RCA Ba2A Pre amp qestions & answers

[emrr]

Not grasping the usefulness of a 2k2 dummy load.  Just start with higher value R8/9 on first power.    Use 10K to be safe, and work down.    Or use stock values and bring up with a variac while measuring, don't go past 290VDC.   

 

[lassoharp]

lassoharp - how did you get the 5.6ma figure?  I'm still getting to grips with Ohms Law, and no matter what figures I try out from the ones emrr gave earlier, I can never get 5.6ma.  I'm keen to learn so your workings out would be of interest.

From the factory chart you're given a cathode voltage (V).  You then have to refer to the schematic to get the cathode resistor value (R).  Since RCA has meter insert points in the cathode circuit the total cathode resistance is broken up into two portions.  So for this calculation just ignore any small effects a meter may have and just add the resistances to get the total.  I used a standard value (2.2K) for V2 even though  it's actually 2270ohms.  So now it's a matter of just solving the equation for current (I) through each tube and adding them together for the total.

 

[letterbeacon]

Thanks both of you - you both make perfect sense!

 

[letterbeacon]

So I'm just sitting around waiting for all the parts to turn up now -slightly frustrating!

In the meantime, I'm looking at adding an output attenuator after the transformer so I can drive the amp a bit if I feel the urge!  I was thinking of building NYD's design from this thread: http://www.groupdiy.com/index.php?topic=4825.0 and adding another pole to make it balanced. 

The OP transformer on the BA-2 has an output impedance of 500ohms, so if I change the 600ohm resistors on the attenuator schematic to 500ohm, then is should work, right?

 

[lassoharp]

Forum member nielsk had some nice balanced T attenuators for sale.  I've used one for the output of an Altec 436 clone I built and they work great.  Price is great too.

http://www.groupdiy.com/index.php?topic=44746.msg560548#msg560548

 

[letterbeacon]

Great, thanks for that.  Looks like they're 600ohm, will that work ok with the BA-2C?

Cheers!

 

[lassoharp]

It should work fine.  I think mine actually measured about 570ohms with a DMM.

If you have a software setup for measuring freq response you can always compare a 600r termination to a 500r termination and see if there are any differences worth worrying about.

Check emrr's thread here for some relevant info on differing terminations:

http://www.groupdiy.com/index.php?topic=41032.0

 

[emrr]

500-600; effectively the same thing. 

 

[letterbeacon]

Still twiddling my thumbs until some essential bits turn up.  I've soldered the turret boards though, so that's some progress.  Starting to think I should have done it just P2P as the turret boards take up a fair bit of space!

C-2 - the coupling cap between the two tube stages, what would be a safe voltage rating for it?  I've got a .47uf Vitamin Q that I was thinking of using, but it's only rated at 100v.  R-10 and R-3 should be dropping the voltage a fair bit though, shouldn't they?

 

[lassoharp]

C2 must be able to handle the full peak voltage of the PT that appears during power up before the tubes start conducting.  (way way >> 100V)

Now, if you were to install a stand-by switch, this would prevent C2 from seeing such a high spike in the voltage at power up, but it will still be seeing the full plate voltage of V1 plus a smaller momentary rise after the stand-by switch is hit.

100V is within limits for the ~73V plate voltage of V1 but the momentary surge would probably be over 100V.  It may take it for hundreds of turn ons or it may start leaking oil after a few hits.  Again, this is for a stand-by switch case - normal turn on would greatly over voltage a 100V cap.  I'd save that Vit-Q for an EQ project

 

[letterbeacon]

Thanks lassoharp, I've managed to get hold of one rated at 630v now.

I've been reading up on grounding and I've come up with the attached simplified diagram which shows how I'm going to attempt to wire the grounds in the pre amp.  It's extremely simplified and only shows the ground wires.

I've been using this page as a reference:  http://www.aikenamps.com/StarGround.html

I have two turret boards in the enclosure, one containing the pre amp parts and the other the power supply parts.  The 40uf cap shown on the power supply board is the first filter cap in the power supply circuit.

The dark green line represents the bus ground, while the lighter greens represents the ground wires going from the off board components to the turret boards.

Have I done anything terribly wrong here?  I'm not entirely sure I've got the shields on the input/ output cables wired correctly.  I'd really appreciate it if someone could look over this diagram and flag up anything that doesn't look right.

Thanks a lot!

 

[lassoharp]

The sec of your OT should be floating, so the "lo" side to pin 3, not earth - if that's the way you had it shown.  You would usually want to either connect pin 1 (shield) of the output jack directly to the chassis (ground tab or screw on jack) or just leave it disconnected.  I've done it both ways. 

Same thing with pin 1 of the input jack.

The other grounds look reasonable.  You may want to try grouping the two 1620 stage grounds and the input transformer sec ground to a single short local point and then running a single longer wire from that back to the main star - as opposed to running the individual long wires from each stage back to the star.  It may not create any problems as is, but the reasoning behind the shorter wires makes sense for heading off any loops. 

 

[letterbeacon]

Thanks for the feedback.

So for the 1620 shields and the input secondary I should make a sort of mini star point which should then connect to the main star?  I just realised I missed out the shields on the wires leading to the grid caps, I'll connect that to that mini star too I think.

One thing I don't fully understand is why, in the article I linked to, it says that the star chassis ground point shouldn't be connected directly to the earth ground point.  If they're not directly connected, they're surely still connected via the chassis, so why does it make a difference?

 

[DaveP]

Letterbeacon,

Remember that all connections whether bolted or soldered have a resistance even if milli-ohms.

It is good practice, not to use the chassis as a universal ground but to use it as an extension of the XLR in/out shielding, so that a screened cable comes in, extends around the circuit in the chassis then goes on out via the XLR out.  So the XLR's pin 1 needs to be soldered with a thick wire to the lug on the socket that connects to the chassis.  Any screened wire within the circuit can also have the screen connected to the pin 1.

Star grounding works, as does a large dia busbar, both should be connected to the chassis at the same point with the Mains earth/ground wire.  The logic is that due to the star, every circuit's earth travels directly to earth, it does not have to carry the earth currents of another part of the circuit, or indeed any hash or RF picked up by the in/out cables or chassis.  A busbar works slightly differently in that the sensitive parts of a circuit are always "upstream" from the heavy current parts.  Electron earth currents are like water, they don't run upstream, but always towards the nearest earth.

When you take these principles onboard you can confidently design your own layouts for any circuit.

best
DaveP

 

[letterbeacon]

Thanks Dave.

I've attached another (scruffy!) Paint drawing taking on board yours and lassoharp's notes.  I don't quite understand why the noise picked up by the shields which is supposed to flow out of pin 1 of the output XLR won't flow out of the power inlet if they're both connected to the chassis.

I've attached all the shields to the shield of the input 1620 which then goes to pin 1 on the input XLR.

I've placed the components on the pre amp turret board so the sensitive input components are over on the left and is far away from the power supply grounds.

As before, I'd appreciate any notes - thank you!

 

[DaveP]

Letterbeacon,

I can't make out what all those green wires are doing, there does not appear to be a star point.

This Jensen schematic is the business:-

Maybe the picture will explain it better

best
DaveP

 

[lassoharp]

I think what letterbeacon is asking - and it's a logical and natural question - has to do with why the "common Z coupling" is prevented when everything ultimately references the chassis. 

They speak of separation - keeping shield noise out of signal commons - and it's natural to ask how this is achieved when electrically they are at one level all equivalent.  When you put your DMM probe on the chassis at one end and measure either to the other end of the chassis or to any of the ground points the meter says:  "Same Place".

I like to think in terms of loop potentials.  If you ground pin 1 XLR shield direct to chassis at POE and also run a wire to the 1st stage ground bundle you have a potential to get that noise going around in circles though some of it still going to ground.  With a single path to chassis only it's more likely to go straight to ground and stay out of everything else.  If pin 1 is ran straight to circuit grounds a similar thing could happen - part of it goes to ground and part of it loops around the amp circuit - and it only takes a very small amount in a gain stage to be heard.

Sometimes anomalies crop up.  I've had to break away from otherwise good star grounding in guitar amps before - separating the V1 cathode ground from the star and placing it on the input jack ground - because nothing else would work to stop the humming.

 

[letterbeacon]

Between your two replies, I think I get it now.

lassoharp, that is exactly one of the questions I was asking, thank you.

DaveP, thanks for the diagram -I think that is pretty much what I have.  All the green lines that go to one of the 1620 stages are the shields from the signal carrying cables which are going to a mini star point (isolated from the chassis) and then go to pin 1 of the input XLR.

There is no star point because, for some reason, in the article I linked to earlier it said don't link the signal ground to the safety ground, whereas in the Jensen diagram the two are joined.  On my diagram, if I join the points labeled 'bus bar ground' and 'earth' then I think it will match the Jensen diagram.

Thanks a lot!

 

[DaveP]

Ok I see what you are getting at,

The meter will register lots of points as at ground potential when in fact there are subtle differences as I indicated in my earlier post.  Most DMM's don't register milli ohms and the subsequent voltage drops, but the amps amplify them up to nuisance levels.  As Lassoharp says, the last thing you want is a loop, its a recipe for hum.

Here's an analogy, if a piece of guttering is dead level, water may collect in some parts because there is no incentive to move in any direction, when its full it will flow but when the currents are low it doesn't flow.  By putting a slight fall from one end to the other there is always the incentive to move to the lower level, that is what a busbar or a star ground does because it is the lowest level possible.

One other point, if there is a decoupling cap for each stage, return the negative as close as possible to where the cathode resistor is grounded, this reduces the length of current carrying wire to a minimum and there is less chance of inducing a current into sensitive areas.  Same goes for large dia. wiring, there is less of a voltage drop in heavy wires to be picked up and amplified (milli ohms again)

best
DaveP

 

[letterbeacon]

I'd be very interested in seeing that!