Alembic Bass Preamp F2B project - Power Supply questions (Moosapotamus design)

[rantony]

Hi everybody,

I'm planning to build an Alembic F2B clone based on the schematic and layout made by Moosapotamus

(http://moosapotamus.net/IDEAS/F2B/alembic.htm)

I have some questions about the power supply section:



First, I question the 470k resistor in the high voltage area. What is the purpose of this resistor ?    There is no
similar resistor on the original schematic:




Next, I'm trying to find proper transformers for the project. Moosapotamus's design uses two back-to-back 'Toroid' transformers:



The specs are:

Series Output Voltage: 24.0 VAC @ 1.458 A

Parallel Output Voltage: 12.0 VAC @ 2.916 A


At a local store, I did not see this kind of transformers, but I found something that might do the jod as well: Low profile, split bobbin, semi-toroidal design transformers. They look identical to this one:



You can find more info here:

http://www.hammondmfg.com/229.htm


The specs of the transformers that I found at a store in my area are the following:

Series Output Voltage: 24.0 VAC @ 500 mA

Parallel Output Voltage: 12.0 VAC @ 1000 mA

I know this is much less than the Toroid tranformers suggested by Moosapotamus, but I don't think that the Alembic F2B requires a lot of current even though it runs at high voltage...

What do you guys think about it ?

Thanks a lot for your help !   

[rantony]

Hi everybody,

I'm planning to build an Alembic F2B clone based on the schematic and layout made by Moosapotamus

(http://moosapotamus.net/IDEAS/F2B/alembic.htm)

I have some questions about the power supply section:



First, I question the 470k resistor in the high voltage area. What is the purpose of this resistor ?     There is no similar resistor on the original schematic:



Next, I'm trying to find proper transformers for the project. Moosapotamus's design uses two back-to-back 'Toroid' transformers:



The specs are:

Series Output Voltage: 24.0 VAC @ 1.458 A

Parallel Output Voltage: 12.0 VAC @ 2.916 A


At a local store, I did not see this kind of transformers, but I found something that might do the jod as well: Low profile, split bobbin, semi-toroidal design transformers. They look identical to this one:



You can find more info here:

http://www.hammondmfg.com/229.htm


The specs of the transformers that I found at a store in my area are the following:

Series Output Voltage: 24.0 VAC @ 500 mA

Parallel Output Voltage: 12.0 VAC @ 1000 mA

I know this is much less than the Toroid tranformers suggested by Moosapotamus, but I don't think that the Alembic F2B requires a lot of current even though it runs at high voltage...

What do you guys think about it ?

Thanks a lot for your help ! 

[soulsonic]

The extra 470K resistor is what they call a "bleeder" resistor. It drains of filter caps after the unit is turned off. It can acts as part of a voltage divider which helps set the proper voltage in the power supply. It's a good thing.

[rantony]

Thanks soulsonic, that sounds like a good explanation !

[snoof]

I built mine with this tranny...

http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=thispage&thispage=parts1.htm&ORDER_ID=666205901

I then used a reg old 12v ac wallwart for the 120v to 12v tranny.  Doug has super fast shipping too.

[CLAAS]

Hi
There is a MISTAKE in the Amp- Schematic
You have to CHANGE PIN 6 + PIN 8 !!!
Have Fun with it!
Thilo

[snoof]

Oops, yeah, I should have added that to my post above.  Luckily, I noticed that error before I built mine.  I'm surprised that RG hasn't caught that yet, it's been almost 10 yrs.

[R.G.]

I'm surprised that RG hasn't caught that yet, it's been almost 10 yrs.

Hmmm... I guess my regular review of everything I've ever done before for any errors is not working that well. 

[snoof]

No offense man, I know you've got alot on your plate.... all the time!  I just figured someone might have brought that to your attention at some point over the intervening years .   It totally slipped my mind after the proj was complete.

[moosapotamus]

The specs of the transformers that I found at a store in my area are the following:

Series Output Voltage: 24.0 VAC @ 500 mA

Parallel Output Voltage: 12.0 VAC @ 1000 mA

I know this is much less than the Toroid tranformers suggested by Moosapotamus, but I don't think that the Alembic F2B requires a lot of current even though it runs at high voltage...

That looks like it might be a nice option for a power transformer. Probably more affordable than the toroids that I used. But, I would also like to know the current requirement for the F2B circuit. I think I may have gone with more than I really needed, but I'm not really sure. My criteria for picking the transformers was to supply enough current to power two F2B circuits, and to fit inside a 1U rack enclosure.

~ Charlie

[Rmanen]

Nice timing, I am going to build this after my meatball project. I've been trying to start this for ages, just never had time

Peace

PS: Post work in progress pics please

[rantony]

Hi guys,

Sorry not to have follow more closely my own thread...

Thanks to everybody for the replies. 

My criteria for picking the transformers was to supply enough current to power two F2B circuits

What you're pointing out is very important. I too would like to know the current requirement for the F2B circuit because I want to build a two-channel preamp just like the real thing.

---> snoof and moosapotamus:

Can you tell us how silent are the units that you built ? Is the dual-transformer solution noisy 

I've been trying to start this for ages, just never had time

Same thing for me !    I hope I'll be able to start my project soon...

[snoof]

the unit that I built is with my buddy in his bass rig.  He has not reported any noise issues.  I went with a 12v ac wallwart (used outside the unit) that plugs into the hoffman tranny(housed inside).  Just look up the 12ax7 spec sheet, it will tell you the current draw for the tubes.  IIRC, the heater current is 150ma per triode using 12v.  I don't think the plate current is much of anything at all.  looks like the hoffman tranny is rated at 1.6a (.8a per side?).  See my link above for reading the specs.

[moosapotamus]

No issues with the two transformers. Mine is not noisy at all. I'm using 110VAC mains power for mine.

Oh, yeah... and, I'm not using 12AX7... I'm using 5751, which I think needs ~350mA on the plate.

~ Charlie

[snoof]

plate current is only 1ma @ 250v, heater current is 175ma @ 12.6v per triode.  Your 350ma draw is HEATER current @ 6.3v, not plate.

http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5751WA.pdf