Alembic F-2B mods?

[SpufuZ]

Im planning on building a Alembic F-2B preamp and mod it for guitar.
Ive read that David Gilmour modded his F-2B with an extra tube for more gain, lower output impendance and changed the cap in lower end to cut som bass.

Would someone like to help me figure out how to do these mods?

Heres a links to the schematic:
http://moosapotamus.net/IDEAS/F2B/alembic.htm


//David

[Jarno]

I think he added the extra tube not for gain, but just for lower output impedance (cathode follower), but I'm no expert. If you want more gain, just add another one of the two existing stages in front of the first one, with a volume control.
I'm a bassplayer, and I actually added a pad in front, and lowered the gain of the second tube stage, because it had too much gain for my active basses.
You can cut lows to make the overdriven sound less muddy, you do this by lowering the value of the electrolytics on the cathodes. This will reduce the gain on lower frequencies.

[SpufuZ]

How would the circuit look like if i want to add the first stage in to the second one?

Why would i want to add another tube to lower the output impendance and how do i add it in the circuit?

//David

[Jarno]

Like this?


There's no necessity to add a cathode follower to this if you have the power amplifier close to the preamp.

[alfafalfa]

That could do it and else add this stage  wired up as a cathode follower:   http://www.tubecad.com/2007/04/blog0104.htm

The signal should be taken from the cathode for low impedance output.

Read the article.

[POGFROGULPOP]

Hi, I'm also interested in a 'Gilmouresque' Alembic F2B Preamp. The addition of a Super Symetrical Cathode Follower looks like the go, on the www.tubecad.com link. It doesn't show the value of Rk (Cathode Resistor). Any ideas as to what that value would be?

[Jarno]

Actually, a simple cathode follower will do the trick, nothing super symmetrical about that one.
Sadly though, a ECC83 is not a good choice for a cathode follower (and you have one half left from the extra booster stage in front), ECC82 is better. In a preamp I'm building I'm using a JJ ECC832, which has one half of a ECC83 and one half of a ECC82, using the latter for the output stage and the first one for the booster stage in front of the standard F2b schematic.

[POGFROGULPOP]

After reading some of the interviews from Gilmour's guitar tech., Pete Cornish, he isn't specific about the exact method he uses. I guess without his input, we would all be guessing to some degree to how he goes about it, and what components to use. Feel free to correct anything I believe at this particular point in time, but I think the signal goes from his preamp output, to his volume pedal and from there it goes off, splits and goes into his 3 delays. So would this mean, in effect, the 3 delays are in parallel, with a volume pedal in series? I guess he would match the output impedance of the cathode follower stage to drive this chain. Unless your rich like Dave, and feel the need to be a 'Gilmour' Purist to have such a exotic rig, you might feel the necessity to create the cathode follower, low impedance circuit. Otherwise, if your on a budget, and want to emulate his tone by spending your bucks on his Effect Pedals used pre- input Alembic, and instead, just run straight from the output of the Alembic to say 1 Delay Unit, or just connect straight into the Poweramp input, like a normal Alembic using Bass Player would do. What would be the point of changing the standard Alembic design by incorporating a cathode follower in this case? Anyone care to comment?

[Jarno]

Like I said earlier, if you have a single amplifier hooked up to the (relatively high impedance) output, on a short cable, you will have no problems.
If however, you want to hook up several things on the output, and/or the equipment is more spread out over the stage, you can add a cathode follower. There's nothing esoteric about it, and no matching is needed, just build one like in all the old vacuum tube manuals, and you're golden, it's about 6 parts, including the tube. So no big bucks are needed. If you look at the other stuff Pete Cornish has been doing, his primary aim is to make things robust, and he uses tried and true methods (sometimes a bit overkill and heavy, which can lead to other problems, if you ask me), the addition of a low impedance output is no different.

[POGFROGULPOP]

Yeah I agree about your ecc82 idea. If you check this valve wizard link, http://www.freewebs.com/valvewizard/accf.html
it shows about 6 components. Would be really interested to hear back from you on completion of your Alembic project, to get some idea of success with the particular components you ended up choosing. How far away are you from completion?

[R.G.]

I guess he would match the output impedance of the cathode follower stage to drive this chain.

More accurately, he uses the low(er) output impedance of the cathode follower to drive the capacitive loading of the cables in this relatively complicated setup.

There's nothing magic about the cathode follower, nor the "Aikido" follower, especially since it's claim to fame is that the distortions of the top and bottom tubes are supposed to cancel.

A good way to get this same effect (i.e. driving cables without losing treble) is to use either an opamp follower or if you want to use the same power supplies, etc. already in the tube circuit, a MOSFET follower. See http://geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm, MOSFET Follies at geofex for how to do this.

[Jarno]

I've already made one, single channel, without CF, and it sounds fantastic on bassguitar (which is what I use it for). It's got a DC regulated heater supply, and 3x450uF of caps on the B+ (because I have a truckload of those caps, it is waaaaaayyy overkill). I ended up with too much gain for my basses, so I added a pad in front, to lower the signal level, and I've also put a 220k resistor in parallel to the 100k of the second stage, lowering the gain.
The trick is to use a really good tube, in my opinion, the lower the parts count, the more you hear the difference between one part or the other, especially the tubes. I use a Tung Sol ECC803s, and I really really like that tube, haven't tried it on guitaramps where it's overdriven, and it may very well be too "bassy" for this, but for a clean preamp, look no further.

The preamp I'm building now is for a friend of mine, it'll be two channels, and it will also have some extensions to the tone stack (shift and presence). One of the channels will have an extra tube stage in front, to get some overdrive. I think it'll be ready in a month or so, I have the design pretty much figured out (not that there is much to figure out, it's pretty much all been done before), it's now down to getting all the parts in (heck of a job).

I would really like to try some of the suggestions of RG in mosfetfollies, adding a fet stage somewhere is easy enough.

[SpufuZ]

Sounds like i wont need the cathode follower then. Output from F-2B will go directly to G-major and continue to power amp. Short cables.

Thank you Jarno for the schematic!

But what about the power supply?
Ive seen different supplies where both AC and DC are used both for the 300V supply as well as the 6.3V for the heaters.

This one uses DC.
http://www.moosapotamus.net/IDEAS/F2B/alembic.htm

Would it make any difference what i use? AC or DC?

[Jarno]

AC heaters = possible hum due to layout
DC heaters = no hum, slightly more complex/ more parts

I'd go for DC, it's not THAT hard to do, and although I usually put in a regulator, you'll be fine without.
B+ regulation is something I would like to try, I already bought high voltage regulators to this end. But like I mentioned before, I have a lot of BFC's I scavenged out of MRI equipment, so up until now I just used those.

[Lorcan]

I actually built an F2B with standard size caps (47uF if my memory is correct) and standard AC supply, I get no hum or buzz whatsoever. The trick is to make sure you twist the AC heater cables properly and avoid coupling by running any signal wires parallel to AC cables. I read in a few places that DC heater supplies can lead to a shorter life-span of tubes, I'm not quite sure about the explanation behind this, but in the end, given AC is cheaper and simpler to do and working with no drawbacks I decided DC was overkill.
A problem I did get was radio pick-up, but I solved this by putting an output volume control pot (50k I think) which seems to give a sufficiently low output load to the preamp, and the interference was drastically reduced.

[SpufuZ]

I actually built an F2B with standard size caps (47uF if my memory is correct) and standard AC supply, I get no hum or buzz whatsoever. The trick is to make sure you twist the AC heater cables properly and avoid coupling by running any signal wires parallel to AC cables. I read in a few places that DC heater supplies can lead to a shorter life-span of tubes, I'm not quite sure about the explanation behind this, but in the end, given AC is cheaper and simpler to do and working with no drawbacks I decided DC was overkill.
A problem I did get was radio pick-up, but I solved this by putting an output volume control pot (50k I think) which seems to give a sufficiently low output load to the preamp, and the interference was drastically reduced.

Did you use your for guitar or for bass?
Feels like AC supply is ok then, after all its closer to the original circuit.
But would it make any difference to the tone itself?

[POGFROGULPOP]

It looks like it's a Bass rig only, standard Alembic clone. Saw the pictures of Lorcan's Alembic build on his website, and I must say the Aluminium faceplate, and general layout looks sexy. Also easier to drill to mount the panel pots, compared to steel. Thought you would've used shielded cables earthed at the star point to avoid RF interference, rather than twist wires? Gilmour definitely uses an extra stage for gain or 'bite' as Cornish quotes. I guess due to the fact the F2B is stereo, would you run the output of 1 channel into the input of the channel 2, (only using the first ecc83 triode of the 2nd stage for gain)? Then by using a ecc832, use the 2nd ecc82 triode for the cathode follower mod? Tempted to buy a 2nd hand Alembic to do this, although building from scratch would be a rewarding achievement in itself. As Lorcan states, the hardest part is getting the materials sourced from various vendors and pcb fabrication (which I'm still to learn about). The assembly itself would be, in comparison, more of a straight forward process. In my particular case, I'm looking at running a wet/ dry rig. So split off from the preamp output, into the wet 'delay'  effect input, and the other into the dry power amp input. Hence cathode follower is probably required. Anybody know of a worthy 'Dummies Guide' Book to DYI PCB manufacture? We all started somewhere!

[Jarno]

But would it make any difference to the tone itself?

Probably not.

[Jarno]

Tempted to buy a 2nd hand Alembic to do this

Well, they don't exactly go for peanuts this side of the ocean! I'd say build your own. Especially since, IF you find a comparably cheap one, it'll need new caps.
I think I saw some pictures on the alembic forum of a guy who did modify his alembic preamps (think he had the powersupply for his bass, a F2b and a F1x) put shielding inside, not sure if he changed the circuits at all.

[Lorcan]

Did you use your for guitar or for bass?

I use it mainly for bass, but I also plug my Strat into it from time to time, using as a DI for quick takes, because I can't be bothered to switch speakers when changing instrument.
It gives a nice Fender tone, great for cleans, as you would expect since the Alembic is mostly a Fender preamp clone