RCA BA-6A from scratch

[Walter66]

To me, the 6386 (or a pair of 6BA6) do have a "sound"...Call it a coloration if you like.

All this makes me wonder if there's some specific sound or coloration that the 6SK7 tubes (in the BA-6a) have?
The Gates SA38/39 limiters seem to sound more like the RCA BA-6a to me., and those use 6L7/1612 tubes for the gain reduction.
Both are octal tubes.
They both have a little more 'in your face", forward kind of sound than the 6386 based compressor/limiters.

Of course transformers and other passive devices add some color of their own, but I wonder if the gain reduction tubes hold most of the coloration in these compressors.?. It might be interesting to hear an RCA BA-6a clone with 6J7/1612 gain reduction tubes. Perhaps I should try to make up and adaptor for my BA-6a limiters...to be able to plug in 6J7 tubes. Would the Gates SA39 sound more like an RCA with 6SK7 gain reduction tubes? It actually sounds kinda similar anyway.

I guess I should take a closer look at the specs and curves for those octal variable mu tubes.

Just some speculation and things to ponder from an old guy with old ears.

Dave O.

Of course, every active and passive parts in electronic gear do produce "sound". We don't know why every single part inside of an amp makes a different sound, but in summary, those puzzle pieces do result in a "sound impression". Electron tubes do sound different, from types, working points and manufacturer differences, too.
It wasn't about sound compression at all in the studios, it was about sound effects, too. And they did know how to enhance original sound to let it improve, just the way like today in digital, but they used analog only.

 

[xazrules]

thank you for the sa39 schematic. 6j7 are the same as 6sj7 and it is possible to use them as remote cutoff? i can't wait to try it, because i frikkin love the sound of that family of tubes, like 6sj7, 6ag7, 6ac7.

 

[xazrules]

i wonder if there are differences too beetwen 6H6, 6AL5 and solid state diodes, in the knee? slope?

 

[DaveP]

I made a SA-39b and was surprised by how well it worked without an interstage, it did not thump at all. The 6L7/1612 tubes may be part of that but later designs stopped using them due to the increased partition noise from all those extra grids.

best
DaveP

 

[Dave Obergoenner]

thank you for the sa39 schematic. 6j7 are the same as 6sj7 and it is possible to use them as remote cutoff? i can't wait to try it, because i frikkin love the sound of that family of tubes, like 6sj7, 6ag7, 6ac7.

The 6L7/1612 is quite a different animal than the 6SJ7. 6SJ7 is much like a 6J7...Not a 6L7.

You can wiggle the grid voltage in just about any tube and get gain reduction. That trick can even by played with a 12AT7. MarkBass does that in their tube compressor pedal, and it does work for about 10db of gain reduction. I had one, and liked it...till it was stolen.

The 6SJ7/6J7 is a "sharp cut-off" tube according to GE. So you might get an odd compression slope using one for gain reduction. I don't know. I've never tried one of those in a gain reduction stage. It does make a dandy high-gain amplifier stage (after the 6L7 in the SA39) however.

The 6L7/1612 is an odd tube (with 5 grids no less!) which can be "remote cut-off" on one of it's grids. It was really designed to be used in the mixer or front-end of radios back in the 1930s. Although it has 5 grids, it's not a pentagrid converter tube. It required an external oscillator to work that way. It's an odd duck to say the least, but it does work well in the Gates SA38/39 limiters, and they last a very long time. Years of 24/7 service, and they aren't very expensive.

Back in the early 1970s when I was using/listening to Gates SA38 and SA39 limiters everyday in radio (both AM and FM stations) those tubes were something like $1.70 each. It was a very popular compressor/limiter. Partly because it lasted forever, was very cheap to maintain, and easy to repair if it ever broke (most didn't). There were still quite a few on the air into the 1980s. In the Midwest, for every RCA BA-6a that was in use there were probably 4 or 5 Gates SA38 or 39. Gates was in Quincy IL., so they sold tons of them there. The typical processing set-up for stations in the Midwest in the 50s, 60s and into the 70s was a Gates Sta-Level, followed by an SA38 or 39. And they sounded great together, as long as the audio chain was fairly quiet. Since they were un-gated, they would really pump-up the noise during quiet parts of songs. Better have good idler-wheels in the turntables.

I have tapes of all Gates broadcast audio chains (CB500 turntable, phono pre, air console, processing, transmitter, modulation monitor, and then through the production console) at some stations, and they sound amazing. You'd never know you were listening through about 30 audio transformers!
But I digress...sorry.

Dave O.

 

[xazrules]

it could be amazing if you can share some of those tapes! especially some funk, soul, garage rock music.
i've looked on internet but at the moment those 6L7 are a bit pricey. 6SK7 still cheap as 6SJ7 i'm going to try them, i'm always on the fence looking for cheaper and cheaper tubes and i've found this page:

https://www.angelfire.com/electronic/funwithtubes/rc_pentodes.html
it's fun to see that some unknown tubes are super cheap

 

[Gavin Pursinger]

I built a single ended comp using a 6L7 as 1st tube...just for fun I put a well used 6J7 in its place...it worked in my build and I liked the sound of the 6J7 without much gr vdc applied to the screen...it did NOT hold up well as control voltage was increased.....pls do not buy up all the 6L7/1612 tubes !

 

[emrr]

6J7 used for a little preliminary gain reduction in the Gates 17/27/28 limiters, good for about 3dB there, from my tests. Otherwise never seen in that usage.

 

[Dave Obergoenner]

BTW...Just to add to this compression discussion mix a bit...

Seeburg (and perhaps others) also put compressors in their jukeboxes.
My late 1950's VL200 uses an octal 6SK7 (single-ended) for up to about 18db of GR.
The later tube jukes used the 6BJ6 (7-pin).

Slopes were shallow at about 3:1 and time constants quite slow at 10-16 seconds for recovery.

They were just trying to keep some hot-cut 45's from blowing the windows out of places, and soft ones from being inaudible.

Dave O.