Sounds like you just want to upgrade the reverb tank. With a real spring reverb there's no way to control the decay time. The way you describe the reverb knob working is pretty typical. They usually just serve as a mixer between the "dry" and "wet" signal. Most are set up to keep the dry signal at a fixed level and just bring up the reverb level. This means you can go from all dry to half dry/half reverb. Some units, like the fender stand-alone reverb units have a true blend pot setup so you can go from all dry signal to all wet signal. These stand-alone units also have a "dwell" control which is basically just a volume control for the reverb driver amp. Increasing the dwell just drives the tank harder causing more "dwell". Fender doesn't include this control when it builds reverb into an amp so all you get is a level control.

-Matt

Sounds like a plan. You're probably better off choosing the amp you want and then finding a cabinet/speaker (assuming it's a head, not a combo amp) that best suits the amp and your playing style as the design of the cabinet and speaker have a significant impact on the overall sound you will get.

When you go out to shop for the amp I wouldn't concern yourself with the wattage too much. Just plug in a guitar and try it out in the store. Any good music store will let you test drive the amp and most will let you crank it a bit to see what kind of volume you can get while still keeping things clean. You may be surprised that you can get away with a much lower wattage amp than you think. Be sure to branch out from the more obvious brands (Fender, Marshall, etc...) and try other stuff.. Many Roland amps are well known for producing great cleans (Roland Jazz Chorus, or the cube 60, for example).

I noticed in your first post that you expressed some confusion over the impedance rating (ohm value) of speakers and what they mean.
Hopefully this will help:
http://www.rmcybernetics.com/science/cybernetics/electronics_volts_amps_watts.htm#ohmslaw

In the most basic terms an amplifier is simply a good clean power supply generating voltage and a "valve" to modulate that voltage to the speaker. When the "valve" is all the way open (almost) all of the power supply voltage is being delivered to the speaker when it's closed, none of the voltage is being delivered.

So, using one rearrangement of Ohms law we can see that power (P in watts) is equal to the square of the voltage divided by the resistance (or impedance) in ohms of the load its driving (the speaker). So, lets say an amp has a power supply voltage of 10V and for the sake of argument it can deliver all of that voltage to the speaker when running at full volume. If you have an 8 ohm speaker you would end up with the following P = (10^2)/8  or 100/8 = 12.5 watts.

Now, if all you do is change the speaker from 8 ohms to 16 ohms, then the maximum voltage output stays the same but now you're dividing it by a larger number. so 100/16  or 6.25 watts.

So, the reality is that your amp doesn't "produce" wattage, it produces voltage and its the speaker that actually determines the wattage (or work done) by the total system. Of course, that's not to say you can simply keep lowering the impedance of your speaker to make more wattage. There are limits. Most amps are designed for a particular minimum speaker load that they can safely drive. As the load driven by the amp is reduced more and more current is being drawn through the amp circuits generating more and more waste heat. If the electronics cannot safely dissipate this heat they will burn up.

Note: While the basic concepts hold true for tube amps, the fact that they use an output transformer to drive the speaker makes things more complicated. Tube amps need the speaker impedance to be a specific value in order for the power amp circuit to work properly. The transformer also acts as a sort of resistor in the power amp circuit. However, being a transformer, the "resistance" of the primary coil (the part hooked up to the power tube) will change depending on what impedance speaker it's driving on its secondary coil. Tube amps need the primary impedance to fall within a fairly narrow range in order for the power tube to function as designed. This is why speaker impedance is such a big deal with tube amps and not so much with SS amps.

Hope that helps more than it confuses

with a solid state power amp you don't need to worry too much about matching impedance. There are limits to what you can do, but generally speaking as long as you stay at or above the rated output impedance of the power amp. So you could drive your 16 ohm speaker using pretty much any head. Of course, the amp will only produce its rated output wattage at the specified output impedance. So if you buy a 50W SS head meant to drive an 8 ohm load it would only generate 25W into a 16 ohm load.

You could swap in the next highest impedance speaker. If the amp has an 8 ohm put in a 16 ohm. Assuming everything else stays the same that will cut your output wattage roughly in half. You could also just put in a low efficiency speaker assuming what you have is high efficiency.

The "best" solution though would be to add an attenuator in front of the speaker. Then you could make the amp as loud or as quiet as you want without swapping parts. The MASS series by Ted Weber look interesting: http://www.tedweber.com/atten.htm

I was crunching some numbers to determine what size power transformer I want to buy for my LM3886 so I decided to create a spreadsheet to make life easy. Sine so many people here are interested in the LM3886 (and other chip amps) I decided to share in case anyone finds it useful.

I provide two types of calculations:

The top table allows you to specify how you want the amp to perform (output wattage into what speaker impedance and with what input sensitivity) and calculates what your rail voltages should be and what to set your feedback-to-ground resistor to in order to achieve the desired characteristics.

The bottom table allows you to specify how your amp is configured (rail voltages, speaker impedance, feedback loop resistors, etc...) and it will tell you what your  output power, gain and input impedance are.

For all calculations I also provide gain in Av and dB as well as peak output voltage and peak output current.
Simply type your known/desired values into the top (blue) section of either table and the values will update in the bottom section.

Please see the notes for additional info.

I would appreciate any comments and/or suggested improvements.
-Matt

You could run the output of your CHAMP into an attenuator box (both for output impedance matching as well as overall signal attenuation) and then run that output back into your higher powered SS amp. Ted Weber sells a wide variety of nice attenuators that would do the trick. Of particular interest are the "MASS" models. They use a real speaker motor as the dummy load rather than a power resistor. This provides a more realistic load simulation to the tube amp.

http://www.tedweber.com/atten.htm

Yes and no. I plan to build a stomp-box version with an additional gain stage in front (to avoid low-clipping the DSP's DAC) and maybe an output buffer. However, the primary purpose is to integrate it within an amp.

I guess you could modify my footswitch to also short the output of the DSP to ground when the effect is turned off. That will ensure that there is only dry signal making its way out. Another option if you really must have only dry signal is to use standard 3PDT true bypass wiring (assuming a stomp box build) Then the reverb circuit is taken completely out of the loop. As for being able to get a full wet signal, that shouldn't be an issue with my setup. Sure, some of the dry signal will bleed through, but very little. You can increase the size of the blend pot to reduce it even more.

-Matt

Here's another resource with several cab sims ranging from simple to complex.
http://www.herby.kielce.pl/~piter/hexenew2/ie.php?c=cabsims_e&l=diy_e

Edit: Oops.. I just saw your name in the article, I guess you translated it I'll leave the link here since it's pertinent to the discussion though.

-Matt

wow, yeah that is simple! might just do the trick. I'm pretty sure I have all the parts here to throw one together and try it out.

Thanks so much for the quick reply. I suspected this was the case but I wasn't 100% sure (The condor cab sim from ROG uses a similar design with only the first two stages biased directly).

What size cap would you use on the output? Presumably something large enough not to affect the frequency response maybe a 0.47 uF? Or bigger... 1.0uF?

I would love to take a look at those schematics. I'm trying to come up with something super easy to build but also something that I can squeeze into an empty space in my preamp box. I built a little preamp for my friend to record with. He's using software amp sims currently but I would like to filter out some of the highs in the box so maybe it could be used with a PA. I hear what you're saying though about simple designs not being realistic. There's no way a couple of filters are going to reproduce the complex frequency response of a guitar speaker in a cabinet. My hope is that I can get "close enough" with the overall frequency response that the PA speakers will sound somewhat convincing to the ear (rather than just sounding tinny like a guitar plugged directly into a PA does)

I've been looking for a decent (yet simple) speaker/cabinet simulator circuit to integrate into a little preamp project I'm working on. I want to keep the part count low and the board small and am willing to compromise some (ok, quite a bit!) of accuracy to do it. In other words, I want something passable, not perfect. I've come across a couple of designs that seem to fit the bill but have a question.

http://www.diystompboxes.com/pedals/schems/msim.jpg
This one is a direct rip from a Marshall amp with an XLR output. I do have one concern though, only the first two op amp stages have the non-inverting input referenced to VB (bias voltage, presumably). Why is that? Is it because the second two stages are not AC coupled to the first two stages? Does the bias voltage carry through? I would think the voltage would be off because the first two stages have gain. Will the design work as-is? I also believe I should add a DC blocking cap on the output of the last stage, correct?



http://www.schematicheaven.com/marshallamps/jtm30_30w_jtm310.pdf
Here's a link to the full schematic of a Marshall amp that uses this circuit. I'm not finding any answers there because it uses a bipolar supply rather than a bias voltage like the first link I posted. The section of interest is IC3 (b - e) which is on the right side of the first page about half-way down. You can see right above it that the IC is powered by two voltage regulators (78L15 for positive supply and 79L15 for negative).

If anyone is curious, here's another design that is even simpler (using only one dual op amp) I don't have a schematic for it so I'll have to create one from the PCB design. http://www.radmer.dk/billeder/amz-lay.gif This seems to be an AMZ design but the schematic is no longer on www.muzique.com

[edit: Oops... I forgot to include a link to the real marshall schematic ]

Seems like it... But if it is it's bad spam... no link to a mystery product that will increase the size of your manhood or anything? They're getting lazy.

I've attached a rough schematic of my idea. I removed the 1M pull-down resistors after the buffers since there shouldn't be any situation where the outputs are floating (I'm assuming that's why they're there, to eliminate problems when nothing is plugged into the green or red channels). I also added a foot switch to cancel the effect and ensure that the full dry signal is passed to the output.

I'm not sure if this is what the OP is after. A circuit like this is not as fool-proof as the runoff groove design and should probably only be used for internal effects where you know all the variables (Impedances, if the effect inverts the output, etc..) It does however eliminate a lot of components (hopefully without any issue! that's the feedback I'm hoping for ) and should be a pretty easy build.

I'm in the same boat at Marek. I have a DSP board I need to build a mixer for. I like the blend pot approach because it allows you to get fully dry or fully wet signal. Most reverb mixer circuits assume you always want full dry signal and then just allow you to control the volume of the wet signal. Some of the effects on my DSP board (like rotary speaker cabinet) only make sense without the dry signal.

So... For the runoffgroove design can you get away with just the two "send" buffers to split the signal? My DSP board has unity gain so Vin should = Vout therefore no recovery gain is needed. Also, it's meant to drive a load greater than 2K Ohms so it should be safe for just about any power amp input without a buffer. So I'm thinking of doing the "send" half of the ROG circuit. The "Green Send" would be my dry signal and run directly to lug 1 of the blend pot. "Red Send" will feed the input of my DSP module with the output of the DSP board running to lug 3 of the blend pot. No recovery buffers (and one less IC!) Do you think this would work? Should I buffer the output of my DSP board anyway? I know I don't need any of the phase inversion stuff (JFET, switch, etc..) since this will be an internal effects "loop" so there's no opportunity for anyone to invert the signal.

Thanks for the info on the speakers. I'm not sure I want to take on this project just yet. The thing is that my friend wants to retrofit a circuit that has the same controls as the original, that means two preamp channels (bright and normal) with a gain control for each. Also, a power amp section with a "presence" control. I figured around $200 in parts to do this all with a tube pre and an LM3886 power amp. I'm not willing to risk $200 of his money or mine on a project that very well could fail I need to get a few simple tube amp builds under my belt before taking on something like this.
-Matt