Controlling brightness

[dude]

In general, which is best way to cut an overly bright treble? I know it depends on the circuit, the type of tone stack and different fixes for each. Say a customer brings in a Deluxe Reverb, Super, Plexi  or an amp with a simple tone pot, what would be the first thing a tech might do?


Would he lower the cap on the treble or tone pot, lift a leg and try different values or go to the board and change a coupling cap for that channel?
 
I know speaker changes and even pre-amp tubes can do wonders to an overly bright amp and brightness is such a subjective thing. But what would be first thing a tech might do?

[sluckey]

If it's a push/pull amp with LTP PI would first try a small cap between the plates of the PI tube. Something between 47pF and 500pF. Fits easily right on the tube socket.

You can also put a small cap parallel to the plate load resistor in a preamp stage of any amp. Something between 47pF and 500pF. I think tubenit is fond of this method and he calls it a smoothing cap (I think).

[shooter]

What Sluckey said!

I do parallel the treble cap, easy with a gator-clip for experiment
I'll also bump up in value bypass caps PAST V1A, so if the 2nd gain stage has 4.7uf, again double it with gator clips.
no solder, no mess.  The key ALWAYS test each change using the same song, rif, etc you used to decide it's too treble-y

[dude]

Thanks, wasn’t getting the results l wanted changing the treble cap or coupling caps. Remember reading tubenit posts before, the 6G3 has both PI plates cap of 100p (.0001), think that converts to 100p, also has the parallel.003 across 220k plate on the normal channel, that cap really took a lot of brightness out, too much, maybe l should have used 47p there. Shooter , alligator clips seems like the way to go, fast and easy, thanks

[shooter]

ya gator-clipping while tuning makes it go quick, you might get some hummy, but you can ignore it 
the bypass caps don't "trim" treble, they add bass which more often then not makes treble "less relevant"

[d95err]

Excessive treble can often be due to stupidly large value on the ”bright” cap over the volume pot.

Cutting out or reducing the value of such bright caps can often help control an overly bright amp.

[dude]

After numerous changes to cut some ice-picking, most not giving me what I wanted I found the real culprit. Sometimes taking out or reducing the cathode cap on the second stage, which is usually a 25uf cap in parallel with the usual 1K5 R, cleans up the amp.  Lowering or removing this cap his cap can also cause overly brightness, sometimes a lot. I had replaced the 25uf with 1uf, no matter what I did the overly brightness remained. I thought about what once Shooter said, "sometimes adding bass you don't hear the piercing highs". Sure enough I put the 25uf back and the overly brightness disappeared. Yeah, more distortion but in a good way and turning up the treble helped cut some of that extra bass if wanted. I had read somewhere that lowering this cap cleaned up the amp, it does but  also can rob the Mojo. I also did this with Sluckey's single channel Deluxe Lite, which was kind of too bright (too), back to a higher cap in that second stage K, and the mojo is back. Live and learn.


Lesson learned: Take a lot of mods with a grain of salt, some might not be what you're looking for.

[sluckey]

Here's how the Texas boys cut the brightness...

[dude]

Could you imagine the tone if they took that cap out in the 6G3's there playing...

[Willabe]

After numerous changes to cut some ice-picking, most not giving me what I wanted I found the real culprit. .....I had replaced the 25uf with 1uf, no matter what I did the overly brightness remained.  ....Sure enough I put the 25uf back and the overly brightness disappeared. Yeah, more distortion but in a good way and turning up the treble helped cut some of that extra bass if wanted.

You went from a full frequency bypass cap value, 25uF, to an almost no bottom end bypass cap value, 1uF, then back to full bypass value. (One extreme to the other extreme.)

Now try something in the middle, a 10uF and then try a 4.7uF, might be mo' better! 

[pdf64]

I think that for guitar, 10uF can usually be regarded as fully bypassed.
See http://bmamps.com/CapCal.html for a peer reviewed rdh4 method calculator.

[Willabe]

I think that for guitar, 10uF can usually be regarded as fully bypassed.

That may be so, and I didn't play with the link's calculator (thank you for posting that  ), but.....

On my bread board, I wired up a Fender Harvard amp (Champ with a single tone control) with a 6 position rotary switch on the 1st/input gain stage cathode; open, 1uF, 2.2uF, 4.7uF, 10uF, 25uF.

Definitely could hear a difference between the 25uF and the 10uF.

The 10uF sounded good with my Strat/single coils, the 4.7uF sounded better with my Les Paul/HB. 

The 10uF stripped out enough bottom mud yet still sounded full/fat. The 4.7uF sounded a little too thin with the Strat.

I was very pleasantly surprised.   

I would say maybe 20% to 25%(?) difference in tone on the bottom end between each of the 25/10/4.7uF.     

I should say, I was testing through a 12" and 15" speakers, might have been a little different with a 10"?

[PRR]

> 10uF can usually be regarded as fully bypassed.

For small signal.

In OVERdrive, cap size controls attack and recovery time constants.

> Definitely could hear a difference between the 25uF and the 10uF.

So I am wondering if this observation is from small or large (clipping) playing?

Since both conditions are typical for many guitarists, both are valid.

[Willabe]

So I am wondering if this observation is from small or large (clipping) playing?

Amp volume was set anywhere from 10:00 to 1:00. 

No pedals.

[jjasilli]

Amp volume was set anywhere from 10:00 to 1:00
Typically, guitarists don't get up that early! 

But seriously folks, in my humble experience, sometimes small value changes have made a way larger than expected difference.  Other times, big value value changes surprisingly made no difference.  So, I think A/B experimentation is worth a try.

[dude]

I usual use a 4.7uf, first stage, a little thin with a strat, 10uf with a strat isn’t too bad with humbuckers it’s ok too, a 25uf can be too much with HB just hoping for something in the middle with all guitars. I do hear a difference between 10 an 25, even 4.7 and 10, slightly. A rotary switch would solved the problem for all guitars but then you have speakers coming into play; so many ways to skin a cat (terrible expression,  ) but l usually find 10uf the best overall for all guitars. But every amp is different, Sluckey’s Fender deluxe lite a 22uf sounds best for 2nd stage and 10 for first to me. Sometimes, l need to put the guitar down and wait a day, my ears get fatigued. I set up a amp, think it’s not quite right, then the next day after two beers, it’s killer..., go figure.

[Williamblake]

If you have reverb and a cap across the resistor connecting reverb send and return you might also tinker with that cap. Also variable caps with 5pF to 25pF are very common in old service equipment.