RCA BA-6A from scratch

[DaveP]

Well done!

At that price you can risk it.  Build your confidence by checking it all through yourself, its the only way to learn.

When you get them, check out the contacts and make sure the internal ring will be there for the number of contacts you need.  Wiring switches is tedious painstaking work, but it has to be done right.  Best to wire up with coloured wire and make a note of what's what before you fix it in the panel, not a job for late night work.

good luck

DaveP

 

[joe-electro]

Great work, Dave! It motivates me to build one of these, one of these days.

Joe

 

[Kid Squid]

Awesome work, Dai 

Best,
Steve

 

[DaveP]

Diolch Steve,

Daffyd P

 

[buildafriend]

Has anyone every seen this schemo? or heard of it being built?

http://www225.pair.com/audio/waltzingbear/Schematics/RCA/BA-6A.htm

Dave I'm still waiting anxiously to see your completed project.

 

[lassoharp]

It is just a simplified version of a BA-6

 

[buildafriend]

Thanks for replying lassoharp!

What I'm curious about is if that simplified BA-6A schematic is trustworthy, or if anyone has tested it.

 

[DaveP]

Buildafriend,

I have attached my re-drawn schematic with voltages, it is trustworthy and works.

The project was finished a year ago, what else did you want to know?

best
DaveP

 

[buildafriend]

How much did this build approx cost you in components? (excluding the metal chassis which is beautiful and kind of priceless)

 

[DaveP]

It cost me about £400 which is $640.

You could probably do it less than that cos your components are cheaper than ours.

For example. an edcor $39 TX cost me $89 due to shipping costs, but thats still less than a Sowter would cost.

best
DaveP

 

[buildafriend]

That's not that bad.. It makes me wonder if anyone has ever taken a stab at a stereo unit.

How happy are you with the sound of the unit? I know I love Jonny Cash's voice.. the BA-6 is partially responsible for that.

Oh crap.. I'm making a parts list from your schemo.. did you trick me into this?

 

[DaveP]

The sound is fine, but to be honest I don't use it that much because I keep forgeting how the controls work!!  In order to save on the special switches I used several others in combinations...big mistake.  I will probably rebuild it sometime without all the current measuring circuits, it just needs the two balance voltages thats all, everything else is rocksteady.

best
DaveP

 

[letterbeacon]

I'm in the middle of building mine (although it's on hiatus due to work commitments) and I'm leaving off the measuring circuit.  Once you get rid of the measuring circuit, you can just use a 2x6 rotary, which is a standard component.

Mine's probably cost me about £800 so far, but then I am using original RCA i/o transformers, a Sowter interstage, NOS RCA tubes and PIO capacitors where specified in the original BOM.

 

[klbr_di]

sañudos amigos de groupdiy  8) disculpen soy novato  :'( por favor ayudenme con estos valores que estan en un circulo verde les agradeceria mucho.

 

[RuudNL]

En inglés, por favor...
I suppose you are asking for the values of C10, the choke and the interstage transformer?

 

[klbr_di]

sorry.. right so, please help with those values...

 

[DaveP]

The capacitor on the diode is 10nF at 2000V ceramic type like Murata.

The choke would be any 10 Henry type with a DC resistance of ~150 ohms.

The interstage would need to be 50k on the primary or thereabouts but these are not really obtainable or affordable.

best
DaveP

 

[klbr_di]

dave thanks for the answer , could you help me with which I can use as interstage which I can get you recommend me .. greetings from Ecuador

 

[gridcurrent]

The interstage would need to be 50k on the primary or thereabouts but these are not really obtainable or affordable.

I've installed UTC A-18's (80k:15k) in a few units, measured and sounded fine.

 

[emrr]

I've installed UTC A-18's (80k:15k) in a few units, measured and sounded fine.

All indications are for a 1:1.  Unless that is wrong, an A-18 would make the unit function at a slightly lower ratio like an 86-A which has a similar step-down ratio in the interstage.  The step-down reduces signal level into the output stage and likewise the side chain, which means the gain reduction stage will run at higher levels before onset of limiting along with a softer slope due to the reduced side chain voltage.  So, all the above being true, it will work, but not quite like a BA-6.    If my info is wrong, I'd love to hear more details.  In a pinch with a dead original, an A-18 may sound more correct than a modern replacement, even with the shift in operation.