RCA Ba2A Pre amp qestions & answers

[My3gger]

Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

Electrically it should work and pass signal,  but it will not give bass response using it in this circuit.

Hi,

i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense. What do you guys think about using one of transformers i mentioned, anyone tried something similar, what are your thoughts about it? I red some good posts about this matter in RCA BC-2B thread, as i understand it i could loose some bottom, but ungapped model should otherwise act similar to gapped with little current as here and big core.
IIRC UA originally used ungapped tx in 610 and there is 8mA in primary, here we have 4mA which doesn't seem much. But i can't really measure what i get with ungapped, this is why i would like to hear opinions. If any of my transformers could work ok for test and i like the sound i'm sure i will somehow buy proper transformer.
Thanks for any help!

 

[My3gger]

I hope i'm not asking too many questions and too much. I really searched well to find at least some answer which don't tell me much. The problem is that i can't measure what i get in lows; i have a scope and am collecting money for good generator.
I would be very happy to know about experiences with dc in ungapped tx, or maybe what you think about it.

 

[buildafriend]

In the kindest possible way, what is holding you back from getting a proper OP transformer? mine is listed here a 1 or 2 pages back. I believe the UTC A25 is an option as it is for the BA-2C.

I like people who ask questions.

 

[lassoharp]

Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

Electrically it should work and pass signal,  but it will not give bass response using it in this circuit.

Hi,

i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense. What do you guys think about using one of transformers i mentioned, anyone tried something similar, what are your thoughts about it? I red some good posts about this matter in RCA BC-2B thread, as i understand it i could loose some bottom, but ungapped model should otherwise act similar to gapped with little current as here and big core.
IIRC UA originally used ungapped tx in 610 and there is 8mA in primary, here we have 4mA which doesn't seem much. But i can't really measure what i get with ungapped, this is why i would like to hear opinions. If any of my transformers could work ok for test and i like the sound i'm sure i will somehow buy proper transformer.
Thanks for any help!

For the ungapped Sowter you'd want to go to AC coupled (cap feed) output stage for best results.  That would require a redesign.  There are some potentially simple ways to do that.

If you want to stay DC coupled then you're narrowed down a bit on choices.  UTC A-25 and LS-27 . . . they're out there but already in the nutball price zone for the most part.  Cinemag has the one 7:1 that's similar to recreation of the one for the old UA preamp.  I'd email David at Cinemag and tell him what you're looking for - he may already have something in the files.  You can also try Lundahl.  They have a couple different ones in the right ratio range.

http://www.kandkaudio.com/transformers.html 

 

[My3gger]

In the kindest possible way, what is holding you back from getting a proper OP transformer? mine is listed here a 1 or 2 pages back. I believe the UTC A25 is an option as it is for the BA-2C.

I like people who ask questions.

Money is holding me back, i wrote i can't afford another transformer at the moment. Someone just asked me if i would sell one or two of my ungapped OTs i don't really need now, so it seems i have a solution.
Thanks for replies.

 

[dmp]

i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense.

I don't think you'll be building a BA-2 unless you have the right kind of transformers. But you may still build a great preamp, though.
Look at Gates designs for alternatives that are resistor coupled (SA-70, for instance). It's a very similar preamp to the ba-2
Lassoharp describes the situation well with crazy prices for the relatively few options out there for transformer coupled..
Of course, you can try if you want and I would be interested to hear how successful you are.  You sometimes learn the most from trying things that don't work out.
This forum needs people to start winding there own transformers for this application!

 

[peters]

Lundahl LL1680 might be an option. Its advertised as "made to match or exceed the specs of the UTC transformer LS-27". Its not cheap though.

I'd be curious if anyone has used it...

 

[My3gger]

i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense.

I don't think you'll be building a BA-2 unless you have the right kind of transformers. But you may still build a great preamp, though.
Look at Gates designs for alternatives that are resistor coupled (SA-70, for instance). It's a very similar preamp to the ba-2
Lassoharp describes the situation well with crazy prices for the relatively few options out there for transformer coupled..
Of course, you can try if you want and I would be interested to hear how successful you are.  You sometimes learn the most from trying things that don't work out.
This forum needs people to start winding there own transformers for this application!

Yes, SA-70 is another cap coupled option. I built one with EF86/half 6CG7 and later changed it to EMRR's WE 141-A mash-up (also EF86 and 6CG7). Since then it stayed this way, i like it because it sounds good and different from anything i tried (no OT). A friend who has it now uses it all the time.
I thought about just trying ungapped OT in BA-2, but it would only make me think obsessive about how it would sound with right OT It will take some time until i get it together with gapped tx and compare to ungapped. I will ask guys at Cinemag about BA-2 OT this weekend and report back about model and price.

 

[lassoharp]

There's also NYDave's 6SN7 One Bottle, and while on the subject of mash-ups - I don't see any major reason why subbing NYD's OB output stage onto BA-2 wouldn't work just fine.  That would keep output stage open to subbing any non gapped OT of choice.  May have to make some minor tweaks in the power supply.



Also - Ian's(ruffrecords)  mu-follower mic pre is an excellent sounding design that uses no OT, or can use a 4:1 if desired.

 

[My3gger]

6SN7 One Bottle mash-up is a good idea, i've just done some work about it. It is incredible  how solutions are found in this place For mash-up i should probably leave 10K R (from reg. dc) for BA-2 input anode, do you maybe know which R is needed for One Bottle output part?

Do you find Ian's mu-follower very clean in use even with transformer, could you explain that a bit? I built a few European classics and many of them are pretty clean. This, interstage pot and trioded pentodes made me explore American no-nfb designs. I red Bob Olhsson's post where he says RCA and WE controlled most of audio patents worldwide until late 40's, which is interesting because not long ago people in Europe didn't know much about American electronics.

 

[lassoharp]

do you maybe know which R is needed for One Bottle output part

Plate load resistor?    Think Dave's SN7 one used 10K. 


I built up Ian's circuit on breadboard and played with for a few weeks.  Yes, it's a very clean low distortion design but the distortion profile leans toward predominant 2nd order.  I'd suggest going to Ian's website for in depth reading.  He explains that particular circuit. 

Are you looking for something to give more distortion?  The Collins 6Q is good one for that and use of non DC coupled output.  Interstage vol is easily added.   

 

[ruffrecords]

Do you find Ian's mu-follower very clean in use even with transformer, could you explain that a bit? I built a few European classics and many of them are pretty clean. This, interstage pot and trioded pentodes made me explore American no-nfb designs. I red Bob Olhsson's post where he says RCA and WE controlled most of audio patents worldwide until late 40's, which is interesting because not long ago people in Europe didn't know much about American electronics.

It is pretty clean and for two reasons:

1. The 6CG7 tube is electrically identical to the 6SN7 which well known to have the lowest intrinsic distortion of all the popular audio triodes.

2. It is no good using an inherently low distortion tube if you put it in a circuit that prevents it achieving its potential - like putting an Olympic swimmer in a pool of treacle. The main cause of distortion in triodes is loading the tube's plate resistance ra. The mu follower uses a clever bootstrapping technigue to make the plate load look like a much higher value resistance which reduces distortion to close to the tube's intrinsic level.

The distortion produced by a mu follower not surprisingly depends on the load. That's why I recommend only feeding it into 10K bridging load. If you want to make it distort more then load it more but be aware this also reduces headroom.

A circuit closely related to the mu follower is the SRPP. This has much better drive capability than the mu so it is ideal if you want a 'dirtier, sound. If you look at the REDDI thread you will will see where I gave details of converting the mu follower to the very similarvSRPP.

Cheers

Ian

 

[lassoharp]

Since Ian's here to explain the details I'll just say that it's a very good sounding circuit and I have it on my to build list of preamps.

Good versatility on output options - transformerless or use of non exotic low ratio stepdown.

 

[My3gger]

Thanks Ian for explaining it. I followed your writing when you told us how to change mu-follower into srpp, it was in another thread some time before REDDI. Your pdf's about 6SN7/6CG7 differences and mu-follower distortion are reference reading for me, very revealing. I learned a lot and it also made redesigns easier because i didn't have to change sockets to octals.
Lassoharp, i meant R8 (4k7) where HT comes into NYD's Simple6SN7 preamp (sorry i can't add schematic for you, i'm having some problems). I think it should be different if i put BA-2 input in front.

 

[lassoharp]

Yes, it will likely take some redesign on the PS decoupling resistor.  Dave's design calls for a higher supply voltage.  Should be no be problem as there are a wide range of PTs readily available.  Might be good to start that as separate thread with both schematics on hand.

 

[peters]

This is my first attempt at a turret style build. The plan is for 2 amp channels with shared power supply in a 2RU case. I'll be using 5693 (6SJ7) tubes, Lundahl LL1680 and LL1578XL transformers. This will be kind of a modern version. I did splurge on some Jupiter Beeswax caps for C2 for some old time flavor.

 

[emrr]

Cool.

I wouldn't waste a 5693 on V2, only on V1.  It will make no appreciable difference at V2.  In fact, I'd be sure to check and see if it even really does at V1.  A good 6SJ7 may equal it at this gain. 

 

[lassoharp]

When I get time I'll try and post my turret layout.  Just finished a single channel of these.  Took about 15 slots I think.

 

[buildafriend]

When I get time I'll try and post my turret layout.  Just finished a single channel of these.  Took about 15 slots I think.

That would be great! I would definitely like to see have a print out of it in my files.

 

[CJ]

you might want to mount the 6SJ7 tube sockets on rubber grommets, or mount the sockets all on one plate and then mount the plate on rubber grommets,

this is because the 6SJ7 can make a lot of noise when you tap on them.

they make a glass version that sounds real smooth,