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Author Topic: alembic f-2b suitable for non-bass preamp?  (Read 9871 times)

armstrom

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alembic f-2b suitable for non-bass preamp?
« on: October 25, 2008, 08:08:34 PM »
I ordered a set of Real McTube PCBs from Tonepad and after studying the schematic for the Alembic F2B I noticed that could build either circuit with just a few component changes. The alembic seems to get rave reviews as a bass preamp (and some good reviews for guitar as well). Since the McTube doesn't have a tone control I'll just build a separate board for the tone stack components and tap in to the signal using the pads for the gain control pot.

So, would this preamp be worth while for "regular" electric guitar? I'm not interested in any kinds of effects in the preamp like overdrive or distortion since there are plenty of pedals for that. I just want a nice, warm, clean tone to work with. I'll be running the output through a digital effects board (reverb, rotary speaker, chrous, blah blah...) and adding a master volume control before running the whole shebang into an LM3886 power amp.

So, any thoughts? Is the schematic found here: http://www.freeinfosociety.com/electronics/schemview.php?id=1191 suficient to recreate the much acclaimed tone of the F2B or is there some other black magic "mojo" involved that I'll be missing?

Regards,
-Matt

Jack1962

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #1 on: October 26, 2008, 07:56:08 AM »
I would change that first bypass cap on the cathode of V1 to a 1Uf polorized and the bypass cap on V2 to the same , this will yeild a higher gain(not distortion but gain(will have a slightly higher volume)).

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danV

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #2 on: October 26, 2008, 10:20:16 AM »
It's almost the same as a Fender preamp. The only mods would be replacing the 1st and 2nd cathode caps by 2.2 uF. Basically it's what Jack1962 said (2u2 caps will give a larger frequency response, mostly in the bass section - I'm not sure wether it'll make such a great difference for guitar though).

armstrom

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #3 on: October 26, 2008, 10:31:06 AM »
do you think that's needed? On the mctube design there's a 10:1 voltage divider just before the master volume pot. These tubes produce so much gain you have to bleed most of it off to prevent clipping the power amp.

danV

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #4 on: October 26, 2008, 06:40:42 PM »
I'm not sure wether on the schematic it says 50nF or 50uF (I guess it's 50u, since it's for bass and the requency response must go very low). In any case it'll give a gain of roughly 60-65 for guitar frequencies.

About the voltage divider in the Real McTube, it really depends on what kind of amplification you use after the pre. If it's a tube power amp, large signals are going to be OK. On the other hand, if the power amp is solid state, or you use the circuit as a pedal (like the Real McTube), you have got to limit the voltage swing to prevent clipping.

If I remember correctly (I calculated it some months ago), voltage gain for tubes without the bypass caps are around 20, and you may still need the voltage divider network (maybe with another ratio). Sound was also a bit different. I'll check if I can still find the calculations, computer simulations and samples.

armstrom

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #5 on: October 26, 2008, 09:48:22 PM »
Thanks for all the info. I plan to verify the gain with an oscilloscope before I hook it up to my power amp. I l'll probably incorporate some clipping diodes on the output to cap the maximum voltage swing and avoid damaging my power amp. If I do it right I might even get a nice soft clipping sound out of it when cranked. Probably use germanium diodes, I hear they have nice soft clipping characteristics.
 

Jack1962

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #6 on: October 27, 2008, 05:11:43 AM »
I'm not sure wether on the schematic it says 50nF or 50uF (I guess it's 50u, since it's for bass and the requency response must go very low). In any case it'll give a gain of roughly 60-65 for guitar frequencies.

About the voltage divider in the Real McTube, it really depends on what kind of amplification you use after the pre. If it's a tube power amp, large signals are going to be OK. On the other hand, if the power amp is solid state, or you use the circuit as a pedal (like the Real McTube), you have got to limit the voltage swing to prevent clipping.

The gain of the first stage is 62.5 , if you add clipping diodes it will clip all the time. the type of driver and power stage (solid state or tube) is not important , if you place a cap between stages and and a dropping resistor( 47k or so) in the circuit this will drop the voltage (if needed) to your driver stage.

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If I remember correctly (I calculated it some months ago), voltage gain for tubes without the bypass caps are around 20, and you may still need the voltage divider network (maybe with another ratio). Sound was also a bit different. I'll check if I can still find the calculations, computer simulations and samples.

armstrom

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #7 on: October 27, 2008, 01:15:28 PM »
I'm not planning to just use a clipping circuit to control the voltage, as you say, this will clip constantly with such a high-gain preamp. My plan is to use a simple voltage divider after the preamp (similar to the McTube circuit) as the primary method to bring the output voltage "in line" with what the power amp wants to see. Then, I will add a second line of protection against spikes with a series of clipping diodes. This should, in theory, allow the circuit to operate normally under 90% of circumstances, but if the user gets overzealous with the volume, feeds a higher voltage input signal to the preamp (active pickups, pedals, whatever) then the diodes will step in to clip the higher voltages rather than fry my Reverb DSP and/or power amp chip.


Jack1962

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Re: alembic f-2b suitable for non-bass preamp?
« Reply #8 on: October 27, 2008, 04:25:40 PM »
well Dan I'm not using a simulator , I using a paper and pencil and over 20 years of working with tubes.

 Sounds like a plan armstrom, lets us know how it works out.

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