DaveP

RCA 96-A from scratch
« on: February 13, 2020, 08:42:03 AM »
I have a few months before bookings start in ernest, so I am going to tackle this beautiful work of art from the 1930's.

Together with my friend Ian Barter,  we have been exploring vintage sounding equipment, the OP-6 was the last project in this vein .

The balancing update from the 86B has been incorporated  but the rest of the audio path will be just as it was 83 years ago.

Obviously, chrome plated transformers are no longer available, but I will be using  similar transformers to those I used in the OP-6 with good results.  Here it is:-



This is the basic circuit:-



I have added modern attack, release and threshold controls to the circuit with a modified side-chain.

I shall be posting pics as soon as the construction starts.

DaveP

Soundcloud: Delayed Action.


Ricardus

Re: RCA 96-A from scratch
« Reply #1 on: February 13, 2020, 10:56:55 AM »
Oh my.
Audio mastering for hire..

EmRR

Re: RCA 96-A from scratch
« Reply #2 on: February 13, 2020, 10:24:02 PM »
For those who don't know, the 96 is the predecessor to the 86, the difference being an external regulated PSU on the 96, with the 86 having a standard onboard CLC unregulated PSU. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

rackmonkey

Re: RCA 96-A from scratch
« Reply #3 on: February 13, 2020, 10:47:45 PM »
Is there some lore behind the higher numbered unit coming first?
Whether you think you can or you think you can't, you're probably right.

EmRR

Re: RCA 96-A from scratch
« Reply #4 on: February 13, 2020, 11:06:19 PM »
Is there some lore behind the higher numbered unit coming first?

86 is part of the 80 series, economy line which came slightly later.
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

mjrippe

Re: RCA 96-A from scratch
« Reply #5 on: February 13, 2020, 11:33:21 PM »
86 is part of the 80 series, economy line which came slightly later.

And the 86 later became the BA-6A, just to complete the family tree.

DaveP

Re: RCA 96-A from scratch
« Reply #6 on: February 14, 2020, 02:56:22 AM »
Quote
the difference being an external regulated PSU on the 96, with the 86 having a standard onboard CLC unregulated PSU.
Doug is correct, they were catering for unstable power lines back in the 30's.  We don't have such problems today.  However, I will be retaining the 10mA voltage divider for the G2 current as it affects the audio path.

DaveP
Soundcloud: Delayed Action.

scott2000

Re: RCA 96-A from scratch
« Reply #7 on: February 14, 2020, 06:07:49 AM »
Quote
I will be retaining the 10mA voltage divider for the G2 current as it affects the audio path.


Is the consensus that this is the best way to do this? Just curious ....

And This is how the Federal does it too?

DaveP

Re: RCA 96-A from scratch
« Reply #8 on: February 14, 2020, 06:47:31 AM »
Quote
Is the consensus that this is the best way to do this? Just curious ....
Other designs use a  voltage regulator tube (which also needs high current) but I am not looking for the best way to do this, I am looking for the sound character of this design.

DaveP
Soundcloud: Delayed Action.

scott2000

Re: RCA 96-A from scratch
« Reply #9 on: February 14, 2020, 07:08:14 AM »
Quote
Other designs use a  voltage regulator tube (which also needs high current)


Thanks.  I guess I was meaning to ask , outside of a regulator tube. Like  this...

http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_032.html

Compared to a voltage divider, is it just a matter of character or are there pitfalls? Sorry to derail. I have had this on my mind for some time and your mentioning of it brought it to the front......

edit...I see I may be on the wrong process.... hmmm... 
« Last Edit: February 14, 2020, 07:18:32 AM by scott2000 »


DaveP

Re: RCA 96-A from scratch
« Reply #10 on: February 14, 2020, 08:47:13 AM »
Quote
Compared to a voltage divider, is it just a matter of character or are there pitfalls?
Designers keep the g2 voltage reasonably constant to get a sharper knee on the compression curve.   The Altec  436, for example, has no means to hold any voltages constant so the knee is very soft, this is fine for recording guitars, but it would not be any good for radio where there are penaties for exceeding your transmission power.   I don't want to use a tube regularor because it will affect the knee and change the character of the compression.   I hope that's helpful.

DaveP
Soundcloud: Delayed Action.

scott2000

Re: RCA 96-A from scratch
« Reply #11 on: February 14, 2020, 08:58:15 AM »
Ok Thanks!

IBart

Re: RCA 96-A from scratch
« Reply #12 on: February 14, 2020, 04:55:46 PM »
Can’t wait to see a picture  :)

DaveP

Re: RCA 96-A from scratch
« Reply #13 on: February 16, 2020, 12:40:32 PM »
Here is the first of many!

I started to cut metal today, this is the faceplate cut to size, but lots more to do on this, as you  will see.



DaveP
Soundcloud: Delayed Action.

EmRR

Re: RCA 96-A from scratch
« Reply #14 on: February 16, 2020, 01:47:27 PM »
Are you going for semi-accurate look on this one?  Surprised to see 8RU. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

DaveP

Re: RCA 96-A from scratch
« Reply #15 on: February 16, 2020, 03:25:58 PM »
Quote
Are you going for semi-accurate look on this one?  Surprised to see 8RU.
As accurate as I can make the front anyway.  I did not take the project seriously, until I had worked out how to do that.  It will look quite spectacular in black and chrome.  I would have made it full size but 11.75" is the widest sheet I can sensibly source.  It will not have the same round VU meter (never seen one for sale) but it will have a vintage 4.5" meter, they always look good.

DaveP
Soundcloud: Delayed Action.

5v333

Re: RCA 96-A from scratch
« Reply #16 on: February 18, 2020, 03:26:23 PM »
hi Dave!

do you have the complete schematic with values and voltages as well? i cant find this info..

i f i understand it right, rca made the only comp/lim with varying screen voltage instead of varying grid voltage for controlling the gm.
is this right?
wonder how it differs in sound compare to every other vary mu comp?

how much does the screen voltage vary? im guessing 100v when not compressing and down half to zero voltage when compressing...


DaveP

Re: RCA 96-A from scratch
« Reply #17 on: February 18, 2020, 06:25:03 PM »
Quote
do you have the complete schematic with values and voltages as well? i cant find this info.
My guess is they are the same as the 86A,

Quote
i f i understand it right, rca made the only comp/lim with varying screen voltage instead of varying grid voltage for controlling the gm.  is this right?
   I've not heard of this but Doug might know.  You may be confusing this with the use of 6L7 tubes on several early compressors, these used extra grids.

Quote
how much does the screen voltage vary? im guessing 100v when not compressing and down half to zero voltage when compressing...
  The screen voltage does not vary because so much current is passing down the voltage divider.  The plate voltage does of course.

DaveP
Soundcloud: Delayed Action.

5v333

Re: RCA 96-A from scratch
« Reply #18 on: February 18, 2020, 07:43:56 PM »
Ok im prob seeing this the wrong way but it looks like the sidechain signal gets rectified and mixed with the screen voltage via resistor net.
The grid is just fixed bias instead of being used as the control input.

Oh yes the 86-a is more complete.

ruffrecords

Re: RCA 96-A from scratch
« Reply #19 on: February 19, 2020, 03:58:52 AM »
Ok im prob seeing this the wrong way but it looks like the sidechain signal gets rectified and mixed with the screen voltage via resistor net.
The grid is just fixed bias instead of being used as the control input.

Oh yes the 86-a is more complete.
There is a big pot divider that provides various dc voltages. One is for the screen and another is for the bias to the side chain.

Gain reduction is achieved by changing the grid bias on the first two tubes only.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


 

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