF? Could I run it to a switch to use or not use? Is it simply lifting it from ground?

[Kap'n]

Nope. That's the top of the tone stack.

The cap is off the board, and runs from pin 5 of the power tube socket to ground. It should be a small yellow cap, probably identical to the one you circled on the board.

Obviously, all the safety rules apply - unplug the amp, discharge the caps, etc.

[refin]

On my schematic,it show pin 5 going straight to the board.The first thing it sees is a 220k resistor and a .022 cap.I haven't cracked open the amp yet,will probably do that tonight.

[Kap'n]

On my schematic,it show pin 5 going straight to the board.The first thing it sees is a 220k resistor and a .022 cap.I haven't cracked open the amp yet,will probably do that tonight.

That's the blackface schematic. The cap was added to mid/late period SF versions. Sometimes you can see it handwritten in on schematics.

[cdw2000]

If this is the schematic (my work computer now blocks photobucket) , it looks like it is a 330pF cap between pins 5 and 8 on the power tube socket:

http://tdsl.duncanamps.com/link.php?...mode=schematic

I think CBS added this to cure any parasitic squeal due to sloppy wiring during that era. You may or may not get away with removing it.

It looks like the midrange resistance is already 15K (If I referenced the correct schematic). You can try to verify this by reading the color bands on the resistor. It would be Brown-Green-Orange-Silver (or Gold) and would be connected between one side of the bass control and ground. This can be reduced to 10K to reduce mids or increased to 22K to increase mids or replaced with a 25K linear pot for a midrange control.

Another mod is to put in a toggle switch between this resistor and ground. Opening this connection effectively bypasses the tone stack for a high gain sound.

This is the BF schematic for comparison. The tone stack is the same and this schematic is a littler easier to read:

http://tdsl.duncanamps.com/link.php?...mode=schematic

Happy modding!

[refin]

Thanks guys,I really appreciate it.
I had a feeling that I might have had the wrong schematic.
Like I said before,it is what it is,a small wattage jewel---but with all the good amp gurus on this forum I wanted to make sure I wasn't missing anything.

[Kap'n]

If this is the schematic (my work computer now blocks photobucket) , it looks like it is a 330pF cap between pins 5 and 8 on the power tube socket:

http://tdsl.duncanamps.com/link.php?...mode=schematic

Shoot, you're right. It's been a while since I've seen the schematic.

[steve25]

You might also like the sound/responce of lifting the small feedback wire off of the speaker output jack that goes back to the board is the sound of lifting the tone stack ground is too much.
So this is another point where you can add a switch if its not a all the time sound that you like, in fact you can use a mini DPDT switch wired so one side lifts the tone stack ground, and the other throw position lifts the feedback connection.

[cdw2000]

That's a good mod Steve, subtle but still noticeable. Provides just a little added resonance to the sound.

I played with the feedback connection in my home-built 6V6 SE amp and decided to remove it permanently.

[refin]

You might also like the sound/responce of lifting the small feedback wire off of the speaker output jack that goes back to the board is the sound of lifting the tone stack ground is too much.
So this is another point where you can add a switch if its not a all the time sound that you like, in fact you can use a mini DPDT switch wired so one side lifts the tone stack ground, and the other throw position lifts the feedback connection.

I did that mod once to a BF Bandmaster head and ran it to a footswitch........sounded great.