Modding a Fender style 'bright' input.

[The Captain]

Hi- would anyone know whats added to an input resistor for a 'fender stlye' bright input (like on a 6w Champ for eg)? Ie Im sure my 68k r2 can be bypassed with a switch on my stock 5w tube amp, but what caps/resistors for a good bright difference? cheers for ideas- captain.

[stokes]

In some Fender designs the bright cap went a cross the volume control.It was usually 100 to 150pf.You could try it on your input resistor as you suggested but I think it may be too early in the preamp stage and may make it too brittle,or cause oscillation problems.The main purpose of that resistor is to block RF signals from getting into the preamp.By bypassing the highs here you may defeat the purpose of this component.But give it a shot you can always reverse it.

[The Captain]

I had a feeling 100-150pf was the ballpark figure- if say I try bypassing the R with switch with a 100pf, would I need a resistor in series with it; what sort of value am I looking at? or if anyone knows if adding things on the vol pot as Mr.Stokes suggests can acheive added brightness.. thanks for the help.

[TD_Madden]

think that the value originally used across the volume control was 47pf...

[The Captain]

That could be worth a go thanks; forgive my ignorance but which pin to which on the vol pot?

[Enzo]

COnnecting a cap from either leg of a pot to ground would serve as a rolloff, the opposite of bright. SO connect it to the two legs not grounded. Thus os serves to bypass the highs around the control.

Just throwing a cap in there doesn't make things brighter. Of course you can tack one in anywhere you please and in seconds hear what difference it does or does not make in any particular spot. You could even short across those 68k resistors to see what highs they cut. uncutting highs is the whole point of brite caps.

Which Champ chassis had a bright jack?

[stokes]

I always thought the idea of the cap across the pot was a high pass filter.The highs that normally would be attenuated by the pot or voltage divider pass thru the cap,bypassing the resistor thereby not being attenuated,no?Putting it from the wiper to ground would cause the highs to be grounded and is called a low pass filter.Most Fenders I've seen have the 120pf,but I suppose you could experiment with different values and see what you like.I have never seen a Champ with a bright input.Like I said earlier,it may not be the best place to put a high pass,could end up defeating the purpose of that resistor.

[The Captain]

Maybe its not the Champ with the bright input I was thinking of, but other ones maybe called a high input? Anyway cheers, Ill give the vol pot a go as described across the pair not going to ground and have a listen. Any other ideas for brightening a muffly bassy 5w would be helpful. Ive tried silver micas of values up to 680 pf paralled with the odd r- without ANY difference in sound at all..all back to sq 1 now- very odd.

Incidentally- anyone know if I could change the coupling caps that were 22uf, to 0.1uf film caps- & would I have to change ant corresponding R values if an ok idea? Ive heard lowering their values may decrease the wooly muffly bass Ive got, and so perhaps making the highs stand out a bit more.

Thanks alot for the suggestions- captain.

[stokes]

22uf coupling caps?Do you mean .02uf?Which Champ are you using.The muffy bass you describe wont go away by adding a bright cap.Tell us which champ you have and I can give you some ideas to improve your bass response.Changing your .02 caps to .1 is going to have the opposite effect.As for the high input,it refers to one input being higher gain than the other,not a higher frequency thing.

[Enzo]

Unless maybe by coupling caps he means the cathode bypass caps. ALthough .1 seems a bit small for that.

Stokes, we said the same thing in different words. The cap across th top of the pot is a high pass filter. That is what I meant when I said the highs bypass the pot. They go past it without going through it. And shunting signal to ground with a cap would roll off the highs - same as calling it a low pass filter.

And I agree, too much bottom is not cured my more highs. it is cured by less bottom.

[The Captain]

Cathode caps it must be, not coupling caps sorry for that- so they were 22uf originally and (Ive changed to 10uf now- no difference in the bass) so Im going to try 1uf tmrw, BUT considering 0.1uf film caps as absolutely none of caps/ rs/ switches/bypasses/pots Ive tried have made ANY difference; its really if the 0.1 uf is a no-no, or if its ok to have a go-?

tweaking guitar amps seems a whole lot less rewarding/ fun than hifi amps.. Ill persevere for a bit then Im binning it- the sound is completely useless!

[stokes]

The Champ was originally designed to be a practice/student amp.Not meant to be played at volumes we tend to use it for.The power supply is not stiff enough to reproduce a solid bass especially with todays more powerful pickups.The first mod you should do is to improve the power supply.If the amp you are using is over ten years old your power supply's filter caps need to be changed and the first cap which is a 16 or 20uf should be upgraded to a 40 or 50uf,I like the 80uf in this position.And forget that 5Y3 rectifier,get a NOS 5v4 or a SS rectifier.You can keep the same value for the rest of the supply caps,but put fresh ones in.These changes will drastically improve the bass response and tighten up the overall tone.Any other mods will be pretty much useless until the power supply is improved.I think after this mod you will not need to mod it any further.

[The Captain]

I will consider different stage mods then. Its not a champ I have, but the Champ as a rough guide, ie 'Fender style bright input' I was hoping to replicate as apparantly my amp is similarly basic- its the ubiquitous Epiphone Valve Junior head, which tho many pages of mods there are I cant access to reply to/ forums blocking my registry; but this site seems very good and Im grateful for the help. I think Ill just stick those 0.1 uf film caps for the cathode caps and hope I dont damage it, unless anyone advises against the idea. If my patience lasts with this amp I may try revamping the transformers/ other stuff- I cant see why such a simple circuit/ amp cant at least sound balanced with a few basic tweaks though.

[stokes]

I dont have a schematic for that amp.But I can tell you that the preamp tube bypass cap is frquency responsive.I think .1 will be too small for your purposes.With no bypass cap it is harder for certain frquencies to get thru the cathode resistor,the cap makes an easier path for these freqs.to follow.The effect of this is more noticeable with a larger resistance.The 22uf is used with a 1.5k resistor to allow all the frequencies a guitar produces to bypass the cathode resistor.Going too small may thin out your sound too much.If you look at some Marshall schems they used a .68uf,but you may not want to go that small as this mod will also reduce the gain considerably in the circuit.I would suggest trying somewhere around 5uf and then work your way down,but dont go below the .68uf or so,you will most likely lose too much gain.Another thing I have used is something you can find in the old RCA Recieving Tube manual called a Corrective Filter,it involves putting a resistor and capacitor in series across the primary of the OT.But try the bypass cap first,it is less complicated.If you have a link for the schem on that amp let me know I'll take a look at it to see if I can come up with any other ideas for you.

[The Captain]

Marvellous Mr.Stokes. I'd be interested in some fresh ideas- I hope here is a schematic of the stock head: A good job I found it as I may go back to beginning again..

http://i59.photobucket.com/albums/g2...chem_Stock.jpg

Ive a feeling the mufffly bassy sound is stuck firm- and may require a new transformer of some variety to boot it to touch! I may as well tweak as far as I can before I consider whether its worth it. Ill try that cap across the vol pot legs that dont go to ground -for yet another try now. Thanks, capt.