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

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Shattersignal

Well-known member
Joined
Jul 14, 2012
Messages
114
Location
Left Coast, USA
Hi all!  I'm building some RCA BA-2C mic preamps.  I'm speaking with Cinemag, who will be building the gapped output transformers for the project.  They need to know how much current will be passing through the primary of the OT so that we can gap it as little as possible.  The output tube is a 6J7.  I'm not sure what the plate voltage is supposed to be, but it's something around 200vdc. 

Here are my questions:

--How would I go about calculating the current flow through the primary?  I am familiar with Ohm's law, but I'm not really sure how to piece together the values that I'd need to perform the algebra.

--Does anyone have any special advice for this build regarding the spec' of the OT for a BA-2C, in particular?  I want to get as many Henries as possible. 

Thanks a ton!  I've learned a ton from this community and I always greatly appreciate those who have taken the time to assist me!
 
Shattersignal said:
Hi all!  I'm building some RCA BA-2C mic preamps.  I'm speaking with Cinemag, who will be building the gapped output transformers for the project.  They need to know how much current will be passing through the primary of the OT so that we can gap it as little as possible.  The output tube is a 6J7.  I'm not sure what the plate voltage is supposed to be, but it's something less than 200vdc. 

Here are my questions:

--How would I go about calculating the current flow through the primary?  I am familiar with Ohm's law, but I'm not really sure how to piece together the values that I'd need to perform the algebra.
Idle current is a design parameter, which governs the maximum output level and THD of the stage. RCA1620 typical idle is about 2mA, and there is no evidence to suggest that this is not the case in the BA2C. Idle current is governed by the total resistance between cathode and ground. Decreasing this resistance increases idle current. Ohm's law alone is not much help tehre; you need to familiarize with tube's operating graphs.

 
abbey road d enfer said:
Idle current is a design parameter, which governs the maximum output level and THD of the stage. RCA1620 typical idle is about 2mA, and there is no evidence to suggest that this is not the case in the BA2C. Idle current is governed by the total resistance between cathode and ground. Decreasing this resistance increases idle current. Ohm's law alone is not much help tehre; you need to familiarize with tube's operating graphs.

Thanks a ton!!  I figured it had to to with the tube spec, itself.  So does it seem safe to inform Cinemag of the 2ma idle current rating?  That might be small enough to attempt using a butt-stacked OT, which would be great.

Thanks for steering me in the right direction!
 
lots of RCA BA series info on this site, try the search thingy at the top,

might have been around the block already on this,  recall a thread with 7 or 8 volts on the cathode so 4 to 5 ma might be a safer bet,

took apart an RCA OPT the emrr graciously sent me a while back,  :D

turned out to have a higher ratio than expected, (10:1)

seen A-25 (5 to 1) mentioned in some RCA threads,

we will calculate some flux figures on this old print,

" is it possible to use a butt-stacked OT, for instance, in this build?  I want to get as many Henries as possible..." 

a butt stacked core will give you less Henries, as it is a gapped transformer,


https://groupdiy.com/index.php?topic=26675.0
 

Attachments

  • rca ba OPT.jpg
    rca ba OPT.jpg
    66.9 KB · Views: 83
pulled from that monster thread, not sure if this is the BA-2c or not but seems like it>

"So I've powered up the unit and have the following voltages with the schematic's voltages in brackets:

v3 plate = 295 (288)

after r8(4.7k) & r9(3.9k) = 247 (248)

v2 plate = 211 (232)
v2 cathode = 7.83 (8.8)

v1 plate = 80 (73)
v1 cathode = 2.3 (2.4)"

you know that output, wondering if the Reddi  DI output could be used, just add a few more turns and take a few off the secondary,  don't know what Cinemag is gonna hit you with, 

PM me and we will tell you the price of a Reddi/BA-2c    (fifty bucks)

FY Ithe  XT  OPT in the print is seeing 3600 Gauss of DC flux and 2100 AC Flux (4 volt rms output)

 
Officially 232VDC on V2 plate.  8.8VDC at cathode. 

The same output transformer is used on other RCA single tube 6J7 preamps that spec as 3.8mA. 

A hand held inductance meter indicated 210H at 120Hz.  Obviously not a complete spec at operating conditions. 

I believe Lassoharp has already been down this road with Cinemag, and had some results to compare against a real RCA output.  I may remember incorrectly. 
 
here is a schemo, unofficial of course, some things don't add up, cathode current thru V1 would seem to indicate a larger voltage drop across 120 K V1 plate R  resulting in 40 volts on the plate instead of 73, but hey, jus sayin, wtf,

V2 plate V calculated from cathode current and DCR of xfmr,  hmmm, forgot meter chewing up V2 cathode current, wonder how much?
 

Attachments

  • ba2c13.gif
    ba2c13.gif
    228.9 KB · Views: 123
CJ said:
might have been around the block already on this,  recall a thread with 7 or 8 volts on the cathode so 4 to 5 ma might be a safer bet,
https://groupdiy.com/index.php?topic=26675.0

Speaking with Cinemag, I was steered toward a 5ma-rated gap, so that's what I ordered.  Thanks--that seems to me completely in-line with your recommendation, CJ!

I have the transformers now.  The rest of the info regarding expected voltages and current flow will be extremely useful.  Thanks guys, for all the help!!  I'm sure I'll have lots of questions as the project commences.  Cheers!
 
In particular, I'll be winging it with the spec of the PT.  I think I'm going with the Hammond 300 series, with a step up to 240V.  I want to make sure that I deliver enough DCV to those plates--my original build guide specified an 200V PT, which seems strangely low, given what I now know about the BA-2C.  I'll be using a SS rectifier and I need to do some learning in order to drop the voltage the right amount for those plates. 

CJ, your V Map seems to spec a PT for 230V.  Do you have a recommended HT for use with a SS recto?
 
lets find out all we can,

you can back track from the voltage curve for that rectifier by using the B+ and current from the original amplifier,

figure 288 and about 6 ma, then you find out how much voltage the tube eats

looks like about 230 volts for the orig xfmr, oh wow, that spec is actually on the schemo and matches our expectations from graph extrapolation, , it's a miracle,

so 288/ 1.38 (root 2 minus a small percentage for losses) = 208.7 call it 209 for solid state,

so 230 minus 209 = 21 volts of loss at 288 and 6ma voltage loss for the 6X5 ,

probably wont find 209, i bet you could find a 210 rating, you can always kill voltage but it is hard to make up for lost voltage, so maybe even 215, but be careful, since we draw only 6 ma, copper loss will be light so B+ may be too high depending on the % of regulation from the PT.

so the graph was not needed , since we know the B+ required we simply divide for solid state operation,  but we love tube manuals,
 

Attachments

  • 6X4-6X5.jpg
    6X4-6X5.jpg
    191.5 KB · Views: 27
Shattersignal said:
In particular, I'll be winging it with the spec of the PT.  I think I'm going with the Hammond 300 series, with a step up to 240V.  I want to make sure that I deliver enough DCV to those plates--my original build guide specified an 200V PT, which seems strangely low, given what I now know about the BA-2C.  I'll be using a SS rectifier and I need to do some learning in order to drop the voltage the right amount for those plates. 
In order to produce the expected 288V on the 1st cap, you need 205-210V with a SS rectifier. If you use let's say a 370AX, although the nominal is 240, the actual voltage will be closer to 250-260 because the current draw will be about 1/3rd nominal. That would result in about 350V at the 1st cap. You would need to increase resistors R8/R9 for a total of about 18kohm. The total dissipation would be about 550mW, so I would recommend using at least 1W types. You would also need to increase the rated voltage of the caps.
 
> I need to do some learning in order to drop the voltage the right amount for those plates. 

Learn what?

There's no "right voltage". You need enough to meet spec, and more just in case. I'm guessing that circuit will run on 100VDC, though maybe not clean at +18dBm. 250V will be fine. 350V will be fine. THD at given signal level will drop as B+ rises but not all that fast. Some extra voltage is fine, way-excess voltage won't be way-excess cleaner, just more heat.

Remember this is NOT 1950, you are not trying to meet an FCC/BBC spec on an entire signal chain, and if you were you would throw op-amps around like candy. As a tube-dude today, you probably do NOT want dead-clean. Lower voltage and higher THD may be "better" for your purposes.

In small work you just aim for a high raw voltage, then use a handful of 10K and 1K resistors to bring it down to some reasonable voltage. Now you have a string of 10K resistors, hang 40uFd to ground from each junction. A 4-stage C-R-C-R-C-R-C-R-C filter will give super clean output DC.
 
What PRR said, though I'll add you want to be sure that increased voltage does not take current through the output transformer over-spec.  You may also watch response while changing voltage and see current saturation impact low frequency in the transformer.    RCA used a 5K rheostat in the stand-alone PSU that fed BA-1/11/21/etc to trim B+ to the number of preamps connected.  With original RCA modules, I run 250 versus 285, for the simple fact I don't feel like pushing old irreplaceable transformers around  and there's no noticeable hit on sound. 
 
emrr said:
I'll add you want to be sure that increased voltage does not take current through the output transformer over-spec. 
...
I don't feel like pushing old irreplaceable transformers around  and there's no noticeable hit on sound.

This is all awesome advice!  Exactly what I needed to hear.  I'll order the 240V PT and experiment with dropping R values. 

Thanks a ton, guys!!
 
PRR said:
> I need to do some learning in order to drop the voltage the right amount for those plates. 


Remember this is NOT 1950, you are not trying to meet an FCC/BBC spec on an entire signal chain, and if you were you would throw op-amps around like candy. As a tube-dude today, you probably do NOT want dead-clean. Lower voltage and higher THD may be "better" for your purposes.

In small work you just aim for a high raw voltage, then use a handful of 10K and 1K resistors to bring it down to some reasonable voltage.

Thanks a ton!  This is just the encouragement I needed.  I think I'm going to order the Hammond 269 PT, which is 250vac, center tapped.  That ought to put me well within the workable B+ range after rectification and filtering.   
 
CJ said:
if the B+ is too high, you can punch a socket hole for a 6X5,  might get you closer to the sound of the original also.

yeah, I'll do that if necessary.  My HT will be lower than the original, and after dropping a little more I think I'll be right on target.  I hope so, anyway, haha.
 
Back
Top