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

I have the circuit assembled and I'm seeing strange behavior--I wonder if any of you can recognize this symptom and would steer me in the right direction to find a cause.

Plate voltage is steady on V1.  Plate voltage on V2, however, climbs rapidly at power-on, hits close to it's target voltage, and then steadily drops to the vicinity of about 40vdc, where is stays.  The buildup and drop happen within about 10 or 15 seconds, so I first guessed that it might be a short inside the tube, perhaps related to the warm-up time of the filament.  I changed the tubes and the behavior is the same, however.  When I remove the tubes and test, all voltages are steady--the problem doesn't present itself.  I've played with different R values  for dropping the B+ lines, as well as with different values for the bleed R.  The v2 voltage issue seems unaffected by changes to the B+ voltage, except that it peaks and settles at different values, directly related to the B+ that's being delivered.  It's also worth noting that the amp passes audio even with this problem--at least when the v2 B+ settles to more than 40v or so--but that there is little headroom (duh!) and poor bandwidth.   

My guess is that it's either an issue with 1) the output transformer, the primary of which passes the B+ to V2, or 2) that my PS can't supply enough current and that somehow this is only affecting v2 and not v1.

My B+ PT is rated for 80va, which ought to be substantially overrated for the task.  The OT is gapped for 5ma, has a DCR of 1.4k on the primary, and measures about 2.6v of drop while the amp is at idle.  I'm not really sure what other info to relate which might be relevant to diagnosing the problem. 

Thanks again for reading and for assisting, if you think you know where I should look for the issue!

Something is drawing too much current.  Double check V2 cathode R, and bypass cap.  Break the path and insert a current meter, see what you get. 

Thanks a ton, Doug.  I'll look at the cathode resistor.  By the way, what is the minimum wattage rating I should use for these resistors?  I may need to play around with values. 

what is the voltage drop across the transformer when you get 40 volts on the plate of V2?

look for oscillation on the plate of V2 if you have a scope, this can sometimes make your volt meter read funny,

V drop across the OT pri is 2.4v when the v2 plates have settled to the lower voltage. 

Oscillation would be extremely high frequency?  I don't have a scope on hand, but I could try to use a FFT in my DAW, if it were lower than Nyquist and could be seen in the audio range.  What causes oscillation?  Could that be the effect of a bad bias?  Or would it be the result of anything else in particular?

I've ordered some more 2w resistors so that I can play with the cathode R value.  I'm also planning on reworking the PS so that I'm relying less on series R for voltage dropping.  As you may recall, by B+ is a little high and I've experimented with different ways to drop the voltage.  It occurs to me that I might be dropping too much current in the process.  In any case, I plan to experiment further once the resistors arrive in the mail on Wed. 

Thanks for helping, CJ!

CJ said:

what is the voltage drop across the transformer when you get 40 volts on the plate of V2?

look for oscillation on the plate of V2 if you have a scope, this can sometimes make your volt meter read funny,

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eliminate the possibility of having a leaky coupling cap C2 on the schematic on page 1 of this thread,  which would drive the grid positive and therefore turn the tube on all the way resulting in a low plate resistance which would make you see low voltage on the plate, to do this, check the grid voltage of V2 using a Hi  Z voltmeter or scope set to DC.

If you have none of the aforementioned utensils, simply disconnect the cap from either the plate of V1 or  the volume RC filter.

Will do.  That's excellent advice.  Thanks, CJ!

CJ said:

eliminate the possibility of having a leaky coupling cap C2 on the schematic on page 1 of this thread,  which would drive the grid positive and therefore turn the tube on all the way resulting in a low plate resistance which would make you see low voltage on the plate, to do this, check the grid voltage of V2 using a Hi  Z voltmeter or scope set to DC.

If you have none of the aforementioned utensils, simply disconnect the cap from either the plate of V1 or  the volume RC filter.

Click to expand...

CJ said:

...to do this, check the grid voltage of V2 using a Hi  Z voltmeter or scope set to DC.

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My Fluke 87V reads 6 to 7VDC on the grid.  Does that mean I have a leaky blocking cap?  Is that enough positive voltage to turn the valve all the way on and explain the issue?  I'll try swapping out the cap right now and see if that helps.

I'm using brand new Mundorf EVO SGO caps, and I've never seen one fail before, let alone brand new.  There's always a first time, though! 

Thanks again, CJ.

I swapped out the blocking cap and now I have 0vdc on the v2 grid.  That would indicate that the Mundorf cap was indeed leaking DC, it would seem.  The voltage still dives on the v2 plate, but now it settles in the 80v range, instead of the 40v range.

I'm passing signal, too. 

CJ said:

...a leaky coupling cap C2...

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I dare say that was the culprit the whole time. 

I reworked the B+ to v2 and now I've got 140v.  The voltage still rises to about 240v and then dies down and settles at 140v--but it's passing signal and sounding great! 

Should I be worried about the voltage fluctuation at power up?  Is that normal for a tube circuit?  I haven't noticed this behavior on the other tube circuits that I've built--but they were all tube rectified and I suspect that might account for the difference.  I've built a Redd47 and an EQP-1a and I don't think I ever noticed the B+ to fly up and down so much at power up.  Maybe that's the effect of the SS recto's super-fast inrush and zero warm-up time? 

I hope someone can comfort me and say that it's not a huge problem, because I *think* this build might be up and running now   .

The one thing I'm going to play with will be strapping another cap across the c2 position--I think the amp may be a little weak in those bottom two octaves and I'd love to get a little deeper frequency response.  Otherwise, it seems to sound quite nice with the UTC A-10 on the input and the CM-27101 (50/50ni, 5ma rated gap) on the output.  I'm hoping the slightly weak low end isn't due to the gap of the OT--I'll report back about that as soon as I can test. 

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

EMRR, do you have any ideas about how to increase the bandwidth? 

I suppose I could be somewhat limited by the UTC A-10, although it's officially spec'd to be 20hz-20khz.  Also, I was previously using the A-10 as a passive DI and it didn't roll off any low end in that configuration. 

Need concrete measurements or a plot. Voltage at startup is normal.

emrr said:

Voltage at startup is normal.

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Excellent.  Thanks!

emrr said:

Need concrete measurements or a plot.

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I suppose I'll be able to figure it out.  I was wondering if you had any general tips besides upping the blocking cap value.  If I do a full circuit map, I'll post that info.  Thanks!

As I work the B+ higher and higher on v1 and v2, the low end seems to be filling in.  I think I was just a little short of headroom and those low frequencies were bottoming out. 

This is a killer preamp.  It's got a really nice second harmonic excitement.  Bass guitar and bass synthesizers are sounding amazing, even though I don't have the voltages all the way up where I want them, yet.  The midrange is extremely exciting.  Surprisingly, it seems to sound more forward than my Redd 47--I was expecting it to seem more relaxed than the Redd, I guess.  I bet the UTC iron isn't hurting, in that regard  

I'm very pleased and suprised, so far!

emrr said:

With original RCA modules, I run 250 versus 285...

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Doug, do you think it's important to the sound of the unit to keep the original ratio between the v1 and v2 plate voltages?  I realize that each stage will run on a wide range of voltages, but I wonder if it's essential to the sound signature of the amp to preserve the original voltage ratio, relative gain staging, and relative headroom for the two gain stages.  I read somewhere on one of the other BA-2c threads that the 72v/238v ratio causes the two stages to saturate at the same time.  Is that something I should strive to achieve, in your opinion?

I have 240v coming out of my last RC filter, 128v on v2, and 71v on v1.  I intend to get the voltages just right if it will create any benefit.  I'm especially interested in getting as robust a low frequency response as possible.

I was referring to the master B+ fed into a stand alone module, in which case the ratios are maintained.  It is insignificant in small degrees. 

Yeah, I understood what you meant regarding adjustment from a remote master psu.  Thanks for all your help, by the way!

I'm wondering if maintaining the voltage ratio is important to the sound of the unit.  If anyone has any thoughts on the matter, I'd be curious. 

I'll make an effort to match the original voltages, and I'll be able to report back on perceived differences in saturation.

I realize that, besides gain staging, toying with the specific plate voltages will also affect bias--which might be why I'm currently finding my low frequencies a little recessed.  To corroborate that theory, I got some strangely low current readings on the v2 cathode resistor path when I tested it last week. 

As soon as my resistors arrive in the mail, I'll be able to exactly copy the original BA-2c schemo.  I'll report on my subjective impressions as well as some tone generator sweeps.  As of right now, my REDD 47 absolutely pummels the RCA in the bottom two octaves.  I'm hoping that will change with a little more work on dialing in the plate voltages.     

From my tests BA-2 sounds quite different from Redd47 because of circuit differences like NFB and gain control, design approach, production period, etc.
DC coupling to output trafo in BA-2 seems to change lows even with more cathode decoupling, higher stages coupling cap value, other anode filter and using different proper 50Ni/Fe, or Fe OT core from Cinemag. Inputs here are UTC, Chicago and Cinemag. As you i find mids interesting, also lows despite missing a part of lowest octave. Iirc highs go to about ~16kHz and then then gentle drop at over 30Khz. Nice. Scope seems good for finding possible problems, not in part sound as for 47 and other parafeed where response can look better.
There was discussion about both stages saturating at the same time in some American designs, don't think models were mentioned. It could be this one.
I can test gain staging in a while and see what happens at few lower than -20dB input pad positions. Proper cathode R values on yours might show similar results faster, a few lower steps along ~ -20dB pad could get you closer to original sound because signals today can be hotter.

Shattersignal said:

As of right now, my REDD 47 absolutely pummels the RCA in the bottom two octaves.  I'm hoping that will change with a little more work on dialing in the plate voltages.   

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This has nothing to do with voltages, everything to do with transformers and circuit design. 

The REDD is a high feedback amplifier, with a very flat response as a result.  Disconnect the feedback and see how it is.    As well, it has a parafeed output with an output cap value that will have been selected to resonate just enough to flatten the lows without providing an obvious boost hump. 

The RCA response plot clearly shows low end rolling off, and a treble boost.  My real BA-2 is about 1dB down at 40.  My 85-X, same output transformer, and a large amount of feedback, is 1dB down about 20Hz. 

You can get very flat response in an amp lacking loop feedback, but you must use top-shelf transformers to do so.  UTC A-10; officially spec'd to be 20hz-20khz is +/-2dB.  LS-10 is +/-1dB.  This is significant when you add input and output curves together.