Amp techs....how do you mod EQ?

[Gearjoneser]

I know a bit about electronics, but want to know more about revoicing EQ.

Let's say you have a JCM 800 that would sound better with less treble, and you don't want to change tubes or speakers.

If you normally set the Treble on 2 or 3, but want it set around 5 - 6 so you get a less choked off sound, but the same tone...

Would you change the pot to a lower value, or start changing out a resistor and cap in the tone stack?

[crusty philtrum]

Re: Amp techs....how do you mod EQ?

http://www.duncanamps.com/tsc/

Download this and then play away to your heart's content.

Personally if i want to make an amp less bright overall, I deal with it via coupling capacitors, but the tone stack calculator in the link will let you see the effect of any changes. There is also a granite version available for hard rock players.

[Gearjoneser]

Re: Amp techs....how do you mod EQ?

I keep throwing Tone Stones at the faceplate yet nothing changes, except for broken knobs...

Please advise. LOL

[MetalManiac]

Re: Amp techs....how do you mod EQ?

Theres a "bright Cap", well, at least in my '78 JMP , that should be identical in the JCM800. Just snip it. Its reversible, no harm , no foul.I did it to mine. Check and see , just google Marshall jcm800 bright cap, it has a number, so its too easy to do.

[Myaccount876]

Re: Amp techs....how do you mod EQ?

The way I'd go about it is basically the Vox Tone Cut control. That way you could keep the treble knob somewhat high to not choke anything off, and slightly damp them down to taste with the Tone Cut. Obviously the amp will need to be push-pull AB, which your JCM800 is. Most classic tube amps are AB.

[El Dunco]

Re: Amp techs....how do you mod EQ?

Theres a "bright Cap", well, at least in my '78 JMP , that should be identical in the JCM800. Just snip it. Its reversible, no harm , no foul.I did it to mine. Check and see , just google Marshall jcm800 bright cap, it has a number, so its too easy to do.

I did a similar thing to my JVM to tame the scratchy high mids and tame the over the top gain somewhat and now it's much smoother on recordings.

[Kamanda~SD]

Re: Amp techs....how do you mod EQ?

Yeah, the 800 bright cap mod is certainly the most common. however, if you run the preamp on 10 it won't make a difference.

[devastone]

Re: Amp techs....how do you mod EQ?

Definitely start with getting rid of the bright cap, it's the little cap that goes across the gain pot. It's called the Warren Haynes mod for Soldano SLOs, so you'll be in good company. The cap is basically doing the same thing as a treble bleed cap across a guitar's volume pot, so as Kamanda said, if you are running wide open, it won't have any effect.

It's the quickest and easiest way to do what you are asking, anything else is going to involve more work and making "real" changes to the circuit.

If that mod doesn't work for you though, let me know, I had a bunch of mods around here and I'll see if I can dig them up for you.

[Lake Placid Blues]

Re: Amp techs....how do you mod EQ?

Be aware that bright cap mods will affect the amount of gain for the whole amp, or for that channel on multi-channel amps. Although it does affect the brightness to some extent it more so affects the gain of master volume amps. I found this out by experimenting with different bright caps on a Marshall master volume amp. I tried it with a 220pf in place of the stock 1000pf cap, and it seemed to kill the gain of the amp. It had rather low gain and didn't get loud until the input gain was cranked up to about 9 at which point it also got a bit muddy. However, the clean tone with a lower input gain knob setting (about 7) and high master setting was excellent. Likewise a 220pf to 100pf bright cap on a plexi, in place of the 500pf stock cap, can be very nice. It tames some brightness and makes the sweep of the volume knob much more even and controlled.

But getting back to the bright cap on that master volume amp, I even tried a 920pf but it didn't bring the gain back. I had to go back to 1000pf and then the amp got its voice back. Of course these caps vary +/- 5% or so. The amp sounded best (to me) with a cap that measured exactly 1005pf. Any higher and it started to get fizzy and buzzy, but much lower it lacked gain.

Nonetheless, the red channel on a DSL, and the like, apparently has enough gain that bright cap mods and even no bright cap leaves ample gain. I changed the bright cap from the stock 470pf (red channel) to 180pf for somebody and really tamed the brightness (and reduced the fizz) of that channel and it still had more than enough gain.

[Lake Placid Blues]

Re: Amp techs....how do you mod EQ?

Go to post 11 here for a schematic of the Warren Haynes mod on a SLO:

http://www.mylespaul.com/forums/squawk-box/128747-warren-haynes-amp-settings.html

It is a switch that cuts out the 1000pf bright cap, or cuts it back in. With out the cap in the circuit, based on my own experience, it would reduce the gain and the brightness, allowing the high master volume settings he uses to push the power tubes a little, without being way too loud.

The SLO uses Tweed or JTM45 slope resistor and pot values in the tone stack. A JCM800 or a JMP uses a lower value slope resistor. Using the Duncan amps calculator linked to by Crusty allows one to see how these different values affect the EQ.

[Gearjoneser]

Re: Amp techs....how do you mod EQ?

Good info guys! I can't believe I forgot about the bright cap mod. I need to start there.

The amp is a 100% pristine 1986 JCM800 2204 w/EL-34's, and it sounds great with the treble at 2 - 3. I also have a mint 100W 212, made in '84, but the EQ sounds perfectly situated on that, since it's got 4 6550's.....loudest combo you've ever heard!

[Chickenwings]

Re: Amp techs....how do you mod EQ?

fwiw nearly every marshall ive played or owned has sounded best with the treble down at 2 or 3, the mids a bit over half and the bass on 10.

[Securb]

Re: Amp techs....how do you mod EQ?

Personally if i want to make an amp less bright overall, I deal with it via coupling capacitors, but the tone stack calculator in the link will let you see the effect of any changes.

I did this with my Fender Quad's bright switch cap, but it made the amp too dark overall. That is saying a lot seeing I play with a very bassy, dark tone. The good thing is it is an easy mod to reverse and give a great insight to the inner workings of the amp.

[LLL]

Re: Amp techs....how do you mod EQ?



This safety warning should always precede any advice along these lines:

Before poking around in your amp, make damn sure you've drained the electrolytic caps first... no matter how simple the job or mod you're going to do; including biasing.

Failure to do so may kill you.

With that out of the way -

Step 1: carefully desolder the bright cap (that is, if you ever want to put it back and keep the amp original)

Step 2: plug in and test - if the amp is too dull/muddy now, try a lower value bright cap

Step 3: rinse and repeat until you find the perfect value that you like


Of course, the bright cap depends entirely upon where you have your (master) volume knob at... if you're "diming" the amp, it won't matter.

But if you're not "diming" the amp, this may just be the ticket without having to noodle with the tone stack on the PCB.

[LLL]

Re: Amp techs....how do you mod EQ?

fwiw nearly every marshall ive played or owned has sounded best with the treble down at 2 or 3, the mids a bit over half and the bass on 10.

The older the Marshall circuit, the thicker the tone.

As Marshall amps progressed over the decades, their tone got brighter and brighter.