RCA BA-6A from scratch

[duffharrisnyc]

Hi Dave -
Been following your posts for a while. One question is bugging me about the original layout. On S1-A the schematic shows 3, 4, and 5 (as well as 1 and 2) connected but the layout shows them being separate. Do you know what I should do if I wanted to keep mine the same as the original?

 

[DaveP]

Hi Dave -
Been following your posts for a while. One question is bugging me about the original layout. On S1-A the schematic shows 3, 4, and 5 all connected but the layout shows them being separate. Do you know what I should do if I wanted to keep mine the same as the original?

They are all the original way of wiring. Look at it this way, S1 is just a five way switch with four poles or four wafers.
best
DaveP

 

[duffharrisnyc]

They are all the original way of wiring. Look at it this way, S1 is just a five way switch with four poles or four wafers.
best
DaveP

I got a 4 pole 6 way switch over two wafers but some terminals seem like they’re unused maybe. Just used as jumping off points? (I.e. pin 7 on s1-A)

So on S1-A there are no jumpers?

 

[DaveP]

I got a 4 pole 6 way switch over two wafers but some terminals seem like they’re unused maybe. Just used as jumping off points? (I.e. pin 7 on s1-A)

So on S1-A there are no jumpers?

Well if you have a 6 way switch and the circuit only calls for 5 positions and if you can fix the stop at 5, then I guess you can use the spares for other connections.

Personally, I don't understand the logic behind making a perfect copy of a BA-6A, it would only make sense if one was setting up a 1950's radio station and the engineer was instructed to go through the switching/checking regime at the start of his shift! With modern components, useage and suitable ventilation, it is much less likely that components will drift much over time, so the tube cathode voltage measurement is probably a waste of time in my opinion. If you want to do some home recording, then a wider range of attack and release timings would be preferable.

With any design, you first have to decide on what you want the product to do, then you can choose components to make it a practical reality.

I mean this with respect because we have all been there, but if you don't understand what the design is trying to do, then you have no alternative to making an exceedingly exact copy of the original. If it is just a challenge to reproduce a copy and sell it on as an original then again you copy every detail.

Sorry to be a bore but the "painting by numbers" approach to electronics just seems wrong to me. Rant over!

best
DaveP

 

[duffharrisnyc]

Well if you have a 6 way switch and the circuit only calls for 5 positions and if you can fix the stop at 5, then I guess you can use the spares for other connections.

Personally, I don't understand the logic behind making a perfect copy of a BA-6A, it would only make sense if one was setting up a 1950's radio station and the engineer was instructed to go through the switching/checking regime at the start of his shift! With modern components, useage and suitable ventilation, it is much less likely that components will drift much over time, so the tube cathode voltage measurement is probably a waste of time in my opinion. If you want to do some home recording, then a wider range of attack and release timings would be preferable.

With any design, you first have to decide on what you want the product to do, then you can choose components to make it a practical reality.

I mean this with respect because we have all been there, but if you don't understand what the design is trying to do, then you have no alternative to making an exceedingly exact copy of the original. If it is just a challenge to reproduce a copy and sell it on as an original then again you copy every detail.

Sorry to be a bore but the "painting by numbers" approach to electronics just seems wrong to me. Rant over!

best
DaveP

Well I’m relatively new to this so I’m trying to use it as a learning experience. I imagine the guys at RCA knew what they were doing! I’m a recording engineer primarily. I have most of the original parts and find the original design interesting. In my line of work I’m aware of how many engineers base their opinions on if a piece is vintage or not so that’s why I’ve amassed the originals for most things. It is dumb but I have heard engineers tell me they wouldn’t buy a piece of gear that is the same circuit as a Fairchild simply because it was on a pcb!

Obviously resistors and capacitors are new, switches will be too, silicon rectifier of course, etc. I just wanted to do the same point to point build as a project in order to learn. Would love to make a stereo one in the future for myself with your layout that can be a bit smaller!

Regarding the switch though, I totally get stopping it at 5. I have done that. Sorry if I’m not being clear. What’s confusing is which positions are not being used as stops at all on the wiper. It seems like some are literally just solder points and are not being touched by the wiper. I just wasn’t sure which. Your schematic actually helped me narrow down the second wafer. It’s just the first that’s a bit confusing.

 

[emrr]

I got a 4 pole 6 way switch over two wafers but some terminals seem like they’re unused maybe. Just used as jumping off points? (I.e. pin 7 on s1-A)

So on S1-A there are no jumpers?

Those are clearly jumpers!

 

[DaveP]

It is dumb but I have heard engineers tell me they wouldn’t buy a piece of gear that is the same circuit as a Fairchild simply because it was on a pcb!

With Point to Point, you can cut out a faulty component and solder another one in without damaging much at all, with pcb you stand a good chance of lifting the foil, I guess this is one of the reasons they say this.

I'm glad to hear you are using it for recording, even just running the signal through it with the original components should sweeten up your digital.

best
DaveP

 

[emrr]

One question is bugging me about the original layout. On S1-A the schematic shows 3, 4, and 5 (as well as 1 and 2) connected but the layout shows them being separate. Do you know what I should do if I wanted to keep mine the same as the original?

the layout drawing - you can't draw it if you don't explode it out.

S1-A/B are the same switch. The dotted line shows they move together.

S1-A disconnects the output during the balance tests and sends it to the rectifier feeding the meter switch. You can't have both connected at the same time.

Only 1 terminal of 20 is unused. Terminals are connected to each other in many cases.


Read the manual about setup adjustments, decide if there're any you can really do without. The choices are well thought out and save a lot of time when there's signs of a problem, many that without would leave you guessing and swapping tubes blindly.

 

[Dave Obergoenner]

I'm SO glad to see a discussion about the BA-6a limiter here! And congrats to all that have built their own.

Own two of the originals, which came from radio stations in my area. I spent a lot of time working on them. Replacing plenty of bad caps, and a few bad resistors, cleaning switch contacts, etc. I also spent a lot of time matching them for stereo performance. They track beautifully now, but did not at first. Time constant components do drift over time.

Nothing sounds like them! No other tube compressor (and I have lots of them) and no plug-in comes close. They are smooth as silk, but also have a bright and lively sound to them. Ask Jimmy Page if he wants to use his BA-6a's, or a plug-in, and see what he says. They deserve the respect they get!

Dave O.

 

[xazrules]

Hello, i'm experimenting with a ba6a topology but with tubes that i have at hand, 6ba6 into ecc81 into 6sn7. i can reach -70db in the sidechain before clipping, but i can't make the sidechain (100nf as smoothing cap and 3.3meg CT resistor) to release slower, it is always full throttle even with a 10meg resistor, i can make the attack slower with the typical series resistance. but i've found that if i add a 22k feedback resistor from the plate of ecc81 to the plate of 6ba6 it works as it should, and now i can make the release variable from very slow to super fast. how does it possible? i really like to know why! thank you

 

[DaveP]

I would say that your 22k feedback resistor has reduced the gain, so you have less side-chain voltage to charge the 100nF. You could probably achieve the same effect by increasing the smoothing cap from 100 to 220nF or even 470nF. You could make a few tests to check this out.

Best
DaveP

 

[xazrules]

Hi Dave, yes upping the smoothing cap slowed the release but the attack too. Yes i have less voltage going to the sidechain, but if i Remove this nbf resistor and use the nfb like the ba6a, 6sn7 plate to ecc81 cathode, i have less voltage at the sidechain but the release is always in the fastest setting. Maybe i’m changing the operating condition of the 6ba6? But how this conditions affect the time costant? Aren’t they two “independent” part of the circuit?

 

[DaveP]

How are you measuring the release time?
Also I think you meant that the side-chain voltage was -70V not -70dB

best
DaveP

 

[Dave Obergoenner]

Hello, i'm experimenting with a ba6a topology but with tubes that i have at hand, 6ba6 into ecc81 into 6sn7. i can reach -70db in the sidechain before clipping, but i can't make the sidechain (100nf as smoothing cap and 3.3meg CT resistor) to release slower, it is always full throttle even with a 10meg resistor, i can make the attack slower with the typical series resistance. but i've found that if i add a 22k feedback resistor from the plate of ecc81 to the plate of 6ba6 it works as it should, and now i can make the release variable from very slow to super fast. how does it possible? i really like to know why! thank you

Just a few observations from someone with a wide variety of tube compressor/limiters...

I've done a lot of experimenting with the 6BA6 for a gain reduction tube in my Gates Sta-Level compressors. Running one Sta-Level with a pair of 6BA6's (with an adaptor) and the other with the original 6386. I couldn't tell any difference in the sound of the two. I even did out of phase comparisons, then summing, and adding gain to the result. There was really no audible difference. I like 6BA6 tube a lot. It's cheap, and easy to match a pair.

Perhaps it's a difference in the tubes, transformers, etc, but my Gates Sta-Levels sound nothing like my RCA BA-6a limiters. The Sta-Level compressors are mellow, and laid-back sounding, while the RCA's are much more "in your face" sounding.

Some of this may be the time constants, and the compression slope, but seems like what you are building will sound much closer to a Sta-Level clone, or a Collins 26u clone than a BA-6a clone. I have pairs of all three of those, and the RCA sounds completely different from the Gates or the Collins (which both use 6386 for gain reduction). The Collins 26u sounds more like the Gates Sta-Level btw...just faster and steeper sloped. Things run through the RCA really jump out more in a mix. I don't have a real good explanation for why...it just is what it is.

Dave O.

 

[xazrules]

How are you measuring the release time?
Also I think you meant that the side-chain voltage was -70V not -70dB

best
DaveP

Yes i meant -70vdc, i’m measuring voltages with a multimeter injecting a 1khz tone to the input. I’m measuring release time by ear! I’ve already built a proper ba6a and comparing the two.

Just a few observations from someone with a wide variety of tube compressor/limiters...

I've done a lot of experimenting with the 6BA6 for a gain reduction tube in my Gates Sta-Level compressors. Running one Sta-Level with a pair of 6BA6's (with an adaptor) and the other with the original 6386. I couldn't tell any difference in the sound of the two. I even did out of phase comparisons, then summing, and adding gain to the result. There was really no audible difference. I like 6BA6 tube a lot. It's cheap, and easy to match a pair.

Perhaps it's a difference in the tubes, transformers, etc, but my Gates Sta-Levels sound nothing like my RCA BA-6a limiters. The Sta-Level compressors are mellow, and laid-back sounding, while the RCA's are much more "in your face" sounding.

Some of this may be the time constants, and the compression slope, but seems like what you are building will sound much closer to a Sta-Level clone, or a Collins 26u clone than a BA-6a clone. I have pairs of all three of those, and the RCA sounds completely different from the Gates or the Collins (which both use 6386 for gain reduction). The Collins 26u sounds more like the Gates Sta-Level btw...just faster and steeper sloped. Things run through the RCA really jump out more in a mix. I don't have a real good explanation for why...it just is what it is.

Dave O.

Thank you very much for this! This is really precious for me, i’m really into tube comp because every topology has its own sound with a unique footprint, except the 660 they are on the dirt side and makes everything sound more present and euphonic and you can use them as mic pres!
I have a lot of fun get inspired by this classic designs, and i think that the ba6a topology is the best sounding and not so expensive to build and now i’ve found that one can scale the power down and have a beast of a compressor. Do you think there could be diffences beetwen 6sk7 and ba6a?
Please report other findings and experience i can listen for years!!

 

[DaveP]

Do you think there could be diffences beetwen 6sk7 and ba6a?

I think you mean the 6BA6, If you download the charts of these tubes for comparison, you will see that they are both capable of operating down to very low voltages on their grids. The proper way to check is to redraw the data of one tube over the other. There may be slight differences in the curves, but these differences may be within normal tube to tube variables of the same type. One thing that has been proved and documented on this forum is that the 6BA6 has an identical curve to the 6386 in triode mode as Dave O has said.

If you want your new circuit to perform like an RCA BA6a, then your output stage will need to have the same output impedance in order to charge the capacitor with the same power. The feedback circuit lowers the output impedance. You can check the output impedance with two voltages and this formula Zout = Rtest (Vout - VRtest)/VRtest

Measure the output voltage with 600 ohm load (Vout), then add say 4700 ohms (Rtest) across the load and measure again, (VRtest)

You can compare the output Z of your existing BA6a to your new circuit.

 

[xazrules]

Thank you very much Dave! This is gold. Yes i’ve learned from building a 660 that the Zout has to be low to charge the cap fast, but in the ba6a the cap is 100nf and in the 660 is 2.2uf, the lower the capacitance the easier it is to charge, isn’t it? With 6sn7 biased at around 25ma 300v i hear no differences in the attack and release timing in respect to 6v6. My next project is a stereo ba6a clone in a 3u rack with external psu, i’d like to use 6bw6 as output because are really cheap on ebay, do i need to had one or two fan to keep the temp in check? If yes is it better to pull air in or push it out?

 

[DaveP]

No fan required, just mount tubes outside vertically or horizontally.

 

[Dave Obergoenner]

To me, the 6386 (or a pair of 6BA6) do have a "sound"...Call it a coloration if you like.
I make this judgment because (to me at least) the Gates Sta-Level, and the Collins 26u have a similar sound or coloration of their own. This in spite of dramatically different time constants and slopes.
Both are 6386 based for their gain reduction.
It is btw a coloration that I like...generally mellow, fat, or laid-back sounding if you will.
I have no Fairchild or any other 6386 compressor to back this up...but the Gates and the Collins do share a similar sound.

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.

 

[Dave Obergoenner]

Just for grins, here's the Gates SA39b schematic. Outside of the inter-stage transformer in the RCA, they are more similar than different.
Dave O.

View attachment SA-39b with DC Fil - 4150163073_b7c2b5b94a_o.jpg