Alembic F2b preamp clone

[Lorcan]

Hi,

I made myself a clone of the Alembic F2b bass preamp, which is itself mostly a clone of a Fender front-end.
I used schematics and info from these sources as a reference:

• http://www.moosapotamus.net/IDEAS/F2B/alembic.htm
• http://www.frontiernet.net/~jff/af2b/Dual_Channel_Blackface-style_Preamp_for_Bass_Guitar.html
• http://music-electronics-forum.com/t2442/

I then designed the pcbs to fit all of this into a 1U rack case and accomodate for the components i used.
I used a transformer from tube-town.de which is originally designed as a replacement for an Engl 520 preamp, and it works great (http://www.tube-town.net/ttstore/product_info.php/info/p782_Torodial-27VA-Preamp---Engl-520.html).

The sound is really great and alive, and the controls are very effective. It works wonders on guitar too, giving the nice Fender treble sheen.
For good signal-to-noise you should take extra care with the wiring, applying the usual recipies: star grounding, short cables to the pots, shielded wire for the signal path, twist the AC heater wires, and not having psu wires parallel to signal wires.
Also i found out the output was high and adding a volume pot to my power amp lowered the noise a lot.

So here are a few pictures of the little beast:


One channel with the 12ax7 socket fitted on a right-angled pcb to fit inside 1U case

Simple psu design (no voltage doubler)


The preamp coupled with my DIY mosfet 2x70W power amp, fedding an Ampeg 15" cabinet - rumble in the jungle neighbor's ceiling



The layouts for one channel


The layout for the HV PSU


And finally, an archive of hires pdfs for the channel and psu (schematic + pcbs)
http://lorcan.me/guitar/alembic/Alembic F2b LMD'09.zip

Hope you enjoy it,
Lorcan

[Ripthorn]

What does the pot epoxied to the inside of the case before the speaker out do?

[Lorcan]

What does the pot epoxied to the inside of the case before the speaker out do?

It's a dirty temporary hack for a volume control until I fit one on my power amp Just blue (well yellow) tack, not epoxy though

[Jarno]

Looks great! The pcb's are great as well, I recently built one as well, but doesn't look as tidy as yours!
Ahem:

It's only a single channel. The big caps are 450uF for the B+ (soldered them of industrial equipment), the heater of the single tube is 12v DC regulated with a 317.
I added a input attenuator as the gain is way higher than I would like it to be (I only have active basses), to this end I also lessened the gain of the second stage it's got 70k on the anode instead of 100k (I think?).
Also, there's a switchable bypass cap on the first cathode (Mesa Boogie calls this "bass shift") and switchable cap for the treble control ("treble shift").
At the moment, I have a Tung Sol ECC803S in there, that sounds wonderful.

[Taylor]

Very nice! I applaud your rig. How do you use the 2 channels? Bass guitar/doublebass, or....?

[Lorcan]

Looks great! The pcb's are great as well, I recently built one as well, but doesn't look as tidy as yours!
Ahem:
It's only a single channel. The big caps are 450uF for the B+ (soldered them of industrial equipment), the heater of the single tube is 12v DC regulated with a 317.
I added a input attenuator as the gain is way higher than I would like it to be (I only have active basses), to this end I also lessened the gain of the second stage it's got 70k on the anode instead of 100k (I think?).
Also, there's a switchable bypass cap on the first cathode (Mesa Boogie calls this "bass shift") and switchable cap for the treble control ("treble shift").
At the moment, I have a Tung Sol ECC803S in there, that sounds wonderful.

Thanks, yours looks great too. More vintage with the point-to-point wiring  
In fact I don't really need the 2 channels, but the idea is you can use the first to overdrive the second and also get even more tone control.
I wondered about regulating or not the heaters, but it seemed simpler to find a transformer with windings for HV and 6,3v ac. I also read DC is not necessarily better for heaters, which seems a little counter-intuitive. I think I read the tubes might last longer with AC too ...
What's the noise floor like on yours ? On mine it's very satifactory, much better than the solid-state Peavey I had before.

On mine I've also got a 'Brite' switch that emphasizes lows and highs (also called treble-shift I think) and a treble cutoff switch.
What do the 2 other switches do on yours ?

Cheers,
Lorcan

[Lorcan]

Very nice! I applaud your rig. How do you use the 2 channels? Bass guitar/doublebass, or....?

Atm I mostly use one channel, but in the future I could switch between the two with a footswitch, using them as 2 presets, or chain them to get more 'grit' 
I haven't played bass live yet, only at home, I'm more of a guitarist really 

[andrew_k]

VERY nice build 

I built one (single channel) a couple weeks back, but mine's far more ghetto than the two in this thread -



External PSU w/ 3 power transformers (2 in series through quadrupler for HT), choke, and regulated DC heater supply


As you've both noticed, there is WAY too much gain in this preamp. ~60dB. The following mods were suggested on another forum to reduce the gain:


I haven't done it to mine yet as I've been too busy, but I plan on making it switchable with a 3PDT.

[newperson]

What type of AL are you using for the panels?  6061 .1?

[Lorcan]

Thanks Andrew !
I like yours too, making the best of what's at hand is a very good skill to have and requires more ingeniosity than online ordering hehe
Should there be a nuclear blast sometime in the future, we won't be left without rock'n'roll and amps that go to 11' 
I'm going to try the mods you show in the schemo. It might further reduce the noise too, which is a always a good thing to have.
But I think I'll leave the first channel as is, just for the sake of overdriving the hell out of the second if I want to 

[Jarno]

As you've both noticed, there is WAY too much gain in this preamp. ~60dB. The following mods were suggested on another forum to reduce the gain:


I haven't done it to mine yet as I've been too busy, but I plan on making it switchable with a 3PDT.

Hmm., negative feedback is also a way to reduce gain, I would be worried about the tone consequences though, it might change it a bit (not necessarily a bad thing). I just tacked a 200k resistor over the second 100k plate resistor.
That's a nice box for the preamp, I had mine in another box which was also really small and with an external powersupply, but decided it was too much work to make a proper interconnect between the two.

By the way, I think the spec for ECC83 explicitly says you can feed it DC, so that should be alright. I just rectified the 12,6v and then regulated it.
The noisefloor on my unit is really really low, but several sources mention AC heaters being a cause for some noise, and mine are DC. Also, the insane B+ supply helps, 3x 450uF is waaaayyyy overkill. The other thing is, I bought a toroid on eBay and it had 270V secondaries, which is way too high if you want to end up with 300v. So the series resistors are also quite high, further lowering the ripple.
My switches are, from left to right, "-10dB pad", "bass shift", "treble shift", "bright".

regards,

Jarno.

Edit:
A sideeffect of the big B+ caps is that I really need to put in hefty fuses or they'll blow, despite the fact that I have also included a NTC on the primaries, what fuse rating are you using?

[Jarno]

By the way, does anybody know of a layout for a simple high voltage regulator to use with tube preamps? Something with a pass transistor and a string of zeners or something. Or a regulator lifted by a string of zeners.
Sort of a general purpose tube preamp powersupply?

[Ripthorn]

You could use zeners (if oriented with anode to ground, they should shunt anything over their rated voltage) or they do make some very high voltage regulators (up to 400V or so), but just beware of power and current ratings.  They aren't too expensive, either.  I think less than $2.

[Lorcan]

By the way, does anybody know of a layout for a simple high voltage regulator to use with tube preamps? Something with a pass transistor and a string of zeners or something. Or a regulator lifted by a string of zeners.
Sort of a general purpose tube preamp powersupply?

I've seen things such as this kit which use a HV-specced transistor to regulate the output.
Zeners alone would be difficult to find because of the power ratings I think.
But IMO this would only make sense with a HiFi or mic preamp, because anyway the greatest source of hum int the path would be the bass/guitar pickups, except maybe if you have a special pickup (Fender 'noiseless' or humbucker).

Regarding the fuse, be sure to use the 'T' version (temporized) or slo-blo to allow for the capacitor initial current-inrush, but I guess you've done that already.

[Belt]

Great building!

I would like to try an amp project in time.

very inspiring guys

[Jarno]

That's a helpful link, only a handful of components as well. I agree that pickups are noisy, but since most of my basses are active and equipped with humbuckers, they aren't that noisy. Also, when you are in a low-nnoise environment like a studio, a low-noise preamp helps.
And, I am thinking of experimenting with small signal pentodes, which are also a bit more susceptible to power supply noise.

I saw your remark about you being a gitarist, Lorcan, you know that Dave Gilmour used (or is still using) a (heavily modified) F2b?

Regards,

Jarno.

[Lorcan]

That's a helpful link, only a handful of components as well. I agree that pickups are noisy, but since most of my basses are active and equipped with humbuckers, they aren't that noisy. Also, when you are in a low-nnoise environment like a studio, a low-noise preamp helps.
And, I am thinking of experimenting with small signal pentodes, which are also a bit more susceptible to power supply noise.

I saw your remark about you being a gitarist, Lorcan, you know that Dave Gilmour used (or is still using) a (heavily modified) F2b?

Regards,

Jarno.

You may want to look at the schematics here for inspiration too. I suspect it can be quite dangerous to experiment with such circuits though, given the voltages involved.
I use my bass in a (home)studio too, so I'm quite picky about noise, but I've also learned to live with it and treat it with a gate after recording if I need to.
With an overdriven guitar amp you have no choice anyway 
I had seen that Gilmour used an f2b live, yes. He drives HiWatts with it too. I drive a 15w Peavey tube amp i got for 50€ 
Whatever works is fine for me ...

[Lorcan]

Sorry for the missing files, they're back up at my site
http://www.lorcan.me/content/alembic-f2b-dual-channel-bass-preamp-clone

[michal_k]

Hi,
I can't find a zip archive on your website. would you mind sharing the link? sorry if it's located in some obvious place and I missed it 

[Lorcan]

Acch ... you're right, sorry, it's Drupal that does strange things sometimes and I must admit I haven't mastered 5% of its features
I've fixed it now