Alembic F2-B Pre-Amp Restoration project

[oatmeal769]

It's for certain, the biggest issue is low heater voltage.  I put a dummy tube in one slot and got the working tube to about 5.8 - 6 Volts.  The sound was markedly cleaner, and distortion free.  Put a working one in there, and it's back to distorted, 'brown' sound.

 I also tried an RCA Red Base 5691 6SL7 which I use for another amp in here, and it wouldn't make a sound.  Popped it right back in the other amp, and it works fine.  The other amp measures a solid 6 Volts.

So, how do I increase the heater voltage?

[DummyLoad]

leave one tube out or get a stiffer transformer.

[oatmeal769]

I was hoping you wouldn't say that.  

Was doing some reading - the Hammond in there has an amperage of 0.3Amps.  each 6SL-7 has a heater rating of .3Amps, I'm wondering if having run them in series, it's over-taxing the PT...

Can I use any PT that has a primary of 120AC, and a secondary of 6 - 6.9VAC?  

EDIT:

OR -  Couldn't I just run the two output leads in PARALLEL rather than series, and use two resistors to get 6.3VAC ??  It has a 12.6V / 300mA secondary output.

[DummyLoad]

I was hoping you wouldn't say that.  

Was doing some reading - the Hammond in there has an amperage of 0.3Amps.  each 6SL-7 has a heater rating of .3Amps, I'm wondering if having run them in series, it's over-taxing the PT...

Can I use any PT that has a primary of 120AC, and a secondary of 6 - 6.9VAC?  

EDIT:

OR -  Couldn't I just run the two output leads in PARALLEL rather than series, and use two resistors to get 6.3VAC ??  It has a 12.6V / 300mA secondary output.

the problem is the specs on that tranny, you are operating it at the limit, so no amount of reconfiguration is going to get you any more power. you NEED a larger transformer.
 

edit: here's a $10 fix... make sure it'll fit into your chassis first though.

[oatmeal769]

Well, I ordered a dedicated filament tranny. - a Hammond P-T166L6  6.3V CT  2 Amps.  I'll have to do some jostling of parts in the chassis, but not a huge deal.


EDIT:  Which may just barely not fit.  Look what I found instead:

http://www.alliedelec.com/search/productdetail.aspx?SKU=2270042

[RicharD]

Well shoot.  I was afraid of that.  Hammond usually rates their transformers conservatively but I fear that isn't the case with this particular transformer.  It's listed on the Hammond page but it diesn't look like a Hammond to me.  Sorry I gave you a bum steer on the tranny.  At least there is a $10.00 fix.

[oatmeal769]

Oh, no worries at all.  Now I have an extra for my next project!

[oatmeal769]

New question -

I have the filament transformer, it's just a bit larger than the P.T.

I'd like to put it in position one.  Is this too close to the audio outputs, or will I be okay?
Position two is an option, but I will need to move and re-drill stuff.  Not an attractive possibility.
Position three can't happen, because that is where the one (and possibly two) impedance matching transformer(s) will go.  Plus, if I decide to go with tone controls later, this space will be even more cramped.

Can I have permission to place it in slot one?

[RicharD]

Position 2 is probably your best choice.  You want all that nasty AC stuff as far as possible from ANY signal.  The only other option is to change out the existing transformer all together.  I'm still kicking myself over that.  A PA211 would do the trick, but you'd need to redesign your B+ supply whereas it's a 330-0-330 secondary.  I didn't select this one in the first place because I didn't know if you should be playing with such high voltage.  Given the initial start up problem, it mighta been scary vs. frustrating.

[oatmeal769]

Yeah, that'd smart a little.  Don't worry about the tranny, What I have now still results at $10 less than the Dynaco one.  But yeah, in retrospect, I think 2X330V would probablybe just enough...

I'll try position 2.  I can get it in there pretty easily, I think without moving the PS board.  I just wanted to see if 1 was a better place, with the higher powered output signal, but I'll take your word for it.

[oatmeal769]

Got the F.T. installed at position 2.  Wasn't as difficult as I suspected it might be.
Here are the results of my new measurements.

- Voltage -
Heaters = 6.8 Volts
B+ = 282VDC
Point "A" = 318VDC

---------------------------

- Variable Resistor Values -
R8/108 = 68K
R10 = 68K / 3W
R2 / 102 = 0/18K/33K Ohms (3-way switchable)

Most likely, I'll just play through it for now, before making any more adjustments, it sounds great!  The output trannies are still a month away, I can adjust then too.

[oatmeal769]

Well, I've been playing and listening to it for a couple weeks.  The only thing I'm missing is an Edcor Output Transformer because I'm still not driving my compressor with much oomph.  If I understand what I've learned here about impedance, I'm basically spinning my engine really fast, to drive a really low gear...

It also has some distortion, but I don't know what I can do to improve that, other than turn the gain down.  -  But then I don't have enough to drive the following component in my rack.

I'm pretty derned happy with it otherwise, I still love the idea of "I made this"  (sort-of)  but there are a couple things I think I'd like to do to at least one of the channels.

1). a tone stack.  I've fiddled a bit with the duncan calc, and I think the James stack would do me - but am wondering about some options.  What about substituting the 'Big Muff' as the midrange control?

2.)  I'd love to have a 'deep' switch.  I feel like the boom in the room is a bit lacking at times.

[oatmeal769]

bump

[RicharD]

If you add a tone stack, you're going to lose gain.  All of these passive stacks are cut only in nature.  If you add a tone stack, you're gonna domino into other issues.

But.....

If you need better bass response, change C2 from a .1uF to something more along the lines of 2.2uF.  Don't forget that needs to be at least a 400V part.

If you need more gain, change R2 from a 47k to a 33k.  This will brighten it up a little bit too but it will not effect the bass response.

[oatmeal769]

Ahhh, not that easy, eh.  Gotta design the whole circuit together... 
Guess I'll have to build another.  Darn   

Here's an idea...
I have the EMG BQC System in my basses.  What I don't like about it is that I can never be sure it's 'set'  I always have to check it if I put the bass down, etc.  I'm thinking about mounting it after the input jack, but before anything in the circuit.  essentially running the EQ circuit 'outboard' from my bass.  Hmmm...

Maybe I should just add to my collection and build your design.  How does it sound?


  I'm also looking for a schematic for a Demeter VTBP-201 S.  I think it would be Da Bomb!

[oatmeal769]

Hey guys,
My Edcor impedance matching transformers finally arrived.  I will use one in my Alembic F2-B, and one in the new pre-amp.  Primarily, they will be to lower the output impedance but a side benefit will be balanced outputs as well.  There's a monkey in my wrench however, The transformer is center tapped on both windings.

I'm finding conflicting info as to what I ought to do with the CT on either / both the primary and secondary windings.

Two Questions:

1.)   I know that Hi-Z signal will go to the (+) side of the primary.  Do I simply ground the (-) side of the primary, and leave the CT un-connected to anything, or should  ground that too?

2.)  On the secondary side, I would go to pin 2 (hot) from the (+) side of the secondary, after that I get confused.  Should I go to pin 3 (cold) from the CT, or from the (-) of the secondary?  If I go from the (-) side of the winding, should I then ground the CT along with pin 1 (ground)?

[RicharD]

Ignore both of the center taps.

Primary: Ground 1 side & feed the other.
Secondary: Tie 1 side to pin 2 and the other to pin 3.  Reverse them and you've reversed polarity.  Shoelace wire a DPDT switch and you've got a phase selector.

Pin 1 should go through a switch to chassis ground.  99 times out of 100, you'll run with the ground lifted to eliminate a ground loop buzz.  You could get away with leaving pin 1 open.

[oatmeal769]

Very cool, thanks!

Does it matter if the polarity is flipped or not?  I know it would matter if you ran the instrument 'direct' first, and then into the pre-amp, but I'd never allow that.   

[RicharD]

Phasing becomes an issue in recording and live if there are outputs.  Consider your bass rig might be out of phase with the PA.  As you turn up, everything gets louder but may appear to get quieter when standing in front of your cabinet.  1 miswired XLR cable from a DI can get funky.  That's where the polarity switch comes in handy.  The ground lift switch is a must.  Again, 99% of the time you'll run with the ground lifted.

I expect your gain to drop considerably from the step down transformer.  You'll probably need to tweak the feedback in one or both of the stages to compensate.  I talked about this a few posts back.

-Richard

[oatmeal769]

Yes, I'm ready for the output gain drop. - But I'm expecting to be transferring power more efficiently, which will enable better gain structure.  Right now, both pre-amps don't have any 'oomph' into 20K loads.
I've been reading that since the Alembic has about 40k output, and the 'oatmeal' has about 15K, it's like trying to start a car rolling in high gear...  My guess is that even though the current (amps) will drop, the volts (pressure) will increase... or something.  I think....