tube solid state hybrid guitar amp, looking for schematic

[OneyedK]

You mentioned parallel configuration, have you tried that with high impedance input? My concern would be stability...

[Wavebourn]

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.

[Ian444]

Originally Posted by jmillerdoc

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

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.

[Enzo]

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.

[jmillerdoc]

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

[OneyedK]

I think the first stage runs on 250V or even less (London City used Fender topology, basically a tad lower as mentioned in the datasheet of the 12AX7), so the 2nd stage (gain stage + cathode follower) would be around 270V max.

[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

[Ian444]

I also sold my Fender Super Six Reverb 15 years ago after I built the Fender preamp I linked to above, and connected it to a homebuilt 100W SS amp. Never regretted it!

I'm just not too sure about overdriving the gainclone, how does it sound when it's breaking up?

Ian.