RCA Ba2A Pre amp qestions & answers

[burdoto]

Output is strapped with 620 resistor. So it sounds like 6j7 and that OT are sucking my bottom end down?

So I should try a different OT? You know off the top of your head of an affordable option to swap one out?

One thing I'm confused about is preservation sound dude built the OP6 with an BA2A first stage and used an edcor wsm 15k:600 matching transformer and didn't seem to have problems like I'm having. As far I know I duplicated that same circuit and used the same OT. So I built bthe BA2A circuit with the SE OT I mentioned earlier and the hybrid OP6 circuit which the wsm OT, but both circuits exhibit the same low end loss. Makes me think I'm missing something else in the circuit. Is there something else I should be looking for?

 

[lassoharp]

I'm pretty sure Cinemag has made at least one SE OT that aspires to work well for a direct coupled output like the BA2.  I would email Cinemag and tell David what you're looking for and see what he has available.  About 4 years ago he was looking into doing a replica with a nickel core but I don't know if they completed that project or not. 

 

[kosi]

Small question, as I'm in the process of building one:

What's the purpose of the cathode resistors in series ? On the joints between the resistors are connections to the  MI-11599 plug, pin 7&8.

Thanks for any info !

 

[emrr]

Small question, as I'm in the process of building one:

What's the purpose of the cathode resistors in series ? On the joints between the resistors are connections to the  MI-11599 plug, pin 7&8.

Thanks for any info !

External tube check meter.  You can combine into one R if you like. 

 

[kosi]

Thanks  EmRR

 

[hylaphone]

May I bump the thread to ask about this Doug? What does this secondary-cathode connection accomplish?
Thanks --

Not much left to do, beyond using an input transformer closer to the original ratio. Cinemag cmqee-3440a is most appropriate in their line.  50k instead of 15K, buys a few more dB. 

Bypass the eq (r11).

Connect the input transformer secondary to v1 cathode through 0.5 mfd or more, rather than ground.

 

[emrr]

May I bump the thread to ask about this Doug? What does this secondary-cathode connection accomplish?
Thanks --

Not sure I recall the full context.  Since the cathode is bypassed it would not seem effective. 

 

[hylaphone]

Thanks --

Has anyone here compared the Cinemag CMMI-10C with that CMQEE-3440A?
I know the turns ratio is closer in the latter..
But I wonder if the former is a better fit for this build, at the expense of a few dB?
Obviously subjective, just looking for some advice..

 

[Winston OBoogie]

Has anyone here compared the Cinemag CMMI-10C with that CMQEE-3440A?

Not in an direct comparison.  But, fwiw, bigger core, more signal level handling before saturation, wider bandwidth with the CMMI-10C.    Not quite the same difference as, say,  between a UTC 0-1 and an HA-10 but, you get the idea.

 

[hylaphone]

Thanks guys. Does anyone know the operating level spec (or ballpark) for the original xt-3635 input?

 

[emrr]

Not published and I’ve never made a test, but they weren’t imagining anything over -30dbm at the time.  It surely does much more.

 

[hylaphone]

Would you say that transformer saturation is part of the sound of an original unit?
Or should I be looking at higher headroom iron?

I'm planning a build, and hoping to learn a bit about transformers

 

[emrr]

Doubtful.  The amp distortion is the main signature. 

 

[dmp]

Would you say that transformer saturation is part of the sound of an original unit?
Or should I be looking at higher headroom iron?

I'm planning a build, and hoping to learn a bit about transformers

The original transformers are big. I don't think they saturated in use.  If you are going to use this as a Studio preamp (i.e. max +4 dB) then headroom isn't a major concern.
Are you asking if the original transformers are important to the sound of the original unit? Probably
The circuit is a no feedback design and has a lot of character. But good transformers are surely in play as well.
If I were to recommend an 'obtainable' input transformer it would be UTC LS-10, as UTC was making RCA transformers, but they are expensive. Size wise physically it is similar. But any 10:1 will work.
The output is more difficult since it has DC. UTC A-25, LS-27, or switch the topology to not have DC in the OPT.
 

 

[IanB]

About to start a 2 channel build. Gonna stick the 6x4 PSU in another box. I think I’m ready thanks to you guys / this thread. Thank you!
I’ve got 2 Simpson meters I was going to run off a jlm buffer board strapped across the output xlr’s. Sound like a reasonable plan?
I’m using A10s for inputs and the gapped version of the 4:1 Carnhills Colin had made for LA2A builds. I’m going to put a 3 way output cap selector on a Tele switch for fun.

 

[rockinrob86]

I'm building one at the moment with a Carnhill VTB2290 10k:600 gapped OT.  Hopefully it works out!

 

[IanB]

I'm building one at the moment with a Carnhill VTB2290 10k:600 gapped OT.  Hopefully it works out!

I think it will work. I used the ungapped Carnhill backwards as input on the B/Z STA build and it sounds awesome! So it should work & sound great here.
Well I’m racing you - ordering cases from Frank this week.
Keep us posted!

 

[rockinrob86]

I think it will work. I used the ungapped Carnhill backwards as input on the B/Z STA build and it sounds awesome! So it should work & sound great here.
Well I’m racing you - ordering cases from Frank this week.
Keep us posted!

The race is on!  I've been working on some other projects, but just need to finish wiring this up

 

[rockinrob86]

I finished mine this afternoon, sounds great!  I ended up using 5693 tubes with a 6x5 rectifier. 

I have been fighting some hum since I turned it on, and it is currently acceptable - until I put the top on my chassis!  Hum shoots up about 20db with the cover on.  Any ideas?  I may just rack it up and see, but I kind of think it is going to hum in the rack, and worried about it causing other gear to hum as well.

Edit: racked it up.  Hums just like with the cover on

 

[rockinrob86]

I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!