Alembic F2-B

[Jarno]

Hello Phil,

Well, if you like point to point wiring, you'll love the way I did the powersupply!
It's somewhat dangerous I suppose, and the "free hand in your pocket" rule is definitely appropriate.
All of these power supply schematics are similar, and I used what I had around, so I used the original schematic mainly for the anode voltage, but I think I got this from the Alembic documentation, not sure anymore it's been a while.
Just use two transformers with the secondaries together, but you being at 240 I think you'll end up quite high (1,4x240=336volt), maybe you could use a 12 volt and 9volt transformer, to lower the voltage a bit.
For the filament voltage I used a bog standard three legged regulator to put out 12 volts, but I lifted that voltage slightly by using a diode on one of it's legs, I ended up with something like 12,6 volts and used two filaments in series. I think, but I'm not sure, you can't do this with all types of valve 12AX7 and ECC83 are different.

I'll make a schematic and a photograph later today, I am at work right now.

Best regards,

Jarno.

[mtyndorf]

Hey Guys, I'm currently in the assembly stage of my own F2-B preamp build, and I found a PSU kit that works very nicely if you're not fully confident working with HV supplies (and if you're not, you shouldn't be.. it can be quite a shocking experience!)

This supply provides enough juice to run six 12A_7 tubes (AT7s, AU7s, or AX7s, etc...) and has selectable 6.3V or 12V heater outputs. Main output is spec'ed in the schematics at 320V @37mA. Comes with all the small components (caps, single diodes, resistors)and PCB, you'd need to get the power transformers (Hammond 229B16 and 229B230 I believe) and a 35mm SIP bridge rectifier.

Cool thing is that everything is low profile and it fits nicely into a 1U project box, but its a bit pricey ($150 for the kit, transformers will run another $15 apiece, I managed to scavenge a BR from an old computer PSU) but all the work is done for you.

ONE THING: I found that the PCB outputs for the heater are not +Vcc and Gnd, they're actually +Vcc and -Vcc. So if you connect the heaters, make sure you use your chassis ground, or else you'll send 24V across the filaments (if you run the heaters in series @ the 12V setting). Another way around this is to use the 6.3V setting, which will give you 12V across both points, if you don't want to run to chassis ground.

http://www.londonpower.com/catalog/p...products_id=62

[Brett]

Originally posted by mtyndorf
This supply provides enough juice to run six 12A_7 tubes (AT7s, AU7s, or AX7s, etc...) and has selectable 6.3V or 12V heater outputs. Main output is spec'ed in the schematics at 320V @37mA.

That will deal with 6 X and probably T, but not U.

[Nordskov]

Oh it's been a long time since I've been here. I thought this tread was long dead.

Sorry, Phil, for not replying to you post.

I'm done with the preamp, but I ran into some difficulties. Actually they are still there


I have a problem with severe feedback in my tonestack.
I did try to disconnect it by make a short between the two stages via a 0.1 uF capacitor.
That did the trick, but I can't find any irregularities with the wiring.
I have been over it a zillion times and everything seems to be connected like in the schematics. No shorts, no nothing.
Does anyone have an idea?

I plugged it into the effect return on my GK1001RB-II, but man... the signal is very weak.
I have to turn the master vol past 12 to get a decent SPL.
Could it be due to a mismatch in input or output impedance? I play a Sandberg bass with preamp.
Would it help if I put some kind of an opamp before the first stage?

[Nordskov]

I did the PSU with two toroid trannies back-to-back. I took the 12V from #1 secondairy and app. 315V from #2 primairy. We run 230V in Denmark.

I haven' installed a bleeder resistor after the electrolytics, - yet.

It fits very fine in a 1 unit box, 23 cm wide and 17 cm deep.

I will post a picture in near future

[Nordskov]

Here are some pics:

The capacitor sticking up on the left is bypassing the tonestack. And so is the unsoldered resistor.

The low voltage supply uses a LM317 regulator (out of image to the right).

The blue ON-led is missing in the righthand socket.

[Jarno]

Looks great, that's a very nice case, love the cute toroids as well. What's the VA rating on those?

I really should finish mine one of these days, too many other projects!

Best regards,

Jarno.

[mtyndorf]

Hey guys, maybe one of you would be able to help me nail down this issue... as far as I can tell, the circuit is wired right, but I'm getting some interesting results here...

Now audio does get from one end of the circuit to the other, tone controls do have an effect as well as volume, however the noise floor is pretty high, but it does something a little weird. I'll hit a note on the bass and it comes through strong, albeit a bit dirty, and as it decays the noise floor will start increasing in volume until there's a steady hum.

I'm thinking its a ground loop somewhere, or maybe AC getting into the plate supply? I'm going to put a larger AC filter cap on B+ (the cap is spec'ed at 22uF/350V, but my supply is putting out 380V due to a hotter-than-normal AC wall output.. around 123 volts). I have a 47uF/400V cap I'm going to try out.

Everything is tied to a common ground (on the AC line itself for testing purposes since its in and out of a metal box often). Should I chassis ground the AC and tie the tube grounds to it on their own, seperate from the signal/tonestack grounds?

It works... but it doesn't. Any thoughts?

Thanks!

[Kramerguy]

could be to much plate voltage for the tube.

Originally posted by mtyndorf
Hey guys, maybe one of you would be able to help me nail down this issue... as far as I can tell, the circuit is wired right, but I'm getting some interesting results here...

Now audio does get from one end of the circuit to the other, tone controls do have an effect as well as volume, however the noise floor is pretty high, but it does something a little weird. I'll hit a note on the bass and it comes through strong, albeit a bit dirty, and as it decays the noise floor will start increasing in volume until there's a steady hum.

I'm thinking its a ground loop somewhere, or maybe AC getting into the plate supply? I'm going to put a larger AC filter cap on B+ (the cap is spec'ed at 22uF/350V, but my supply is putting out 380V due to a hotter-than-normal AC wall output.. around 123 volts). I have a 47uF/400V cap I'm going to try out.

Everything is tied to a common ground (on the AC line itself for testing purposes since its in and out of a metal box often). Should I chassis ground the AC and tie the tube grounds to it on their own, seperate from the signal/tonestack grounds?

It works... but it doesn't. Any thoughts?

Thanks!

[deepthrob]

Don't know the F2 circuit but I remember the Alembic preamps as being a standard Fender front end with a voltage doubler for power. Is that the item ?

I've built a severa; of these and eventualy worked out pcb that I now use. Never any hum problem even with everything out of the box.

Here's a short diy:

http://www7.taosnet.com/f10/fender_pre.htm

Frank