alembic f2b

[stretch2011]

I just built one, well one channel of it. I'll post pictures later. I must say this thing sounds good. Incredible amounts of gain. I actually had to put a 12ay7 in it because it had just too much balls.

I'm thinking about putting in some relay switching for foot pedal operations. : ))

[stretch2011]

As promised

[Colossal]

Very cool!

Did you build it to the schematic floating around? It shows a B+ of 300VDC to each of the plates.

[stretch2011]

The schematic I used didn't have any voltages on it. Its running around 210v on the plates after the 100k plate load resistors.

I used a replacement reverb transformer. 250k on all the pots except for the volume which is a 1m. I thought that 250k for a mid pot was a little strange. Then I found the original owners manual schematic and it should've been a 10k.

I just tested this in the passive loop of my 28 special (gainier 5f6a head) through a EV Tl8 whatever cab with a wgs et65. My thoughts so far:

With the 250k pot instead of the 10k you can get a huge volume boost. Its pointless to have the volume turned up past 12 o clock. Just too much gain after that.

Problems:

I'm getting some high frequency feedback that is directly proportional to the treble and volume pot. Anything past 12 o clock on the volume and it starts, then the treble must be turned down proportionally to the volume to cut it out.

[Colossal]

Ok, that makes sense why you were saying "too much gain". You are stacking it in series in a loop. I was confused by that, as by itself, it is just a Fender preamp. Yes, with 250k as the mid pot, that is basically lifting the tone stack from ground so, yes, a gain boost.

[stretch2011]

Even when plugging into the front end of an amp like a tweed Princeton it can get too hairy.

That's good to know!

[JMFahey]

Of course, you are going through TWO cascaded full preamps
The original Alembic preamp was designed to drive a rack power amp directly; in that case it has the same gain as a regular Fender channel preamp, it is not an "effects pedal".

Try plugging into it directly, and sending its output to the loop return, NOT driving it with the much louder Loop send.

Congratulations on the build, very neat

[stretch2011]

Ah I see! Very interesting.

Thank you!

[JazzGuitarGimp]

Of course, you are going through TWO cascaded full preamps
The original Alembic preamp was designed to drive a rack power amp directly; in that case it has the same gain as a regular Fender channel preamp, it is not an "effects pedal".

Try plugging into it directly, and sending its output to the loop return, NOT driving it with the much louder Loop send.

Congratulations on the build, very neat

Of course if you were to add a master volume at the end of the preamp, you would be able to use it in the EFX loop - just turn the master way down.

[stretch2011]

This brings up a question of mine.

A volume control controls the amount of voltage the power tubes receive in an amplifier. Now when I add the alembic in which boosts the signal (voltage) wouldn't I be able to drive the power tubes with the volume on the amp turned down and the volume on the alembic turned up?

I want to say no. Because its kind of the same as boosting the signal with a pedal in front of the amp.

[JazzGuitarGimp]

If the volume control on the amp is before the EFX Send, and you have the Alembic in the EFX Loop, then yes. Start with the amp's volume all the way down and the F2B's volume all the way up, then use the amp's volume to control overall volume. But two things: 1. You won't be able to turn the amp's volume up but just a tiny bit due to the additional voltage gain of the F2B, and 2. You'll probably get a lot of hiss doing it this way.

This probably goes without saying, but the amp's volume control controls the amount of signal voltage to ALL stages the follow it, not just the power tubes.

[stretch2011]

I thought so, that makes sense.

With the amp at half volume and the alembic at half volume it doesn't hum really, but anything more than that on the alembic and the high frequency squeal starts.

Honestly it is pretty Damn quiet.