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Old 16th April 2012, 05:39 PM   #71
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think i may be able to get hold on a few of these quite easily and cheap, i have got a spare 12AU7 PP amp i could build this in, but i need a new output transformer for it, its getting old and considering it just appeared one day its done a good job, though my other thinking was using an EL84 or 12k5 as a preamp tube, ive done some thinking and looking but ive just never got around to it, 12k5 is specifically designed as a 12v tube, and people have said it distorts like an EL84. Im swamped in school work at the moment though so if anyone has the ability to do this then il mroe than happily share some ideas.
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Old 16th April 2012, 05:40 PM   #72
shanx is offline shanx  Canada
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Hi Razorrick! Yes a new thread is in order for that. If we don't call it a ''pedal''..
calling it a preamp or micro tube amp head it is possibly more interesting for all! As Simon mentioned, might as well add the power output with that amount of work. Can make it small enough to fit in a Hammond 1590DD size case even better! 2 to 4 W maybe? Could even have a Mosfet as power output, or use a pre-reverb transformer..Just kicking some ideas around.
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Old 16th April 2012, 05:48 PM   #73
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I had an Alembic preamp, and with both channels in series it would make any 'fenderish' distortion tone I wanted, but twiddling the knobs was never convenient. I totally seperated the 2 channel outputs, turning it into two complete fender preamp channels, all single-ended. In the first stage, I modified the input resistor network so the guitar pickup went pretty directly to the first 12AX7 grid. Next it burned off the gain in "tone controls" then recovered the gain...with the volume knob very low the "bright" switch is just a cap across the volume so it ends up with a lot of extreme treble boost (and I added a switch to vary the "bright" cap value). Then this treble-boosted signal overdrives the first stage of the 2nd channel, creating most of the distortion. Then the 2nd stage's tone controls roll off that extreme treble.

One intent is of a 'tone curve' around your distortion...kind of like the RIAA tone curve is arond the record and playback process. When the objective is to control the tone of the distortion independently of the main signal, attempts to run the distortion thru a seperate EQ and then mix in a percentage just doesn't work if the distortion really modifies the main signal. But if you want seperate control of the tone of the distortion, you can achieve that by putting the main signal thru pre-emphasis before the distortion is added, then the distortion and main go thru the final de-emphasis filtering. The main goes thru both, but the distortion only goe thru the final filtering. The result is what we used to call a creamy smooth distortion that has its irritating treble and extreme-order harmonics de-emphasized while retaining plenty of treble for main signal intelligibility. Boost treble, add distortion, cut treble = mian guitar tone is boosted & cut and sounds natural, distortion only had its treble cut.

Then I started listening more to some good blues players, Roy Buchanan, that sort of thing. These guys are often playing major against minor in question/answer suspense/resolution and also often playing clean against dirty. When they pick hard (and I mean extreme attack with a HARD pick) it doesn't get louder, it gets BITE. Like a saxophone or a vocal tract distorts when over-driven. Like a tube does pretty naturally, and compressed some like when a tube supply sags.

So I'm running this Alembic preamp like 2 fender channels in series, and a 2nd volume control on the output of each channel. So I have 5 volume controls in this gain line (including one on the guitar). I always run the guitar full-volume into the first tube stage, so that volume control on the guitar is only for 'swell' effects when I turn it while picking. I pretty much always have the first channel's volume control low and the bright switch on because that makes the treble boost. I always run with extreme treble boost at the start, because I can always reduce the treble later. For a really nice clean bright sound, I just run the 2nd channel more flat, all tubes running in their linear range. Turn up the later volume controls and maybe turn down the last one, and get some intense tube distortion.

So...too late to make a long story short...I got a dual-pot Ernie Ball footpedal, and replaced the dual-audio taper pot with a dual-linear and added resistors to make the taper more like an audio-taper, except I made one pot 'reverse taper'. The idea was to make a footpedal that, when pushed forward, reduces the volume of a later stage (think master volume) but increases the volume of an earlier stage. Plenty of intial trim, range, and taper pots to adjust the pedal operation. Then I had a second more normal volume pedal at the output (actually later after the power amp buffer). So the first "distortion" pedal didn't change volume much, it changed how hard you have to pick to get the tone to 'cross the line' from smooth to dirty. This is for highly emotive playing with exceptional 'feel'...and instant ability to match to any guitar, strings, pickups, and pick.

Of course, this is all far divorced from stomp-box distortion devices, which are kind of the opposite. They tend to use a lot more compression with a different intent. Here their objective is a little like a computer's 'spell-check' but more like "pick check"...it makes each note the same volume and same amount of distortion, regardless of whether it's a trill, hammer-on, pull-down, fingerboard tap, etc. It corrects every attack, making them all the same. Less emotive perhaps, but easier to play those Van Halen or metal tricks, and incredibly easy to get weird harmonics if you use 2 pickups out of phase (emphasizing what's different between the two pickups).

Now I have two small tube stomp-boxes, each of which is fundamentally a Fender channel, and I'm in the process of wiring them in series and installing similar pedals. They originally came with a small output transformer and speaker as an inductive dummy load!

I use the Ernie Ball string-type pedals.

I guess my point is that it takes 4 tube stages to make a nice preamp IMHO; what usually makes 2 channels in parallel works much better in series. Then it will do nice clean or dirty.

Just my opinion. Oh, BTW, know what Jerry Garcia used? Alembic preamp with both channels in series...

Last edited by cyclecamper; 16th April 2012 at 06:00 PM.
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Old 16th April 2012, 05:48 PM   #74
shanx is offline shanx  Canada
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OK Razzorick, when you get some available time, let me know and we could get a thread going.
I need to get a few other things outa the way first as well.
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Old 16th April 2012, 06:02 PM   #75
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Of course, adding a tube box to a tube amp gives you those two channels in series. The question then becomes what to put where.

The first tube or FET probalby belongs in the guitar. Maybe phantom-power with a voltage doubler?
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Old 16th April 2012, 06:04 PM   #76
shanx is offline shanx  Canada
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Quote:
The idea was to make a footpedal that, when pushed forward, reduces the volume of a later stage (think master volume) but increases the volume of an earlier stage. Then I had a more normal volume pedal at the output. So the first "distortion" pedal didn't change volume much, it changed how hard you have to pick to get the tone to 'cross the line' from smooth to dirty. This is for highly emotive playing with exceptional 'feel'...
Cyclecamper, that is a cool solution to the setup. That is about what I experienced when playing with these circuits controls. The bias control in this soft clip changes the '' crossing line'', and interacts very closely with the gain drive. At mid point on the bias there is practically no distortion from that section. As you move the bias away you can hit mild distortion, but you have to play the notes harder to get it..it only bites when you play hard..if you soften up the playing it cleans up. But then you can add more gain and you don't have to play it as hard to get that same bite.
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Old 19th April 2012, 02:23 PM   #77
shanx is offline shanx  Canada
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Quote:
Originally Posted by cyclecamper View Post
Of course, adding a tube box to a tube amp gives you those two channels in series. The question then becomes what to put where.

The first tube or FET probalby belongs in the guitar. Maybe phantom-power with a voltage doubler?
Hi Cyclecamper, I guess this starts to become a question of personal taste as to where and how much clipping happens. The reason I built up this soft clipping pedal with a few options was to experiment a bit with the sequence and type of overdrive that could be created. The asymetric clipping is adjustable on the op-amp NFB, and plugged into the input of my tube amp I just give a little bit of either positive or negative clipping and tend to dial up the gain. That setting sounds IMHO more natural with the tube amp,and keeps a lot of headroom. For a solid state amp (on the clean channel) I could bring in more of that soft clip, and/or post clip to get some more drive.
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Old 22nd April 2012, 02:28 PM   #78
shanx is offline shanx  Canada
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Default Tube version softclip

Hi Razorrick, I think that through our discussions and your idea about doing this in a dual diode tube version is brewing up some ideas , or is it the beer(?!) talking..
So I have been pondering the setup of a dual triode,2 stage with the dual tube diode in between. There could be a control to bias the diode clipping points to vary asymetry, adjustable filter, gain/drive on first stage, and adjustable output volume. That would follow the same layout as the original SS version of the soft clip.
You were thinking about using a low voltage triode version, so I may try this with a 12AX7/7025, run off a regulated external 12VDC or perhaps AC supply. I will probably build up in a slightly bigger enclosure at least for ease of prototyping. This setup could be run as a <<hot channel preamp>> for a single channel tube amp or driving an SS amp.

It would be interesting to compare how setups sound:the low voltage triode, pentode and higher voltage triodes.

So let me know if/when you want to start the new thread..will get some preliminary ideas sketched out
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Old 22nd April 2012, 09:07 PM   #79
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i think a 12au7, or better yet a 12u7 would be better for 12 volts. if the diodes were used in a typical shunt to ground configuration, a 10k pot with each side on anode of one, cathode of the other and wiper from a 470k "headroom" pot. might be quite good, with NFB gain control on the second stage using a 2.2M log pot. a starting point maybe?
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Old 24th April 2012, 12:35 PM   #80
shanx is offline shanx  Canada
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Yes for a 12 volt supply I would go with those tube options. Good starting point definitely.
There are a few things I was thinking of, too. Diode anode on grid, cathode tied to variable pot to cathode bypass network of triode stage, then other diode shunting off of output of that stage/grid of second stage..well lots of different things to try anyways!
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