ba71 power supply $5 torriod ok? ALSO:Sifam AL-19?!NEW pics!

[ToobieSnack]

hi peeps
a day or two ago i wrote down a parts list for a ba71 mic pre power supply.
I've searched the forum and my comp and i can't find them anywhere.
the parts list small but complete is:
c1-250-50v
c2- 35-50v
c3- 35-50v
c4- 35-50v

all res 1/4w
r1-750 -/+5%
r2-200 -/+5%
r3-750 -/+5%
r4-750 -/+5%
r5-7500-/+5%

q1-2N2270

can someone please post this schematic

**ALSO** the scemo never indicated the secondary voltage of the power trafo (24v right?)

anyway
thanks in advance
Toobie Snack
r5-7500-/+5%

 

[jensenmann]

You find the schem in my album at:
http://groupdiy.twin-x.com/displayimage.php?album=29&pos=14
the manual and a reversed PCB layout can be found, too.
:sam:
Jens

 

[ToobieSnack]

thanks so much for the schematic.
Does anyone know the secondary voltage on this power trafo
I am assuming 24v...right?
info much appreciated
thanks Toobie Snack
:grin:

 

[fum]

I just picked up 3 BA71's, they are on the way, but the seller listed it as requiring a 30V DC rail.

He's supposed to be sending schematics and pinout. More than happy to share it, once it's in hand.

Regards

ju

 

[hitchhiker]

hi toobiesnack,

here's a link to a very clear copy ...

http://www.waltzingbear.com/Schematics/RCA/BA-31A.htm

These run on minus 30 volts. I've not tried the power supply in that
schematic, maybe PRR could say what that transformer would be.

Lance

 

[jensenmann]

If you need 30V DC just take 30V AC.
:guinness:
Jens

 

[wilebee]

it actually needs to be -30 vdc Wil

Wilebee

 

[PRR]

>> If you need 30V DC just take 30V AC.

> it actually needs to be -30 vdc Wil

So use -30VAC. :shock: :roll: :razz:

The "ACV=DCV" rule works OK for regulated supplies.

This antique is not actually regulated. It has an R-C filter and dropping resistor with a transistor buffer. The supply voltage isn't set by a regulator, but a complicated function of resistances and transformer voltage.

http://www.waltzingbear.com/Schematics/RCA/BA-31A.htm

I figure it runs on 28V AC making about 38V DC at C1. R1 R3 R4 R5 drop that (filtering along the way) to a bit over 30V DC at Q1 Base, so 30V to the amplifier.

A lightly-loaded 24VAC transformer might work well.

There is no ground on the power board, so you can't really say if it is "+30V" or "-30V". The supply is arranged as if it were a +30V supply. And the amplifier is drawn as a grounded-negative affair. However the power supply "-" terminal is not connected directly to amplifier negative, there is a 130 ohm resistor in the "-" lead. This will work fine for one amp. It clearly was NOT intended to connect several amps to one power source this way. If you need to do it that way, skip the 130 ohm resistor in the "-" lines, and add a 130 ohm resistor in the "+" line to each amplifier. However there may be good reasons to use a separate power supply for each amplifier.

 

[ToobieSnack]

ok let me see if i understand
the power trafo is 120v AC primary
-30/CT/+30 secondary
and i would use the -30v (AC? half wave?) and the center tap for my 30v supply to the schematic indicated on the ba31 schematic from waltzing bear?

plz help

 

[ToobieSnack]

PRR thasnsk for the info
I was actually writing my last post as you were posting yours and didn;t see youir info untill afterwards. kinda funny huh?
any way I will be racking a pair of these so the resistor info was great.

A lightly-loaded 24VAC transformer might work well.

Would 2 of these preamps be considered "light load"? (racked pair)
SOoo......I CAN USE A 24V ?___?AMP POWER TRAFO?

BTW: anyone know the load (in amperage) of these units?
don't see it listed anywhere.
more thanks
toobie snack
[/quote]

 

[PRR]

> the load (in amperage) of these units?

48.5mA. (Compute the voltages around the 130Ω resistor that passes all the supply curent.)

Double that to get the transformer AC Amps rating: 0.1A per channel.

A 24V 1A transformer is as cheap as it gets and is ample.

For two amps, I would strongly suggest two transformers (or a 2-winding transformer) and two power supply boards. There is way too much ground confusion here already. (This has to have been adapted from a positive-ground design, though in the form shown in this link it is negative ground.) And it isn't like the parts are expensive. 250uFd??? I have not used a main cap that small in decades.

2N2270 is an old favorite Silicon NPN that has fallen out of style. If you don't need an exact replacement, 2N3053 is about the same die in a different can. But in this circuit, almost any NPN will work. 2N2222 or its plastic replacement will do very nicely. In fact one catalog crosses 2N2270 to 2N2219, which is the original 2N2222. (I remember when ten 2N2219 came nested in a padded box and were worth a day's pay.)

 

[ToobieSnack]

PRR thanks for all your helpful info (ebody elso too)
This is a really simple and cheap design. Since everything else I'm tryin to do cost quit a bit more I'm goin here first. ( being broke sux hehe)
AND NOW I'm crystal clear!!!
Toobie Doobie whooo hoooo
even though this is not an actual toobie snack it is germanium right? hehe

thanks again
Toobie Snack

 

[ToobieSnack]

Hey people
finally got these parts and i am starting this power supply build tonight.
I have the parts positioned in an "off the shelf" Radio Snack project board.
They had one with a ground rails right down the middle of the board, which worked out quite nicely for me. (and only $1.79)
I'll post some pics soon.
I was wondering if the $5 torroids would be a good choice for these power supplies?

ALSO: I have a chance to pick up some Sifam Presenter AL-19WF's with light kits( which kinda have that RCA bc-7 look ...nice...yep..)

http://www.sifam.com/presentor.lasso

can't find prices...at website

what is considered a good price for one of these?
Is $10 good?
what about five al-19's for one UTC a-21?
i had a few of these when I visited him and he expressed interest in trading for one)
thanks
Toobie Snack

 

[ToobieSnack]

ba-71 pics

power supply circuit ala ts

Not too shaggy.....is it?

have fun
ts