Hello,
I have a LM3886x3 150w power amp module that I was thinking of using to build a head unit for a friends electric guitar. I was wondering if anybody knew of a guitar preamp schematic that I could graft onto this module. I would like to find something with the typical 2-3 12AX7 tubes that has the master, gain, treble, mid, bass, and presence features. If anybody knows of a schematic that has a tube front end connected to a LMxxxx chip amp for guitar use I would be greatly appreciative. I have heard of commercial amps that use a series of tubes for the preamp stage that use a chip amp for the power stage but I don't know any of the models. I am sure if I could learn of one of these commercial amps I could find the schematic and easily adapt my module to it.
Thanks,
Jeff
I have a LM3886x3 150w power amp module that I was thinking of using to build a head unit for a friends electric guitar. I was wondering if anybody knew of a guitar preamp schematic that I could graft onto this module. I would like to find something with the typical 2-3 12AX7 tubes that has the master, gain, treble, mid, bass, and presence features. If anybody knows of a schematic that has a tube front end connected to a LMxxxx chip amp for guitar use I would be greatly appreciative. I have heard of commercial amps that use a series of tubes for the preamp stage that use a chip amp for the power stage but I don't know any of the models. I am sure if I could learn of one of these commercial amps I could find the schematic and easily adapt my module to it.
Thanks,
Jeff
I don't know of such a schematic offhand, but, as long as you don't overdrive the chip amp, I think it will work very well. The sound will almost certainly be very gross if you do overdrive the chip amp.
In practice you could just build any tube preamp you like, measure its maximum output voltage swing (with maximum drive or full clipping, preferably with a 'scope) then attenuate the output signal so it ends up just below the input sensitivity of the chip amp.
In practice you could just build any tube preamp you like, measure its maximum output voltage swing (with maximum drive or full clipping, preferably with a 'scope) then attenuate the output signal so it ends up just below the input sensitivity of the chip amp.
I agree, instead of looking for a commercial amp that happens to be exactly what you want to build, just build your SS power amp, then build a preamp you like, and connect them together.
The tube section and the 3886 sections will need totally different power supplies.
How complex a preamp you want?
http://www.schematicheaven.com/newamps/peavey_rockmaster_preamp.pdf
The tube section and the 3886 sections will need totally different power supplies.
How complex a preamp you want?
http://www.schematicheaven.com/newamps/peavey_rockmaster_preamp.pdf
Ask your friend what types of amps he likes the sound of. If he likes overdriven Marshall, I built a Marshall pre using 2 tubes into a SS 100W kit project about 15 years ago, worked great. You won't get a "presence" control as that is usually in a seperate part of the amp schematic. You can use two small back to back mains transformers for heaters and B+. If he wants something "clean", try a Fender Twin Reverb schematic, uses only one valve. Most of the components are in the tone control section and you can hang them from the pots, very simple to build. Maybe these days there's much better stuff around like this this one, scroll down to the bottom.
Can you tell me how you did it? The voltage swing into the chip or SS amp section, isn't it too wide? I thought most of these chip and SS amps only require about 2vrms to drive to full power. How do you step the voltage swing back down to the amount needed for the SS section? Or am I missing something or way off? I am only familiar with tube corcuits, have very little knowledge about SS circuits. I have built a couple of Gainclone kits before but anybody can do that. I had considered just taking, as has been said so far on the thread, the front end off of one of the Marshall or Fender circuits and mating it to the input of the chip amp module via a capacitor. Could a voltage divider be built to lower the voltage swing into the chip amp input? How did you account for this? Or did you even need to? Again, I am very naive when it comes to solid state circuitry and really need some hand holding with this.
Thanks,
Jeff
Ok, I found this and I like it: http://www.drtube.com/schematics/londoncity/dea70.gif
Can I just take everything in front of the phase splitter (right where the "treble" pot is) and couple this to my SS stage with a cap? Also, I can't tell from the schematic what the votage should be on top of the 12AX7's, any clue what voltage the preamp tubes should be run from?
Jeff
Can I just take everything in front of the phase splitter (right where the "treble" pot is) and couple this to my SS stage with a cap? Also, I can't tell from the schematic what the votage should be on top of the 12AX7's, any clue what voltage the preamp tubes should be run from?
Jeff
Nope, you cannot do that, imput impedance of an LM3886 is far too low.
Also, you don 't want to couple a tube amp (high voltage) with an SS amp, doing that with a transformer instead of a capacitor will gain you a lot of reliability.
I am not sure that is so correct. If the LM3886 is connected as non-inverting the input impedance will be very high, megaohms and more.
If you use inverting mode the input impedance will equal the input resistor.
I think my main problem will be the voltage swing still.
Jeff
In 1982 I fixing guitar amp with fried OT put couple of transistor class AB followers after a phase splitter, instead of EL34 tubes and OT. As the result, the amp had bridged output, working on the same 32 Ohm speaker. The sound was very tubey and the owner was happy. Can't post the picture: my DSL is down, a technician will come on Monday only. I'm using some public WiFi now.
Yes. You will probably need some sort of buffer between the treble pot wiper output and the chip amp. There is probably no point in having four inputs, I don't know why the amp makers ever did that, but it was common. You only need one input and that would give you a spare half stage if you need it. Otherwise you could use a opamp buffer. Since I presume you are going to drive 3 x modules a buffer or cathode follower would probably be a good idea.
For some ideas, here's a Fender preamp I built and used for quite a few years, and a Marshall copy stompbox I built, I also built that cct into the 100W SS amp I mentioned earlier, just change the output resistor on top of the final volume control to match the output level to the amp.
Musicman had a SS preamp and tube output stage, what a waste of time IMO (the SS preamp that is). You can get some good tone shaping with a tube preamp. If you build the pre, make sure it is easy to modify as you may want to do some tweaking, or try a completely different preamp schematic. The Marshall preamp I built really did sound like an overdriven Marshall amp, no doubt about it, Have fun
Ian.
There are plenty of fans of the Music Man SS front end. And ther are no shortage of guys who love their old Peavey Mace/Duece amps as well. The Lynyrd Skynyrd sound.
Why does this amp have four inputs? There are two per channel. The input resistors are configured so one jack feeds the grid through the resistors in parallel, while the second jack sees the resistors as a voltage divider - a 6db pad. That explains two jacks per channel. And why two channels? Look at the lower set of input jacks, and follow over to teh volume control for that channel. It has the added "bright" cap. SO you have a normal chanel and a bright channel.
You could replace the bright channel with a bright switch on the normal channel, and for that matter, you can use just one input jack and have a 6db pad switch next to it. But on stage, some guys will have a guitar plugged into each channel, maybe the bright channel for his humbuckers and the normal for his single coils, who knows. And the separate volume controls mean no twiddling at the amp when making a guitar change.
Why does this amp have four inputs? There are two per channel. The input resistors are configured so one jack feeds the grid through the resistors in parallel, while the second jack sees the resistors as a voltage divider - a 6db pad. That explains two jacks per channel. And why two channels? Look at the lower set of input jacks, and follow over to teh volume control for that channel. It has the added "bright" cap. SO you have a normal chanel and a bright channel.
You could replace the bright channel with a bright switch on the normal channel, and for that matter, you can use just one input jack and have a 6db pad switch next to it. But on stage, some guys will have a guitar plugged into each channel, maybe the bright channel for his humbuckers and the normal for his single coils, who knows. And the separate volume controls mean no twiddling at the amp when making a guitar change.
Ya'll think the HT in this preamp is about 280-300v? The 12AX7's seem to be pretty much center biased with the 820R cathode resistors on the parallel sections (820+820=1.6K), pretty typical bias point for a guitar amp as I understand. B/C of this I deduce the HT is going to be pretty typical too, around 300v, but I could be wrong. Anybody here have any insight into what the HT would be for this circuit, it is not noted in the schematic. I will be using the inverting inputs on the LM3886 as that is how the module comes. It already has some NFB built into the module, I think using about 22K feedback resistor. I was thinking of mating the two stages together using a 500K pot following the treble wiper, just like a typical volume control. Once I find the best setting I will replace the pot with a set voltage divider with a couple of resistors. I can't think of an easier way to do it.
Jeff
Jeff
Jeff I worked on your guitar front end
The cathod follower will load into the gainclone unit nicely. The variable resistor can be used to set the voltage swing input to the gainclone. Set it for a clean non-overdrive input first, at full volume. Then increase it to a level that just overdrives the gainclone at full volume. Then increase the variable resistor at CF, until you reach the level of tube overdrive that is desired. Seal the variable with wax. And it should be fine.
Measure your voltage swing closely, starting out at 250 to 500 mv. And increase it until you reached the desired level of clean and overdrive sound and volume.
I do this for my grandson and his band. I build SS amps with tube fronts. I sold my Fenders, because they are asking too much for replacement tubes and tubes in general.
Take Care
Ivey
The cathod follower will load into the gainclone unit nicely. The variable resistor can be used to set the voltage swing input to the gainclone. Set it for a clean non-overdrive input first, at full volume. Then increase it to a level that just overdrives the gainclone at full volume. Then increase the variable resistor at CF, until you reach the level of tube overdrive that is desired. Seal the variable with wax. And it should be fine.
Measure your voltage swing closely, starting out at 250 to 500 mv. And increase it until you reached the desired level of clean and overdrive sound and volume.
I do this for my grandson and his band. I build SS amps with tube fronts. I sold my Fenders, because they are asking too much for replacement tubes and tubes in general.
Take Care
Ivey
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