RCA BA-6A from scratch

[DaveP]

This will be my main project this year.  After discussing what to make in a previous topic I decided on a BA-6A.  These are all the parts ready to work out the layout.

After a lot of thought I decided not to make a straight copy, because the original was not really built for a 19" rack and you needed access inside for the various adjustments.  I have managed to fit all the various controls onto the front panel along with a control to vary the attack.

I've redrawn the schematic to make it clearer.

I couldn't afford the wafer switches so my switching is in stages.
I will make it in pure point-to-point without PCB's and design my own layout from first principles.  The tubes will be run on DC to keep ac fields to a minimum and all the tubes are original metal vintage from James Cross at Vacuum tubes inc, Florida.  Can't think why more people don't use them, they are beautifully made and are screened as stock.  Also using an Edcor transformer for the first time, its really well made (English) Re-all purtty (American).  Sorry bout that!

I decided to live dangerously by showing the project from the start rather than when its all safely finished.  That way I can't back out half way through!
Wish me luck!
DaveP

 

[baadc0de]

I'm going to watch this with interest.. I'd like to make a pair for myself one day. Though I thought of turret boards rather than purely point to point, just a preference I guess. Anyhow, good luck and be sure to post frequently with progress!

 

[Joechris]

Great work on the schematics, thanks for sharing. Witch Edcors did you end up with?
Im gonna follow this thread like an eagle
j

 

[dustbro]

Inspiring!

 

[lassoharp]

Oh yeah!  Been looking forward to this for a while.  Using the Sowter interstage?

Can't think why more people don't use them

Be thankful . .  'tis a blessing of sorts.


Best of Luck!

 

[DaveP]

Hi Guys,

The Edcor is the CXPP10-600-8k at ~40 bucks, lovely little job.

The interstage is a VT1399 15k:15k from livinginthepast:-

http://livinginthepast-audioweb.co.uk/xfrmrvt1399.php

Gone midnight now, time to turn in, will keep you posted.

best
DaveP

 

[Marcocet]

Awesome. I've been considering a very similar project. I'll be excited to hear about your results!

 

[Mike Cleaver]

I have an original BA6A which spent all of it's previous life in a radio station transmitter rack.
Because only one engineer looked after it all of it's life, it's in mint condition and sounds amazing.
The original unit slides into a 19" rack mount tray with connectors on the back.
Good luck with your project, which looks amazing!

 

[DaveP]

Thanks Marc and Mike,

It makes such a difference having the support of people you respect with a common interest.

Marc,

I guess a guy can't have too many compressors!  Looking at your studio outboards, about the only thing missing is a BA-6A!

Incidently, when I was doing the research for this project, I discovered that the BA-6A has almost exactly the same timing on the single setting as the Fairchild on #2 setting.  I couldn't get the maths to square up until I realised that although the attack speeds are calculated on the normal 1 time constant of 63%, the release figures are calculated on 3 Time constants of 95%.
best
DaveP

 

[alexc]

Also following your build with great interest!

Good luck with it all 

 

[Rybow]

I guess a guy can't have too many compressors! 

This is 100% true!

Can't wait to see the finished product. Good luck!

 

[nielsk]

I'm working on one of these as well, using original type audio transformers (a serious investment of time & $ )
I am interested in your solid state power supply approach, have you calculated the current needed on the PT taps? How are you generating the 3.15V AC to the balance adjust circuit? Have you found a suitable choke?
I have been told that these really want a 60K secondary on the input transformer, and life will be easier if the primary is around 10K

 

[DaveP]

Hi Nielsk,

I guess it will be easier using the original transformers, but if you do, you will need to follow the original schematic, not mine.  I have omitted the treble boost RC on the 2nd stage, because I don't expect to lose the top end with modern transformers.

Yes I calculated the current and I have a PT that can give 100mA  and the drain should only be about 80~90.

I have a spare heater winding on the PT which is not shown on the schematic for clarity, I'm going to use one side for the 3.15V with a CT to ground.

The original uses a twin 25k pot making 50k across the input TX.  My budget wouldn't run to a new one and I already had a 1:4 Sowter to hand, but I've kept the 50k giving me an input of 3k which is fine.  I use my own mic pre's and don't intend to use the BA-6A as a pre so there is no problem.  You will need a 1:2.45 CT to get 10K:60k.  If you can get a modern one it will have an excellent frequency response.  The original (like many of that period) was 1:10  600:60k I believe.
best
DaveP

 

[skal1]

Hi Dave


i am curious how are you setting up the interstage tx 1:1  15k:15  or the 3.75 k?

ska1

 

[DaveP]

Hi Skal1,

1:1 15k:15k

With a 44k load on the secondary that will be reflected back as 22k/tube , which will be in parallel with each 4.7k.  Evidently this was a revision which helped the tube balance.

best

DaveP

 

[nielsk]

I see what you mean about the RC network, I have LS12's for the input, which spec out excellently... I'll have to try it both ways.
What is the reason for the parallel 1 ohm & 10 ohm resistors on the heater supply?

 

[DaveP]

Hi Nielsk,

I calculated that I would need less than 1 ohm to drop the voltage down to 6.3, but this kind of trimming can only really be done when its up and running, it may need to be altered.

best
DaveP

 

[DaveP]

Right, its now down to serious metal bashing!

3mm thick aluminum 19"x 7" all the holes drilled for the controls.

I will also be drilling some small countersunk holes for the angle which holds it to the chassis and the big hole for the meter.  This just gives you an idea of the finished article, a bit busy in'it?

The meter is a modern Sifam 100uA movement fitted into a 1960's Sangamo Western case.  As the original meter is 500uA, I have shunted the new one with the 300 ohm resister you see in the schematic.

I will print my own scale from a photoshop off the original.

Nielsk.  The choke is a bog standard 10H job that you would fit in any old amp, its not critical

I'll post more photos next week when hopefully most of the hard graft will be over.
best
DaveP

 

[skal1]

Hi dave


where did you get the aluminum from for the chasis , did you bend it yourself .

cheers

skal1

 

[DaveP]

Hi skal1,

I haven't made the chassis yet, hopefully it will appear on this topic next week.

No, I don't have a bender, I use angle section to join sheets, or I cannibalise other equipment that already has a fold in it.   I buy the aluminium sheets from RS.  Its all basic metalwork that I learnt in school in the early 60's!

Cut it with a padded jigsaw to avoid scratches, file it straight etc, hard work but keeps you fit 8)

best
DaveP