Building my point to point BA-6A with original TX etc.

Inspired by DaveP's great thread, I decided to build myself an RCA BA-6A limiter.  After being lucky enough to buy the original input and output transformers from emmr, I became a bit obsessed with building it as close to the original as possible.  This means using NOS RCA tubes throughout, carbon comp resistors pretty much throughout, and paper in oil capacitors where specified in the BOM.  I know I know, this isn't the most efficient way to build this limiter -there are plenty other cheaper, less noisier ways to do it, but indulge me!

I also love the look of the BA-6A and decided to mimic it as closely as I can.  I know cloning the look of a piece of kit is frowned upon on this forum, but I'm not doing this to impress a client, it's just for my own amusement!

I'm pretty new to tube electronics -the only other thing I've built with tubes is an RCA BA-2 pre amp (also with the original transformers and NOS RCA tubes), so progress has been slow.  I've been asking all sorts of newbie questions over on this thread: http://www.groupdiy.com/index.php?topic=47096.0 and have been learning a lot.

Again, inspired by DaveP, I'm aiming to document the build with photos as I go.  Hopefully this will be fun to read (come on, we all love a bit of gear porn, right?).  As I'm quite new to this, please shout out if you spot me doing anything wrong, or can think of better ways of doing things!

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I'm using a standard Hammond steel chassis for the build.  After about a week of drilling I finally ended up with this:



I'm pretty inexperienced when it comes to metal work, but I'm pleased with this.  I used a hole punch for the tube sockets, and they came out great -never again am I going to use a step drill!  Perfect circles with no burrs or anything!

After drilling the (many many) holes, I started on the power supply.



I've copied the original schematic entirely for the power supply -except the DC circuit.  Rather than use a choke as per the schematic, I decided to build a circuit based around a LM317 chip to give me a regulated 6.3VDC supply to the 6SK7 tubes.  You can see it on the left of the picture hooked up to the separate toroidal transformer.

The only other difference from the schematic is that I've used resistors between the transformer and the rectifier tube.  This is so I can a) drop the B+ if I need to and b) prolong the life of the tube (I believe it allows an even current across it)

I tried to do the AC heater wiring as neat as possible.  They're wound very tightly with a drill and wired in phase, which should allow the push pull circuitry remove as much hum as possible.

After hooking it all up - voila:



All tubes getting what they need!  You can really see it in this photo but the 0D3 glows a rather fetching purple, which is nice.

That's all I've got so far, I'll try and update this as often as possible.

Great project letterbeacon.
I'll follow your project

Very nice work Matt,

Looking forward to the next instalment

best
DaveP

Dude! AWESOME!!

I might follow you guys after I get my BA-2 going and in a nice chassis.

I managed to squeeze a bit of time in front of the soldering iron again today.  Nothing too exciting, but I took some pictures anyway.

Now I've got the power supply going, I'm starting work on the amplifier section.  I'm using a 23 position 2 pole rotary switch for the input attenuator.  I followed the values on the Gold Point website to give me 2x 25k ladder attenuator.



Took me bloody ages!

I also hooked up the input transformer and the balance circuit:



I know, it's a bit OTT using carbon comp resistors in the balance circuit, but I'm trying to be as strict as possible when it comes to following the BOM.  I want to build a BA-6A, warts and all!

I also installed the Sowter interstage as well as the RCA output transformer:



Please excuse the iPhone  camera quality!

Inspirational.

Thank you very much the compliments!

I managed to get some time in front of the soldering iron today, it had been too long!  Especially as CJ is putting me to shame with this P2P 670!  I should be further ahead, but I decided to flip round the 6J7 tube sockets, which meant tearing down a fair bit of work.  Still, I think it'll be easier to debug and less cluttered this way.

I also re-routed the AC heater wiring.  It's a bit neater now, and tighter as well.

Some pics:

Absolutely awesome stuff.    ;D

That's going to be one amazing unit when it's done.

How's it going so far?

Cheers

Thanks mate!

It's actually come on a fair bit since those last pictures. The amp is pretty much done, just the sidechain circuit to go, but I'm away filming in South Africa at the moment (lucky me) so I've had to put work on it on hold for a bit. I can't wait to get back to it, but it'll have to be in December.

I'm starting my one this month I hope.

A thoroughly non authentic diy mongrel build, as is my 

2channels in a 2RU unit, tubes, traffos and caps on top.

Using UTC input and interstage, Edcor psu and outputs.

6SJ7 gr tubes and 12B4A for outputs.

I think this circuit topology will be my biggest diy challenge to date.

Look forward to reading of your progress 

Wow, that sounds awesome!

Looking forward to the build thread!

I've been away with work so it's been a little while since I've updated this thread.  Here's the BA-6A as it stands at the moment:

I've sorted out the meter scale as well as made a bracket to attach the meter to the unit -


You can sort of see the bracket in this shot -


Here's the guts.  I've added a fair bit more since the last photo.  In fact, I think I'm almost done with the amp section, I just need to do the sidechain and the meter circuit.

looks great!

Thanks Doug!  Nearly there now, just got to find the time to finish it!

Awesome work.

Love the PtP

What great build.  Isn't this the Jimmy Page Acoustic Guitar compressor?

Michael

Awesome build is too weak... I am blown!!!!
How did it go since your last post ?

.................(speechless)...............

I’m close to acquiring all the parts to build mine now. I’d like to make it the same as the original. One thing I’m super confused by is the Bal-Limit switch as laid out in the original paperwork. If anyone could explain it to me that would be great.

Let's have a go...........

All vari-mu compressors like this have a way of varying the timing to suit the music, in addition the BA-6a uses the first two positions for the setting up procedure. It uses a 50/60Hz ac signal from the heater supply in which the level is changed by the two capacitors, the 12nF supplies a low level signal and the 24nF a stronger signal. As the system is push-pull, if the tubes are well balanced they should cancel the signal, both at low level and at high level. (Vari-mu tubes have a gradual increase in the interval between their grid wires, so it's important to find a pair of tubes that balance across the entire range, not just at one point).

The third position shorts the timing signal to earth so you can hear what effect the limiting is having and also so you can check the voltages without the timing signal. The fourth and fifth positions give you a very basic choice of timing, single release or double. They were designed for the music played on the radio at the time, not how we record music with a compressor today 70 years later.

best
DaveP