How do I calculate current on the primary for spec'ing a gapped OT? (RCA BA-2C)

[Shattersignal]

1st stage tube is all you should worry about, 2nd hardly matters so long as it works correctly.

Thanks!  I'm thinking of springing for some 1620s for the V1 positions.  That's useful advice.

 

[emrr]

A random 1620 may be no better than a random 6J7 at this point in time, even though in theory it should be better.  I pull from a pile of 100 NOS 6J7's and maybe 30 1620's when putting tubes in things, and it's a toss-up in practice.  If you have a burn-in rack and can properly evaluate 50 tubes at a time, you might see a trend, I don't know.    Glass 6J7's seem to have a high likelihood of the grid cap separating from the body, since it's metal cemented to glass.    A better tube doesn't buy you a lot in a 50dB amp, much more critical in a 70+ dB amp. 

 

[Shattersignal]

Yeah, I have about 16 NOS RCA 6j7s and they don't really sound different as I've swapped them in and out of the three preamps that I've built so far--the voltages do seem to settle differently, though.  It's great to hear your experience--those 1620s take a pretty serious price hike over the 6j7s.  Thanks again!!

I'd love to know more about your studio setup, Doug!  That seems like a pretty fantastic batch of preamps you've got.  What are some of your other favorite pieces that you use?  Do you have other mic pres or do you only use the BAs?

 

[emrr]

I have a lot of tube preamps, about 30 channels have made it over the finish line to being fully integrated into the studio, another large lot are rebuilt and have been used in test bed sort of rigs for temporary recording/analysis, but wouldn't be the sort of thing you'd leave wired up around paying clients.  There are a lot of channels of things I've restored and racked for other people, or myself and then sold on after using for awhile.  I like big heavy crap, so it's mostly octal and older American stuff, though there's a selection of what I like amongst the 9 pin stuff from the end era.  Besides RCA there's Altec, AWA, Collins, Gates, Presto, UA, and others.  I've had most of the Langevin at various times, and a few Western Electric.  There's original transformers waiting for me to build a number of other fairly obscure but significant RCA pieces, such as the RCA Photophone preamps used in a large percentage of american motion picture work.  I'd probably feel pretty good about letting all the other tube stuff go if I was told I had to live with 20 channels of BA-2 for the rest of time, I could make that work.  There's about 20 varieties of SS preamps here too, from earliest germanium to modern transformerless. 

 

[Shattersignal]

Awesome   Sounds like you've gotten familiar with a LOT of coveted and classic equipment. 

I must say, I'm growing increasingly fond of these BA-2As, even though I was perfectly happy with the other preamps here in the studio--I think these new builds will be seeing A LOT of use around here. 

 

[Shattersignal]

I have couple sets of original transformers available:

https://groupdiy.com/index.php?topic=67162.0

Thanks for the heads up, Doug!

 

[Shattersignal]

Hey Doug, what are your thoughts about the R-11/C-8 network?  It seems to be a radio frequency filter for the V2 grid.  There is no such filter (or grid stopper) on V1, and that stage seems fine without it.  I've tried bypassing R-11/C-8 and it doesn't seem to make an audible difference.  Is there a good reason to keep that filter in place that I'm not seeing?  I'm using extremely nice components and it seems a shame to throw some un-needed junk into the signal path, if I'm correct in believing that I can live without it.

 

[emrr]

That's a treble boost.  Depends on the transformers and the desired outcome.  Take it out, gain is slightly higher.  A real BA-2 has an 'air' lift. 

 

[Shattersignal]

Thanks!

That's a treble boost.

 

[lassoharp]

Even with C2 at near 1uf, I'm still not hearing those bottom octaves. 

I'd suspect the Cinemag OT before the A-10.  Maybe not enough Henries.


I spoke with Cinemag a couple of times back around 2010 re a suitable OT for a BA-2 type application.  I know they had at least one model at that time and they sent me some spec sheets.  I recall them saying "Good for up to 9ma idle current" but I was not clear on the conditions then, and am still not.  If yours doesn't measure the same on the bottom as the curves that Doug has posted for real BA2/BA1 then I would suspect the Cinemag OT first.  280 ++ H is what I recall for the stock BA2 OT pri L. 

One thing that seems worth mentioning.  The original BA1/2 OTs load at 30-50K AC Z = lots of thin wire = unavoidable high freq loss = no big deal if you ask me.  If your goal is flat to 30K then I'd so go for one of the tube projects in the Jensen White Paper sections .  Go look at Dougs posted curves and note the how the top end behaves.  Cinemag or Sowter might consider this a travesty and "warn" you of the loss of high frequency response as if it were news of Cancer.

I haven't got to hear Cinemags offerings but the 50/50 Ni core should help it sound like the original.  But I think it will need to be wound like the original as well which means them making you something with "inferior high frequency response".  They are also a little reluctant to give you an Inductance quote as it's condition dependent.  But, crude as it may be, a handheld L meter will show 250+ L @ 120Hz on a real BA2 pri tested out of circuit.  So if the same meter reads a lot less than that for a Cinemag offering I wouldn't expect it go as deep on the bottom as the stock iron.  I have never measured any of the Cinemags.

BA-2s idle at just under 4 ma on V2.    If an OT is "good to 9ma idle" meaning it will show no loss of low end response due to exceeding DC handling, specs, then BA2 should fall well in the safe zone.  On paper this should give you at least as good bottom as any stock BA2.    You can always bypass the A-10 and run another sweep.  That should really clear it up.

*source Z was specified and the value I gave them was 12-16Kish which is about where triode strapped 6J7s fall at that current.  I think David's test spec sheets were for a 20K source Z - so there wouldn't seem to be any extra losses due to too high a source Z.

 

[Shattersignal]

I'd suspect the Cinemag OT before the A-10.  Maybe not enough Henries.

Thanks!  Lots of great info here.