RCA BA-6A from scratch

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lassoharp

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The interstage would need to be 50k on the primary or thereabouts but these are not really obtainable or affordable.

an A-18 may sound more correct than a modern replacement, even with the shift in operation. 

I worry these two statements may cause undue confusion.  The Sowter BA6A interstage is 1:1, is readily obtainable, and performs very well as shown in another thread where it was tested.

As to affordable who knows.  Anything vintage and UTC is now crazy high.  How long will it take you to build your "bargain" BA-6A?  That's a lot of sweat.  Based on what I saw during testing the 1:1 looked best overall for the design.  If you already have an A-18 then why not.  Or you could easily sell it and buy the Sowter or try the Lundahl.  Besides, most people want to lower the threshold on the big tube limiters and you're already contending with a 10W output.  When you consider all the hours of work you'll put into it . . . . .

You can spend 200 on the Sowter interstage and still build a BA6A for probably a lot less than what most typical vintage ones sell for.    That's another way of saying question yourself if the need to constantly swerve from the perceived big wallet bite is running you ragged.
 

emrr

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an A-18 may sound more correct than a modern replacement, even with the shift in operation. 

Which is to point out again, that one is somewhat hallucinating if they think building a clone with modern transformers will sound like an original, while repairing an original may benefit from 'close enough' vintage iron versus modern replacement.  In other words, if you want a BA-6A, buy a BA-6A.  If you want something somewhat similar, there are lots of options without necessarily building a clone of a BA-6A. 
 

DaveP

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I didn't mention the secondary because I wasn't sure what the ratio was.  In the end I didn't use one at all and it worked fine!!
But I did spend a lot of time selecting tubes and getting them perfectly balanced,  maybe engineers back in the day would not have wanted that degree of hassle.

best
DaveP
 

rafafredd

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Sowter lists it at 10k CT primary for the interstage transformer. I also thought it is too low for those pentodes, speacially in gain reduction, but they apparently are making 1:1 copies of these, as far as I understand their website.
 

emrr

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I've said it before; they don't know, that's just what they are making, which will work.  Original is much higher Z and L, and I guarantee more nickel.  Compare our posts about the 86-A limiter original versus their repro. 
 

prescott

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If the sowter interstate transformer works fine, what about an edcor XSM10K/10K?
Its cheap and its 2.5w.
Would it work?
Or any other from edcor's line?
 

lassoharp

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It will probably work - as in get by ok.  I doubt it will have enough inductance for balanced bottom response.

Also, the Edcor line matching series were not designed for push-pull tube amp use.  They will work technically but you may find them a little grainy sounding.  Edcor started making transformers for PP use, mostly outputs though.  Not sure if they made an interstage type.  They do make a nice output that would work for BA-6 though.


It may be worth the time to ask Edcor about getting a custom one built.    Would probably be cheaper than the 200$ Sowter.   

 

DaveP

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If the sowter interstate transformer works fine, what about an edcor XSM10K/10K?
Its cheap and its 2.5w.
Would it work?
Or any other from edcor's line?

It will work, but you will probably lose some bass, the primary has 25H inductance, the XSM15k/15k may well be better, but last time I looked, they had not reported the inductance for that model.  I would have thought the extra turns would have given it more Henries.

DaveP
 

burdoto

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Howdy,

I built a BA-6A after reading this thread and another ba6a thread on here it's mostly a success. Thanks for the guide. However, I can't get the meter to register the hum output when balancing the 6sk7 tubes. I can hear the hum if I hook some headphones up to the ot outputs. My cheap DMM says I have .6v ac on the output. I tried a ic bridge rectifier, a 1n4148 bridge, and a 1n4006 bridge. None give me a reading on the meter. It's as if the balancing hum signal isn't strong enough to get past the bridge rectifier.

All tubes running on ac heaters and I did not elevate the heaters. Is one of these conditions the strength of the balancing hum?

Currently, I'm sending the hum through the main outputs so I can balance the 6sk7 hum by ear. Which works. I might have to leave it this way and call it a success.

Over all with the heater circuit setup the only hum I have is from the cheap power transformer(I had to move the input tran to the opposite side of the unit), and it's down around the level of the noise floor. Am I loading something in the sound of the unit by not at least running dc heaters to the 6sk7 tubes?

Dan
 

lassoharp

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That's a pretty effective hum balancing circuit they have.  It always worked well for me on bench versions.  What level of hum are you actually getting on the output?  If you're down around -65 to - 70 db (whatever) I'd count that a success.  I doubt you'll ever use the BA-6A with the output attenuator wide open either, and when it's lowered to the typical range, that cuts down even more on effective hum.

Not sure what meter you're using but it may be that the hum you're seeing is out of range for the meter scale.  I think the acceptable level of hum was higher in that era than now.  An SA-39 for example, gives -50 db as typical hum level.
 

burdoto

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I have 500ua Simpson . It registers gain reduction just fine.

I should have been more precise about the hum and the meter circuit. The schematic shows a tap off the 6.3v heater winding which is sent up to the Bal-Lim switch to balance the cathodes and screens of the 6sk7 tubes. I believe this introduces a 60 cycle hum into the center tap off the input tran secondary. I can hear that 60Hz hum on the secondary of the output tran. The schematic diverts the secondary of the output Tran away from the output ATT and to a bridge rectifier feeding the meter switch when the Bal-Lim switch is set to the Bal A or Bal B positions. I have quadruple checked the wiring and routing of the Bal-Lim switch and it seems to be set up correctly. The problem is that the meter doesn't register anything in the Bal A and Bal B positions. I suspect the forward voltage drop across the bridge rectifier is too much for the 60HZ hum. In other words, I can't use the meter to balance the 6sk7s. I can balance them by ear if I send the 60hz hum to the output pad. This would be similar to the rs124 balance strategy. Perhaps the 60hz signal is too weak because the 6sk7s have ac heaters.

Dan
 

lassoharp

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Hard to say for sure.  I've never wired up the BA-6 meter circuit for the balance but your description sounds like it may be a wiring problem.  Have you tried it with different tubes or any you know have greatly different idle currents?
 

DaveP

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Are you sure about the values of C12/C13?

The hum is fed to the meter from the ac heater winding via these capacitors, so their reactance at 60Hz decides the level to the meter during balance.  These are not standard values nowadays.

Is the meter still in circuit during balance?  There may be a mis-wire somewhere.

DaveP
 

burdoto

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I scrapped the meter switch and used the panel hole for the vari threshold pot. Saves me an extra hole half way up the panel. I'm just sending the balancing hum through the main output and balancing with my ears. Seems to work fine.

Dave, I noticed your schematic left out the .0015 cap in series with a 1.2k resistor on the cathode of the the 6j7s. Is there a reason why? What's the purpose of that cap and resistor? Lowering distortion?

Dan
 

DaveP

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Dave, I noticed your schematic left out the .0015 cap in series with a 1.2k resistor on the cathode of the the 6j7s. Is there a reason why? What's the purpose of that cap and resistor? Lowering distortion?
The transformers originally used had poor top end performance, so they put treble boost on the cathode to compensate.

Modern transformers did not need that tweak.

Best
DaveP
 

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