bright switch on a SFPR.Want less bright cap-Values??

[sidehatch]

Can anyone suggest a value of cap to put in an early 70's pro reverb that will add a subtle amout of presence but not this ice pick this I have going with the stock switch?

Say Im gigging and have the amp set perfectly for me but as the room fills up I need a little bit more cut. Turning up the treble seems to thin out the tone and the bright cap is horrible.

Which value/makup would you try?

Thanks!!!

[Structo]

Bright caps on volume controls or masters do less and less as the pot is turned up.
Maybe try a different slope resistor to raise the treble knee a bit.

How is the bottom end?

[Bob-I]

The stock bright cap is usually 120pF. Try dropping that to 68pF or 47pF. I have a 3 way switch on one of my amps, center off, down 47pF up 120pF. works well.

[billyz]

I have never cared for the Bright switch on a fender. You could try a different type cap , my favorite is polystyrene. Silver mica is better too. Also, change the treble cap to polystyrene . This will smooth out the treble too. If your still not happy, try a 47pf bright cap or 68pf. Another option would be to use a resistor or trim pot, in series with the bright cap to lessen the effect.

The slope resistor change was already mentioned. try 56k like a Marshall JTM.

[sidehatch]

Great ideas everyone!!

Its so damn hard to find these values here in town. Actually, its immpossible.

Heisthl has been helping me out on this amp as well. Its sounding pretty darn good but I still think it can be better.

Im going to order some of those PF caps and try that first before I branch out on the other stuff. Before I registered on this forum I couldn't even change a bright switch cap. I still dont know much but I am learning.

Im starting to think that a lot of my probs with this amp are the speakers I selected. I hated the oxfords that were in it and I still think they were the worst I've heard but it has an Emi R,W&B and a C-rex in it now. Its pretty dark sounding and the treble has to be ran high.

I have a G12H30 here I could try but I dont know if that would remove bass or which speaker I should try replacing. Now Im talking bass but thats because I'm not getting enought treble and when I do get treble its harsh.

I wonder if a less bass heavy speaker will allow me to lower the treb and get a smoother tone? I wish I had a pair of tonkerlites.

[jjman]

Radio Shack often has ceramic caps in these pf ranges. Originals are often ceramic anyway. Voltage rating is not a concern here since there's no DC on the volume control.

[billyz]

You really must like your speakers first. In a Twin Reverb, I always liked the C12N Jensen. Actually the Italian ones are not too bad either .I just put a pair in a Silver Faced Twinn Reverb and they sounded very good. Even better after broken in. A lot of guys like the JBL d120, but I find them too ice picky.

I have a pair of Older Mojo speakers in my Drip edge Pro Reverb and they are perfectly fine.

[sidehatch]

Would someone please point out the slope resistor and tell m e which values to try?

[IMG:799:599]http://img.photobucket.com/albums/v334/ ... G_8589.jpg[/img]

This old pic has mods that have been done which are highlighted(thanks Heisthl!) I also replaced the vibrato channel tone stack and lightened up the bass a bit too.

[IMG:799:598]http://img.photobucket.com/albums/v334/ ... eisthl.jpg[/img]

Thanks

[Structo]

I circled the slope resistor.

[sidehatch]

Thanks Tom!!!

Now that I know where its at what would you try there? Is that for both channels ?

Does the slope resistor change more than just the gain of the preamp?

Reading all I can and I keep coming up with preamp gain associated with that resistor.

My whole issue I have with this amp is that I have to run the treble near max on gigs and if I want more the bright switch takes it to rasp.

I've never had a fender amp so dark. I've been thru all the tubes subbing them all out with others and I can get subtle variations. All of the stuff I've done is slowly helping. The normal channel sounds like a bass amp now in comparison. Weird problem.

Sorry for all the questions and thanks for the help.

[sidehatch]

I think I answered my own quastion as far as which channel it controls. There is one for the vibrato channel too I can see.

I still need to figure out what to try though.

Thanks

[jjman]

Yes.

[Andy Le Blanc]

Replace the slope resistor with a pot temporarily and dial in the value.
The larger the resistance the thinner the tone. Another way is too replace
the inverters input cap with silver mica caps paralleled, say two 500p to get
around .001, you cant add high's but you can limit the bass response along
the signal path.

I'm running a JBL E120 with my builds for the time being, incredible tone,
but I don't voice with a fender in mind, intentionally.

You can also put a trim pot in series with the brite cap, dial it in.

[Colossal]

You can also put a trim pot in series with the brite cap, dial it in.

Andy, this is a slick idea. I've done this with mid slope resistors and elsewhere, but not a bright cap. Can you suggest an Rk-L value for a trimmer? I'm working on some Fender builds. The Twin has 120pF for a bright cap and folks always complain that it is ice picky. The Deluxe Reverb has a stock value of 47pF (which I'm basing my builds on). Bob-I says he uses 47pF and 68pF on a ON/OFF/ON switch in this situation and is happy with it. I had planned on doing the same, but using a trimmer in front of a bright cap seems like it would allow you to find a happy medium.

[Colossal]

I have never cared for the Bright switch on a fender. You could try a different type cap , my favorite is polystyrene. Silver mica is better too. Also, change the treble cap to polystyrene . This will smooth out the treble too. If your still not happy, try a 47pf bright cap or 68pf. Another option would be to use a resistor or trim pot, in series with the bright cap to lessen the effect.

Billyz, thanks for the suggestions on cap changes for the Treble and Bright positions. Have you tried paralleling say a silver mica with a polystyrene or an polystyrene with a ceramic? Just wondering.

Thanks.