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/g288/LogicBean/Epi_VJH_Schem_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.

[stokes]

Capt,I dont think the cap on the pot is gonna do it for you.That is going to let some highs bypass the vol control but the floppy bass is still going to be there.You will get better results altering the bypass cap on the preamp stages to "plug up" some of the bass.Also looking at the power supply I would increase the size of those first 3 filter caps to 40uf on each.You can do this by adding a 20uf cap at each.This will do more to tighten up that bass response.What is happening is that when you hit those bass notes the amp needs more power to handle the bass and the 22uf just aint gonna cut it.Also I would suspect they used some sort of Jap caps in that amp,they are without a doubt the worst sounding filters.Get them out of there and replace with a good quality Sprague or F&T brand.I would get at least 40 or 50uf for the first 3 and 16 or 20uf for the last one.Dont go any higher than 20uf for that last one marked B3.This upgrade to your power supply is critical to improve the bass response and with out it I dont think any other mod is going to help much.You could even put 100uf for that first cap,since you have a SS rectifier.This is a common fix for the floppy bass you are describing,I can say 99.9% sure it will do it for you.

[Tom Phillips]

I'm surprised to see the input circuit used in this amp. R1 and R2 set up as a voltage divider giving you 6dB of attenuation at the input. This is equivalent to using the #2 input jack on a standard Fender amp. Makes me wonder if the schematic is correct. If it is correct and this the amp was mine to mod, I'd:
1) Move R2 to the other side of R1
2) Change R2 to 1 Meg Ohm
3) Change R8 to 1.5k Ohm
4) Change R1 to 33k Ohm (Optional but this will give you the equivalent of the classic Fender input configuration when the #1 input is used)

Then I'd proceed with the cap changes already discussed starting with C4 and a 47pf bright cap on the volume pot. After messing with the front end I think you will find that you don't need to change C3 or C5.

IMHO, the speaker and OT will have the next biggest effect.

Regards,
Tom

[The Captain]

Mr Stokes Ill definately do those cap changes you suggest: when you say the 1st 3, is that specifically C9, C7, C8 ('B1, B2, B3')? Are the 1st 4700, the next C6 22 and the 0.1uf at the right best left as is? I was eying those caps suspiciously as it was; they look pretty shoddy. I think Ill bin those I redo and replace with these 47uf maybe.. http://www.ampmaker.com/fc01x.asp

Tom I think the schematic is correct, ie 2x 68k as R1andR2. Ive done the 'page 1' mods to these so that R1= 1M, R2= 33k and also R8 to 1.5k: all exactly as you suggested, & exactly as the reams of 'Epi VJ head' mod pages suggest too- this is where the term 'fender input' was used in conjunction with the R1/2/8 changes, and where I probably confused the terminology regarding the 'bright input' idea.

Brilliant help- cheers alot. It may save much farting about with the pesky 100pfs/ 1ufs in the 1st part of the amp. Ill report back when Ive done said mods. Capt

[stokes]

Sorry,Capt,I should have used the numbers marked on the schem.Here is what I would recomend,C6 100uf C9&7 40-50uf C8 16-22uf.LIke I said before upgrading the power supply is the answer to curing the loose bass response,I have done this to many amps,and it always works.Those 22uf filters are just not enough to keep up with the power demand the amp needs to produce a solid bass response.As for those caps in the link you posted,they look like the Jap made variety, I wouldnt recomend them unless you know for sure they are a good quality.The only caps that I know of that are not made in Taiwan are the Sprague and F&T,there is a couple of others the name of which I cant think of right now but they are ridiculously expensive,and used in high end HI-FI applications.If you cant find the 100uf for C6 a 40 or 50uf will do.Also you can leave that .1 on C11,but if you want to change it that one doesnt need to be an electrolityc but use a 600v if you go with a non electrolityc,I had a 400v explode on me once.

[The Captain]

Edit: Just thought: Maybe I could add some tone pots like this 5w kit amp?? This would solve my issues simply if so.. http://www.ampmaker.com/se5av2/se5abd.asp (not sure which bits are the tone pots)

The stock VJ head schematic again.. http://i59.photobucket.com/albums/g288/LogicBean/Epi_VJH_Schem_Stock.jpg

As to original plan, Ive got a plan now for what cap for where- great. Ill have a word with Ampmaker.com (extremely helpful/ nice bloke) about his OT similar to a Hammond SE apparantly (my last resort with this little 5w- not much point spending loads on it). Is there a Uk supplier for the Sprague caps, or F&T I could browse over in Uk? Im not up to speed with guitarry suppliers yet. Thanks- capt.

[Steve Conner]

I live in the UK, and I tend to use two kinds of electrolytics: industrial grade Rifa and Aerovox units that I buy from RS for "Nice" stuff, and these cheap wire ended ones from Rapid for general experimenting:

http://www.rapidonline.com/productinfo.aspx?&tier1=Electronic+Components&tier2=Capacitors&tier3=Axial+Electrolytic&tier4=High+voltage+85%26deg%3bC+axial+electrolytic+capac itor&moduleno=62540

It's difficult to get the big industrial grade units in capacitances small enough to be relevant to tube amps, so these little wire ended guys are great. I know they're Chinese but I could care less: broadly speaking I think the whole capacitor sound thing is hooey.

The last time I used them was a bag of 100 of the 22uF 450V ones when I was building a 10,000V DC supply.

I forgot to say, if you want the premium parts, Watford Valves sell F&T and Sprague Atoms.

http://www.watfordvalves.com/

[stokes]

The cheap ones are okay for "general experimenting",but I wouldnt put them in an amp I wanted to listen to,much less an amp somebody was paying me to fix.As far as the sound thing being hooey all I can say is compare and listen,if you dont hear the difference,then to you there is no difference.But when those Taiwanese craps start spewing thier guts all over your chassis,or humming like crazy a year later,the problem will be obvious.I wont use them and would not recomend them to anybody.

[Bruce / Mission Amps]

For a quick mod, the resistor you want to bypass with a cap is R6.
Use something like 680pF to 1000pF.
Also, dump the first triode's 22uF cap (C4 across R8) and use a 1uF for C4 instead.
Change C5 to about 100uF/50v and C6 to at least 47uF-100uF/450v.

[The Captain]

Bruce, Ive done every quick mod there is: the R6 mod with 47pf, 100, 150 even 680 (all no difference whatsoever) changing c3 &4 both to 10 uf, then 4.7, 2.2 and now 1 uf (no difference whatsoever) and going as high as 1000uf for C5 (no diff..). Changed C1 & 2 to 10 nf orange drops (no diff..). All these caps in various combinations too, until I gave up and swore loudly at it. Also I did a gain pot at R7 (added no gain whatsoever!!). So you can see Im a bit fed up with the hours Ive lost :rolleyes: Especially when I read 'done the mods- wow what a difference' etc.. One success- a master vol pot that works, but as the tone is so useless I dont use the amp. Changing R1 and R2 too made the tiniest difference it was hardly worth it -I feel like the VJ buddha is peeing on my head!

All I want to do is 'turn the bass down a bit' & (heavens above if I could) the treble up/ brighten the tone a bit.
I will do it- its a bugbear now- the only thing I can think of is somehow adding bass/ mid/ treble tone pots. Blimey O' Reilly..
Captain.

[Tom Phillips]

I didn't see mention in the thread but did you try a different speaker with this amp?

[stokes]

Adding a tone circuit you will need another gain stage to bring the signal back up.You could get away with a simple one knob tone control,but it is still gonna suck some of your signal,a full tone stack is gonna need another gain stage.The speaker in that is probably not a very high quality and you would see an improvement as Tom pointed out.What you have there is loosely based on the Tweed Champ circuit and a common fix for the sloppy bass is the upgraded power supply in a tweed.After that you will most likely want to upgrade your speaker to handle the tighter bass response you are going to get.All the suggestions given by everyone are solid,but with that 22uf in the first cap after the rectifier (C6) it is just not going to hold up when you hit them big notes.If you do nothing else at least replace C6 with a 100uf 450 or 500v,good quality cap.Better to do them all as described earlier,cheap caps=cheap tone.

[The Captain]

I have seen tone pot mods for the head, but finding amongst 120 pages is another thing. I chose the speaker carefully enough- a 12" Eminence Legend 125 8ohm at 101 whatsits, bumf stated 'good higher freq characteristics' and basically an even tone; I made a solid (20mm mdf) small cab for it, glued and screwed. Ive not tried another spkr as I dont have one, and shoip says v unlikely to be spkr problem. I did mount the spkr on some 15mm chipboard, which if Id thought better should have been 20mm mdf. The back is 'open'. Thanks again for your input chaps- much appreciated.

[stokes]

Forgot we were dealing with a head and not the combo.Speaker is okay then.I would not even consider any other mods till you upgrade that power supply.Most likely wont even want to.If you brought the amp to me and wanted to pay me to do any other mod to cure what you describe I wouldnt take your money for anything until that upgrade was done.I have done this to many Tweed Champs and Deluxes and have never had a customer want anything else done,it was fixed.This is not a theoretical thing,it is a proven mod for these amps,you can find it in many books.Gerald Weber is one that comes to mind,and I am sure it is in other published sources as well.

[The Captain]

Ok, Ive a second opinion now that its the OT I need to change, 1st opinion was supplier, not so surprisingly.. bit confused as the 65 pages of mods (3 hours to plough thru) on 18watt seem to imply adding a new OT makes the bass BIGGER and bolder amongst other differences. I need the bass SMALLER and the highs zingier: I just need to turn bass down a bit, and treble up a bit. One or two tone pots. I cant get any info in layman's terms on where/ how to stick them in the circuit.. all bits are right here.. argghhh. Thanks again, capt.

[The Captain]

I think Ive cracked it- I added this 'Tweed princeton' tone pot (1st one down) idea to the modded-on master vol pot as a test- bingo! http://amps.zugster.net/articles/tone-stacks#Moonlight

Its done exactly what I wanted; so an afternoon off, 3 hours and much determination later Im a happy chap. Highs zing and bass can be overrided, and its just one knob to fix. Its a bit crap at the mvol extremes etc- but hey.
My final Q just concerns the bits I used: pots are both 470k, pic says 1M. My caps are 680pf and 10nf, pic says 500pf and 4.7pf. Are these ok? or should I use exactly values on pic, or can I change the odd r value to help?

Appreciate your help Mr Stokes and others a great deal- I may well also add a better OT and fiddle with better caps now I know its playable, unless it goes pop with my odds and sods bodged in the amp.. Captain.

[Steve A.]

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.

I haven't worked on an Epi Valve Jr HEAD yet but my combo sounds great with the described mods and a Jensen 8" ceramic speaker:

http://www.blueguitar.org/new/schem/electar/modnotes/bridged-t_mods_for_lp_jr_amp.pdf

The bridged-T circuit made all of the difference, at least to my ears.

Steve Ahola