Another benefit to building an amp based off a single 12V rail is that you can use cheap sealed lead acid batteries to power it. These are VERY easy to charge, cheap, and available in many different sizes. They are pretty much small car batteries. Yes, they're heavy but that's the penalty for lots of capacity with older (but cheaper and easier to implement) technology. The batteries used in computer UPS systems or home alarm system backups would work perfectly. Just search amazon for "sealed lead acid" and you will find a ton of options.

Another thing I failed to mention is that you have to also be careful when discharging Li-Ion batteries. If you drain too much power from them you can damage the battery such that it will no longer take a charge. This means you either have to build a sophisticated power management system that keeps track of the current into and out of the battery and shuts your amp down when it thinks the battery is depleted, or you have to just try to play it safe. "playing it safe" is what RC helicopter/plane pilots do. They know how long they can typically fly on a fully charged battery so they set an egg timer and clip it to their controller. When the time is up they stop flying because continuing to do so may damage the battery.

Just as food for thought. Why not just build yourself a small practice amp? In the long run it would be cheaper and far more rewarding. Several years back I built a practice amp called the Hurricane (it won an award on this very site! ) The whole amp ran off a single 12V supply and could put out about 15W. It had a nice 6" weber speaker in it and sounded great. You can get all the details here: http://www.ssguitar.com/index.php?topic=1237.msg7705#msg7705

It was a fun project. The power amp cost a whopping $10 in kit form and I assembled it in less than an hour. I built the preamp from a proven design using parts from radio shack. The speaker was probably the most expensive part of that build. You could search your local pawn shops to find a small practice amp that you're willing to gut (assuming you don't want to gut your Marshall ) and reuse the cabinet and speaker.

Just putting it out there.
-Matt

Keep in mind you will need two batteries wired up as a dual rail voltage supply. In other words, you connect the negative of one battery to the positive of another to give you a full 30V potential difference (29.6 with the batteries you listed) Then you tap your ground connection into that connection between the battery terminals. That gives you +14.6V from one battery and -14.6V from the other (relative to your chosen ground). One thing to keep in mind is that 2600mAh is not much capacity. It might do fine if you keep the volume down but I wouldn't count on more than an hour or two of run time. If that. 2600mAh = 2.6 Amps of current continuously for 1 hour. I'm not sure how efficient the TDA2030 is but it could easily draw 2.5-3A of current when playing loud. The rest of the circuit has current draw as well but that power amp is your biggest contributor by far. You might want to look for some larger batteries if you don't want to have to recharge it all the time.

Speaking of recharging, you need to keep in mind that any Lithium Ion batteries you buy will require a special  charger. Since you're going to need to charge two batteries at once you should look into special chargers for RC batteries. This will allow you to get faster charge times as well as take precautions to ensure the batteries are charged safely and maintain a long life. NEVER charge Li-Ion batteries with a charger that isn't designed for them.

http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG10CD.pdf

There's the schematic. As was already said, this amp is not a good candidate for battery conversion due to the split rails. You could possibly get away with a pair of large Lithium Ion cells (like those used in RC vehicles) but you would need to take care where you inject the voltage. Or just use an inverter

the LM386 is not a dual op-amp. You used an LM358 which is a dual op amp... That's why your schematic is confusing. A triangle is generally used to indicate an amplifier or buffer (often an op amp but not always). The LM358 being a dual op-amp automatically drops two op-amp symbols into your schematic. You either need to use the correct LM386 schematic component in your schematic capture utility or create a custom one.

-Matt

Your dirty channel likely has significantly more gain than the "clean" channel. That's how all analog amps achieve "dirt" in the sound. Lots of gain and then some manner of clipping after the high gain. Digital/DSP based amps are an entirely different story and can use all sorts of techniques to introduce "grit" into the sound.

The volume control(s) in MOST amp circuits (not all, but most) are simple variable voltage dividers. They are not increasing the amplification of any stage in the amp. This is known as passive volume control. Active volume controls exist but are less common in guitar amps since messing with the gain of an amplification stage can cause other issues in the signal. So typically the gain of a given stage in the amp (preamp, poweramp, pre-pre-preamp, whatever...) is set at design time and the input signal to that stage is simply attenuated to achieve lower volumes.

So don't think of your amp as being "half way up" think of it as "reducing the signal to the power amp by half". (no, being on 5 out of 10 does not mean half signal.. but it works for this example.)

So, if your clean preamp has less gain to begin with (trying to avoid clipping and other gnarly distortions) then cutting that signal "in half" compared to cutting the dirty channel signal "in half" before feeding it into the power amp must be quieter.

Couple all of that with the fact that distorted sounds have a psychoacoustic effect of "sounding louder" than a clean signal at the exact same SPL and I'm sure you can account for the difference in volume you're hearing.

So, to sum up all that nonsense rambling... I agree with the others, your amp is fine There's a reason the amp has an independent volume control for each channel. If they were equally as loud for any given setting then they would have just given you one for both channels
-Matt

Hopefully they fit They are not identical to the pictures you sent, but they look close. Did you take a look at the datasheet for the Panasonic pots and compare the physical dimensions to the part you need to replace?
-Matt

That looks very similar to a panasonic EVU series pot:
http://search.digikey.com/us/en/products/EVU-F2MFL3B14/P3G7103-ND/243621

But I think the shaft on the panasonic pots is slightly longer. Since it's plastic you could probably just trim it to length.

Might be worth a shot.
-Matt

You want a 10K linear taper pot. Is it PCB mount or does it have free wiring between it and the board? The fact that all the pots are 10K linear taper seems to indicate that this is a digital amp and the 10K pots are just providing signals to aux ADC inputs on the board that are used to control the DSP circuitry. Take a look through the list of pots smallbear offers. Find one that looks like what you need to replace. Once you select the style you can change the value using a combo box on the product page. Remember, you want 10K linear.

http://www.smallbearelec.com/Categories.bok?category=Potentiometers+and+Trimpots
(the pots you want to compare against are on the first three pages of results)

Good luck!
-Matt

There are always exceptions, but the "standard" formula for a SS amp does not involve power amp clipping (rather, it tries to avoid it). So you really don't need an attenuator. Just turn down the master volume.
-Matt

I would suggest this transformer:
http://www.antekinc.com/details.php?p=607

I really don't think you can find better prices anywhere. I've bought a couple of transformers from Antek and they are great. All of the mounting hardware is included too. Just double check the size but it will probably fit. (Here's the PDF spec sheet with dimensions http://www.antekinc.com/pdf/AN-1224.pdf )
-Matt

That's what I'm leaning toward as well. There's tons of free space inside the chassis. That 200VA toroid is a heavy beast though Not exactly tube amp PT territory, but big by SS standards. I wanted some extra capacity since I'll be running a pair of LM3886 chips at 50W into 8ohms (well, as close to 50W as I can get, the 25V AC secondaries should get me close to the +/-35V DC I need for 50W into 8 ohms. The speakers are a pair of Weber Ceramic Sig 12B. All wrapped up in a (hopefully!) lightweight 2x12 combo with a fiberglass composite cabinet. My goal is to keep this thing as light as possible for gigging musicians. 40lbs is the target weight, but we'll see.

-Matt

I'm starting to work on a stereo guitar amp using a pair of LM3886 chips. My 2 x 25V 200VA toroid arrived the other day and it JUST fits inside the hammond chassis I plan to use. So, are there any disadvantages/issues with mounting the transformer inside the chassis? Or should I stick to mounting it to the top of the chassis like you usually see.

Thanks!
-Matt

Ok, so there's nothing special going on.. If you were to put the two speakers from a JC120 in two different rooms you would only hear the effect in one of the speakers and not the other... interesting. Doesn't sound like something I want to emulate. I think I'll just stick to my plan of using a mono chorus effect that later gets split to two LM3886 power amps driving individual speakers. It's easier that way

Quote from: phatt on August 16, 2011, 09:10:08 AM
Tremolo and Vibrato; 
On most Amps these two words often mean the same thing, Vibrato where the *Output Level* is Vibrated creating a vibe effect.

True Vibrato is not *Pitch related* it is just the level output is turned up and down via a low frequency oscillator circuit (LFO)

True Tremolo is just the fast Chorus, Pitch shifting effect,,
So yeah,, it can get confusing.

Thanks for the reply!

I think you have the terms backwards. Vibrato is periodic variation in frequency, tremolo is periodic variation in amplitude.
http://en.wikipedia.org/wiki/Vibrato

I believe guitar and amp manufacturers have created much of the confusion between these two terms. A "tremolo bar" (or whammy bar, if you must) is really a vibrato bar since it changes the frequency of the signal. The "Vibrato" effect found on most Fender amps is really a tremolo effect since it simply varies the output volume.

Now, the question remains... Is the vibrato on the JC120 real vibrato, or did they use it in the Fender sense to mean tremolo (I've never heard a good clip of the vibrato effect being used on a JC120... if I could find one it would be instantly obvious which effect they really use). As for the chorus, I'm aware of the different ways to achieve a stereo chorus, my question was which one (or ones, if it varied between revisions) were used by Roland in the JC series of amps.

-Matt