Alembic F2 not so clone

[skyboltone]

I'm going to build an Alembic style stereo rack mount pre with a clean side D #124 pre on one side (simplified, no rock/jazz, midboost or deep) and an AB-763 on the other side. I'll include D style buffered loops in both sides or maybe just one so that the outputs are summed before entering the loop.

How about a good old style arguement on the best way to control output voltage.

My intention is to use this pre with various power amps including HIWATT, Fender, and Marshall styles. Maybe even a SS for pedal steelers. In other words, if my power amp uses 6BQ5s it will require less (but linear) output than KT-88s or even maybe some 807s. I got a raft of those and what's his name who built Standels used them. Some old pedal steel guys really swear by them. I've used them in modulators for AM transmitters and found them less than ideal. Flabby bass compared to EL-34 for instance, but that was with regulated screens.

I'm thinking just a D style pre PI master volume where signal is just dumped to ground or a Dana UR-12 variable voltage setup. The PRE PI master volume can get stinky without something (lightbulb?) to dump it into?

Thoughts please

I've moved more or less permanently to the desert and will be putting my boat up for sale. My shop is in the building stages and everything is half fast right now. I don't anticipate soldering for a month or more.

The other project this summer will be an LA-2A. We'll talk about that later.

Dan

[M Fowler]

Here is some info I found when trying to get info for my own preamp.

Mark

[roberto]

250k mid pot is logaritmic, otherwise you can use a 10k standard fender pot. I've found a pcb for this alembic somewhere, hope to find it again.
I'd another eq on the other section.

[M Fowler]

I have the PCB for this but the original poster said he was going to go his own way non original on this build so the PCB was posted.

[ER]

Take a look at the old Peavey "raxx" tube pre amp, they have a ~1 volt out for SS, and a ~3 volt out for tube. Not sure of the top of my head how they did it, I think it was a resistor network on the output. I have one for bass (T.B.), but it's the same for guitar. Works well for using it into a SS power amp when playing bass, and then can use it with a tube power amp for guitar.

If I remember correctly the alembic stuff is based on the dual showman. I've thought seriously about converting the peavey pre to a #124, but I'm not sure if it would sound good for bass, and as they say "if it ain't broke"...

-E

[skyboltone]

I've seen the scheme and board info on the alembic clones. One guy uses a pair of 117-0-117/12-0-12 toroids back to back for a 234V secondary into a full wave rectifier. Kinda clever way of getting everything to fit in the 2U chassis.

The original Alembic is a pair of Showman preamps.

Gilmour used an Alembic for many years; maybe still, and a whole bunch of HIWATTs with the pre's ripped out. But from the sound of things his amp guys leave the phase inverter in place in the HIWATT. That is, after all, the only really unique thing about the HIWATT anyway. A #124 pre starts from a basic twin/showman power supply. No big deal here.

So a 1 or 3 volt input isn't what we're after is it guys? The PI on a Princeton for instance is biased for 32 or 36 volts PtoP in. It's a unity gain cathodyne or concertina PI. A HIWATT, Marshall, or TWIN/Showman style power amp is usually biased around 36 to 38 volts on the output tubes, so our "Not so Alembic" should be putting out enough signal to produce 76 volts PtoP less the gain of whatever PI is being used. Mostly Schmidt type I think. Though I for sure want to use the full PA of the HIWATT too. Different. I think I need to vary the output from (say) 12V PtoP to 75V PtoP.

In the final analysis, the problem is to find a way to control the range of output voltages so that the pre will work with a variety of power amps from the input to the PI.

If you're not familiar with Dumble technique check the files section in the Dumble forum for a classic pre PI master volume.

In an Trainwreck v.s. Liverpool, the same basic pre is adapted for the much lower power amp bias by adjusting the longtailed pair bias and tail resisitors to affect the ouput voltage of the PI.

Now, with all that clear, I want to build this pre so that I have a tone neutral (or nearly so) way of adjusting the PtoP output of the pre to match the needs of the PI and output pair being used in a variety of different power amp types. El-84, EL-34, 6V6, 6L6, etc etc. Clipper stage forward Express. Straight Fender. Straight Marshall. Cathode biased, fixed bias etc. Soup to nuts. ......Dreamer eh?

If one uses a typical pre PI master volume control the signal form gets compressed radically unless the whole system is designed as a unit. Like the Dumble.

This may not even be possible. If I uses Dana's variable VR on pre tubes only I don't know how the 6 to one ratio of Plate resistor to cathode resistor works out to keep the pre working identically at (say) 100 volts on the plate resistor as opposed to (say) 330 volts on the plate resistor.

Clearer or cloudier? I'm going to see if I can get Dana in here.

Oh, one more thing. If you look at the Alembic scheme posted above and then do the most basic gain calc with 1 volt in from the guitar you've got something well in access of 50 volts out the other side. So the Peavey works different.

Dan

[mhuss]

Gilmour used an Alembic for many years; maybe still, and a whole bunch of HIWATTs with the pre's ripped out. But from the sound of things his amp guys leave the phase inverter in place in the HIWATT. That is, after all, the only really unique thing about the HIWATT anyway.

hey Dan,

I beg to differ. The PI is certainly unusual, but the most important Hiwatt sound differentiator to my ears is the completely unique tone stack. I've never seen another guitar amp design with anything similar, and never heard another passive tone stack that provides as much variation in tone.

DG also used his customized (linked input) Hiwatts preamp & all for many years, the pictures from the last tuneup are still on Pete Cornish' web site.

If you want to experiment with the Hiwatt-style power amp, I suggest you check out the "slave amp" preamp schematic, as it has just what you need in this application.
http://mhuss.com/Hiwatt/Schematics/STAInputPre.pdf

--mark

[Structo]

Hi Mark.

I just wanted to say thank you for preserving the Hiwatt history on your website.
I've spent many an hour reading all the information you have put there.

Cheers!

[skyboltone]

I'll have to take your word for it on the tone stack ingenuity. You have exactly 100% more HiWatt experience than I have.

So that is a Phase Inverter too. Would one put that in each slave amp and then run the Alembic or equal directly into the grid of the first section?

In the original "unbuffered" effects loops, the loop is inserted between the last stage and the PI. D labled that "Power Amp In" in his early stuff.

I guess one could leave the ECC-81 out and take the output of the cathode follower into the PI stage of a slave amp where the PI is intact.

I've got some chassis and face plates on the way. Are you stocking Iron from Heyboer or just having him make the Partridge clones onsey twosey for customers? I really hadn't intended to make more than one and then this question came up about did I want to build a 100 a 50 or a 20 watt HiWatt? So, being who we are I decided all three. I actually only own one amp right now. A Gibson GA19RV. That's it. Everything else is sold.

[skyboltone]

Hi Mark.

I just wanted to say thank you for preserving the Hiwatt history on your website.
I've spent many an hour reading all the information you have put there.

Cheers!

Hey Tom how's the weather in Sasquash country? There's a really easy way to thank Mark right there on the website I found it the last time I was over there.

Dan