How to darken a bright amp

[Leonc]

Well I accomplished a lot with my new (old) Gibson GA-19RVT today. I replaced all the electrolytic caps and some others, a number of resistors (e.g., the bias resistor) and put in a 3-prong AC code. The amp no longer has the hum problem. I also experimented with tubes and speakers, basically looking for a bit fuller, slightly darker sound.

The Raytheon 6V6s and Cannabis Rex really helped move in the right direction. The amp now sounds rich and fuller. It still seems rather bright to me. Brighter than I would think is normal. It has only a tone control. I spoken to another owner, erksin, who told me he keeps his on like 7 or 8. I don't think that's humanly possible on mine. I can't really turn it past 3 or so and it gets crazy bright. So there's some reason to think that some values may have drifted off a bit. Here's the original schematic.



Any suggestions on what to experiment with?

[Affectronix]

I really would have thought that the input resistors and cathode capacitors would help, but they all look very high already (good for dark tones).

[VaughnC]

The first thing I'd check are R9, R10, & C7. If any of these are open or have changed value significantly, C5 (500pf) could be passing nothing but treble frequencies on to the following preamp.These components form a mid cut filter.

You could even install a small switch to bypass the filter for a more even response when desired.

[Leonc]

Well I guess it's possible for some components to have drifted. I'm guessing the first candidate to look at is the tone pot itself, yes? 1M, audio taper. Would it drifting (and even starting out off) in one direction or the other likely result in to much high frequencies getting through?

[da-boogieman]

I think I might try C5 and C7 first. Lift C5 (sort of a bright boost it looks like) and play with C7 values. If it helps to lift C5 but gets too dark, try a smaller C5 (~100pf -250pf). If lifting C5 is still too bright, decrease C7. Coupling cap after V2 (C22?) to the phase splitter might benefit from a larger value? Easy things to try...

[Whiskeyrebel]

Try setting the amp on top of a layer or two of thick carpeting. I am not joking. I have an Ampeg that I thought was muddy til I moved it from the living room to the basement. A friend of mine had trouble tuning the "ice picks" out of the DRRI he had at the time and I told him about my experience, so he put a couple heavy pieces of carpet under his cabinet and it balanced the sound a lot.

[Leonc]

Whiskeyrebel--I'm already playing on thick carpet in a pretty dead room. It's still bright.

Thanks for your input fellows. Much appreciated.

[Swarty]

I would characterize these amps as thin rather than bright... just not much in the low mid & bottom. I'd experiment with some larger coupling cap values; maybe change V1 from .001 to .005 and if that doesn't do it, change the outputs from .02 to .05.

[VacuumVoodoo]

C5,C7,R9 & R10 form what is called a "bridged-T notch filter", it is a mid cut. You can adjust depth of mid cut by adding a variable resistor between ground and C7, a trim pot ca 100-220K should work.
C12 and C22 are the ones responsible for low end cut. C12 sets the initial low cut characteristics and what you cut early on in the circuit is difficult to recover later on. Try increasing this to something like 4.7nF. After that try increasing C22 to 1-1.5nF.
This will lift the low end thus changing proportions between highs and lows. At the same time the tone control which is only a simple treble cut should become more usable.
Another thing you might try in addition to the above is connecting the now removed C22 across R24 to fine tune the high end.

I hope this helps to make your Sunday a great one

[trisonic]

Leon,

Am I the first to suggest: throw a large blanket over it!
Or bypass the tone stack? I just prefer that tone, nowadays.

Best, Pete.

[twangbanger]

Originally Posted by Swarty

I would characterize these amps as thin rather than bright... just not much in the low mid & bottom. I'd experiment with some larger coupling cap values; maybe change V1 from .001 to .005 and if that doesn't do it, change the outputs from .02 to .05.

I think the coupling caps are small also, especially the 500pf after V2.

[Affectronix]

Originally Posted by trisonic

Or bypass the tone stack? I just prefer that tone, nowadays.

Same

[StompBoxBlues]

Pardon the stupid question, and I assume you really want it to sound its best probably with guitar tone on 10, but...

This is a little like the too cold, too hot climate thing for me. In a hot, hot climate...other than air conditioning not much you can do but wilt in the heat, in a cooler climate you can always put on a sweater..etc.

I was thinking, why not turn the guitar tone control down? On a dark amp, I can't do that at all without it sounding muffled, etc, but I would think on a bright amp, it could just give you more options. You might find that what sounds too bright at home, is just perfect in a band situation.

Just a thought.

[Leonc]

Originally Posted by StompBoxBlues

Pardon the stupid question, and I assume you really want it to sound its best probably with guitar tone on 10, but...

Incorrect assumption. Perhaps you should read the original post again. What I said was that it appears that some of the values in this amp may have drifted (or something may actually not be working properly) as I know someone else with the same amp (same year as well) who says his does not exhibit this symptom.

Originally Posted by StompBoxBlues

I was thinking, why not turn the guitar tone control down? On a dark amp, I can't do that at all without it sounding muffled, etc, but I would think on a bright amp, it could just give you more options. You might find that what sounds too bright at home, is just perfect in a band situation.

Uh, you think I haven't tried turning down the tone on the guitar? That's not really a satisfactory solution if the amp isn't performing its best. That's a bit like, "Oh, the track sounds like shit, but don't worry, we can fix it in the mix."

I think maybe you've assumed that I'm an inexperienced 19 year old who doesn't know about the obvious stuff. I actually have an excellent command over the obvious, LOL! Much better than over the not-so-obvious, hahah, particularly when it comes to electronics (where I am indeed very much a novice).

Originally Posted by trisonic

Leon,

Am I the first to suggest: throw a large blanket over it!
Or bypass the tone stack? I just prefer that tone, nowadays.

Best, Pete.

Hey Pete - the blanket is not a good idea! The blanket doesn't give you your low end back, it just muffles everything else. I'd rather figure out how to fix that apparent problem, electronically. That's half the fun of it for me.

Now as for bypassing that tone stack...I like that idea! I've had a number of amps with that option and always found it very useful. Anyone have any directions on how that could be done, perhaps via a switch (preferable a solution that doesn't involve permanently altering the amp)?

Also, I appreciate the excellent suggestions but do want to point out that there are a couple things I'm trying to accomplish:

1) Make sure the amp is functioning properly with respect to this "too bright" issue. So here, I'd kind of appreciate suggestions as to what the culprit might be. Again, I believe there is something in the amp that's not quite "up to snuff" involved here.

2) Learn about a mod or two that might get me a somewhat darker sound; louder is better too (i.e., with little amps like this, I find I always want every last bit of volume/headroom I can get). I'm not expecting this to compete with a 50W Marshall, obviously, but with respect to this amp, more volume (and a bit more substantial low-end) is more better, IMO.

[trisonic]

Leon,
Kidding on the blanket thing - but you realized that, of course.

The other benefit of the tone stack bypass thing is that you get a tad more gain. I got it switchable on my BF VC (and reversible, it's only a weeny mod but makes a big sonic difference) so now it's BF or Tweedy (I read somewhere that that is what EC actually used on the Layla sessions - a modded VC).

I got Jim Somma to do mine as I can barely change a fuse......

Best, Pete.