Marshall tone, in just a preamp

[LeftyStrat]

I've been trying to come up with a design for a multichannel preamp. In addition to a blackface Fender pre, obviously a Marshall channel is in order.

If I have to, I'll build a full mini amp with low power tubes, some type of load, and a post OT line out. But I'm wondering if I can get there with something simpler. The fact that people are pleased with post PI master volumes suggests that the PI distortion is where things are really happening.

A few of ideas:

- Marshall preamp followed by a pentode.
- Marshall preamp and fake PI.
- Combination of the two.
- Steal the 'crunch' channel from something else (x88r, mesa quad, tsl100, bogner triple, or ?).

Anyone have any suggestions?

[Colossal]

Lefty,

I've been entertaining this idea myself for some time. One of those projects for a home studio rig where you would reamp to stereo monitors maybe. I agree that a Blackface type clean would be requisite, maybe a JTM45 crunch channel, a dimed Plexi type channel for more squish, and then of course the over-the-top channel.

I like the notion of a low power output section run to a reactive load so you can get some added warmth. All of this could easily fit into a four space rack with four rows of knobs

[LeftyStrat]

I like the idea of two Marshall channels, plexi-ish and more modern crunch.

I'm planning on using an arduino to manage the relays and MIDI switching.

I also thought about a Dumble preamp. Since the low plate classic uses the same plate and cathode resistors as the Fender AB763 preamp, I could just have a relay to switch between the two tone stacks.

For the Marshall channel, maybe a relay to implement the 'one wire' mod.

[Bob S]

Interesting - been thinking about a dual channel 4 holer.
Independent pre channlels before the PI.
5 or 6 pre tubes, but lots of options available.
My brain hurts!

[Reeltarded]

How about a dummy loaded 6v6 section after a PI that everything goes through? You could make it low B+ and only use those tubes to play through. Around 5w each. Easy to soak 10-12 watts up and still run kind of cool in an enclosure.

[LeftyStrat]

I have some ECL84's that I thought about using. These have a triode and pentode in a single bottle. These seem to have really good bass response. I have a Hammond 270DX (3A heaters, 275-0-275, 104ma) I'd like to use:

- 2xECL84 for PI/PA, 1.44A
- 5x12ax7 for preamp channels, 1.5A

I could use the 5v tap to power the arduino and relays.

- One 12ax7 with switchable tone stack (Dumble clean and Fender).
- One 12ax7 Dumble ODS.
- Two 12ax7 Marshall.

One 12ax7 left over for something. I supposed I could do a one channel Marshall and use the leftover triode for a wreckish preamp. Or an HRM Dumble OD channel.

Have a switched reactive load on the output for when no speaker is plugged in. A line out and maybe the Marshall and Fender speaker sims described here:

http://home.netcarrier.com/lxh2/web/

[10thTx]

Your idea intrigued me, so I drew up a schematic on how I would approach it. I also posted the ExpressSCH version so you can edit it, IF you want.
There is a second page on the editable version.

I find using a mosfet as a CF to be just fine to my ears. Correction on the CF resistor value. It should not be 120k but somewhere between 82k to 68k.

The values in the schematic are ones that I prefer and have worked for me.

Your challenge will be to have voltages high enough for the Dumblish preamp and OD ......... but NOT too high for the ECL84 pentode.

I think I would be inclined to use a VVR for just the ECL84 to get it down to it's max voltages while keeping the preamp voltages high. IF you did it that way than a PT with 150ma and 275-0-275 would be a good choice, IMO.

Honestly, I would lean towards NOT using the ECL84 triodes for the LTPI and instead use a 12A_7 instead so I could experiment with 12AT7, 12AY7, 12AV7 or 12AU7 for the LTPI.

You could just use the ECL84 pentodes alone for the 3-4 watts total.

With respect, 10thtx

[10thTx]

Now having posted that ...........................

I personally would be inclined to build it with a PT that is 275-0-275 150ma and a tube rectifier.

With that you could use 6K6 tubes with 5Y3GT, then 6V6 with 5V4, & 6L6 with GZ34. I have an amp that I can do that with using the 275-0-275 and it allows some cool options, IMO.

With respect, 10thtx

[caphead]

Have you ever seen the schematic for the Laney Klipp? It uses a voltage starved LTPI as its 'fuzz' circuit. With that circuit running at normal voltages, and with a presence control added, I think it might properly simulate a Marshall style PI. Perhaps after the 'fake' PI, you could have a 12AU7 run in parallel into a reverb transformer (I believe the 1 watt marshall uses this arrangement for it's output section), to simulate the sound of the powertubes and transformer saturation. After the reverb transformer (the NFB for the fake LTPI comes from here) use an 8-10 ohm 5 watt power resistor to ground (like the Garnet H-Zog preamp), then a capacitor (value depends on your desired low frequency cut off) to a level pot, then to the output. If you want to go one step further, you can add a speaker simulator circuit, which can be active or passive. For the passive method, you would need a couple of inductors, a few capacitors and resistors and alot of experimentation to find the right combination that works for you. In case you couldn't tell, I've been giving much thought for a while to do something like this myself, lol! I hope this helps.....

[LeftyStrat]

Yeah, I've been investigating reactive loads:

http://www.aikenamps.com/spkrload.html
http://www.thegearpage.net/board/showth ... ?t=1072793

The second link is a long thread where someone creates one.

This post in particular demonstrates the different between a resistive load, a reactive load, and a real cabinet:

http://www.thegearpage.net/board/showpo ... stcount=60

[M Fowler]

I use a 25kL pot between the PI 820R bias resistor and pins 3&8 of PI tube to reduce the voltage and there by reducing the volume for bedroom playing.

Colossal and I took that idea from Egnator Rebel 20. Also, Carlsboro 60tc amp has same idea. Pritchard amps did same thing.

Doesn't sound great in all amps but works nicely in circuits like TW Rocket, Vox AC30, Fender 5f6a.

Mark

[Colossal]

I use a 25kL pot between the PI 820R bias resistor and pins 3&8 of PI tube to reduce the voltage and there by reducing the volume for bedroom playing.

Colossal and I took that idea from Egnator Rebel 20. Also, Carlsboro 60tc amp has same idea. Pritchard amps did same thing.

Doesn't sound great in all amps but works nicely in circuits like TW Rocket, Vox AC30, Fender 5f6a.

Mark

Also used by Mojave Ampworks.

[Cantplay]

I have one of the Frenzel tube pre-amps.

I was a little more than a cheap pedal, and it sounds really nice.

3 12ax7's in the front end, and a jcm800 vioce.

John

[Xplorer]

Wow ! Cool project !

[dave g]

The Guytron GT100 is based on overdriving a pair of push pull EL84s in the preamp. Apparently they have also patented this:

http://www.freepatentsonline.com/6111961.html

I think a lot of the "magic" of the plexi/bassman/AC30/trainwreck sound is the fact that there is a single asymmetrical clipping stage in the preamp followed by the symmetrically clipping power amp section (PI and power tubes). Allow me to illustrate.

The first image is a plot of the fourier transform of the signal on the speaker connected to a marshall plexi output section. But, rather than just feeding a signal generator into the preamp, I bypassed the preamp and fed a clean sine wave straight into the PI. I did these plots a long time ago and don't remember what the amplitude was, but I think it was around a few volts which is about the same you could expect to find on the wiper of the treble pot.

The peaks you see show the amplitudes of all the different harmonics. Notice how the symmetrical clipping of the PI and power tubes results in predominantly odd-order harmonics. The slight imbalance between the two halves of the push-pull power section prevents the even harmonics from being completely cancelled out, but they're still 35 dB down.

The second plot is the fourier transform of the signal taken from the wiper of the treble pot with what I think was a 100mv sine wave fed into the preamp. The only stage clipping here is the 2nd gain stage/cathode follower pair. It's clipping only one side of the wave and so the distortion is predominantly from the 2nd harmonic.

The third plot is the fourier transform of the signal across the speaker, this time with a signal generator at the input of the first preamp stage and the preamp connected to the PI as usual. Notice how all the even harmonics are now filled in, because the 2nd stage/cathode follower is clipping asymmetrically and then the resulting signal is getting clipped symmetrically by the PI/power tubes. If you symmetrically clip a signal that is already asymmetrically clipped, the resulting signal will also be asymmetrically clipped (draw a picture to illustrate for yourself if you don't believe me).

The point is that in the bassman, plexi, ac30, and trainwreck circuits, there is always a single asymmetrically clipping stage in the preamp followed by a symmetrically clipping output section, and I think the plots here demonstrate why that's a good idea. It also means that as you turn the gain up higher and higher, these amps don't fizz out like a JCM800, because the clipping just becomes more and more asymmetrical. In fact, it seems like in all of these amps when you turn the gain up high the lead tone just keeps getting fatter (i.e. more 2nd harmonic from increasingly asymmetrical clipping). As you clip a JCM800 preamp harder, the clipping actually becomes more symmetrical (since in an 800 the 2nd and 3rd stages are clipping opposite sides of the waveform). This is why the 800 fizzes out when the preamp gain is up too high - it sounds great when the 3rd stage is clipping a lot more than the 2nd, but as you turn the gain higher the 2nd stage catches up to the 3rd and you end up getting more and more of a square wave, canceling out the even order harmonics. You will often see the same thing happening in some of the more poorly designed 4-stage high gain preamps (although there are some exceptionally good sounding 4-stage high gain preamps out there) and I believe that this is one of the main reasons why "power tube distortion" has the reputation for sounding better than "preamp distortion", even though the real issue is more complicated than which tube clips the waveform.

Sorry for the long winded rambling post, I'm sick in bed and a little lightheaded. I guess the whole point of this post is to suggest putting an asymmetrical clipping stage before the dummy-PI in your preamp.