RCA Ba2A Pre amp qestions & answers

[letterbeacon]

I did think that was slightly odd. I have a higher B+ than the one specified, if that makes a difference.

 

[emrr]

It shouldn't.  You have measured as much more gain as an additional tube might give.

 

[letterbeacon]

I must have measured wrong or I'm making a mistake somewhere along the line in the set up.  I'll double check tonight whether the interface added any gain at some stage.

 

[letterbeacon]

so.....
.....why does your build have 27 dB more gain than it should?  Am I reading that correctly?

I tried the sine wave test again without the BA-2 in the chain - plugging straight from the out of the MR816 to the in of the MR816 and it looks like the unit adds 19.5dB of gain somewhere along the way.  As it's a loan unit until my RME gets fixed I'm not very familiar with it and can't figure out where this extra gain is coming from.

So it looks like my amp has a gain of 57dB.  Does the extra 7dB sound reasonable?

Seeing as my MR816 has a gain of 19.5dB, then my hum figure of -33dbFS is more like -52.5dBFS, right?  That's with a 200r resistor across the input.

I'm going to attempt to rotate the PT later to see if I can lower the hum anymore.

 

[emrr]

I bet the MR816 has programmable input gain that's set higher.  7 dB sounds like reasonable variance with transformer differences and tube gain differences. 

 

[letterbeacon]

I think I may have discovered where the hum is coming from and it's NOT the power transformer.

I took the amp out of the rack and laid it on the floor so I could reach behind it to rotate the PT at the back.  Even before I had turned the PT I noticed that the hum level was reading -45dBFS (down from -33dBFS).  When I turned the transformer, this figure stayed the same, nothing changed.  When I put it back in the rack, the hum level went back up to -33dbFS.

If the hum was coming from the PT then turning it might have made some difference, right?

This is my home studio and at the bottom of the 10U rack is my PC.  I think the hum is coming from the PC.  I set the BA-2 up at the other end of the room to the PC (about six feet) and found the hum had gone down to -52dBFS.

With the BA-2 still at the other end of the room, I took the 200r resistor out and plugged in my mic turned the gain up to about 2 o clock, and recorded myself playing a bit of acoustic guitar.  The hum was still there, but much less noticeable, you could only really hear it when I wasn't playing.  I A/B'd that guitar recording with one I had recorded when the amp was in the rack, and difference in hum is amazing.

Does my PC causing the hum sound plausible to you?  I suppose the switch mode power supply must be having an effect on my input transformer.

So the hum reading is -52dBFS, take way 19dB which the MR816 adds so that means a hum of about -71dBFS. That sounds about right, right?

 

[lassoharp]

that means a hum of about -71dBFS. That sounds about right, right?

Yes.


I know that both of my DIY Pultecs picked up hum from adjacent other tube gear's PTs.  I felt it that in those cases it was more the passive circuitry via rails and case that was transmitting it to the makeup amps than the input transformer alone which was a vintage shielded type.  Either way, moving out of the rack was the solution.  Glad to hear the problem is solved.

 

[letterbeacon]

Thank you very much to both of you with all your help on this project.  It's been incredibly educational and hugely enjoyable.  I'm going to put a JLM go between in and a nice engraved front panel and then it's all done.  I'll get some proper pics up when I'm completely finished.

lassoharp - when you're recording with your BA-2-esque pre amps, can you hear a hum faintly in the pauses between vocals etc.?

 

[lassoharp]

I can't hear hum on either the BA2 clones or the 3 stage RCA-ish pres.  I don't have an official figure yet on where exactly the hum resides db wise but a rough figure would be somewhere below -80db, or below the hiss level.  My crude method for checking hum involves running the preamp wide open into a power amp that has ~ 60db of gain and pushing my ear to the loudspeaker.  If I can't detect it in that set up I know it won't be audible on recordings.  I hear only faint hiss on BA2 when the preamps are cranked wide open while monitoring on the DAW.

The DIY tube pres (2 at current) I have that do have small amounts of low level hum seem to be caused by PT field pickup to some degree.  On one I had to rotate the PT to reduce the hum and it seemed to be going directly into the output circuit immediately at power on (It was a 3 stage amp and I had the 2nd stage grids shorted to ground).  That being said, if you still find the hum level objectionable, if possible,  you could try rotating the PT and see if it changes anything.  On mine the PT was perfectly square mounted with bell towards the other iron as yours is but it wound up being just slightly off this at an odd angle that gave the best hum reduction for some reason. 

 

[letterbeacon]

That being said, if you still find the hum level objectionable, if possible,  you could try rotating the PT and see if it changes anything.

Unfortunately I've tried this to no noticeable affect.  It doesn't bother me though as I have to have the gain cranked up pretty high to notice the hum.

I installed the JLM go-between the other day and now have phantom power.  Using a condenser mic, I don't need to crank the gain up much and you definitely can't hear any hum.  Very pleased with that!

I did some measuring the other day and I noticed my B+ is about 20v lower than what it should be, due to me not having exactly the right dropping resistors on hand.  Is it worth me trying to get it up to what it should be?  It works as it is, but will it make a difference if I get it up to what it should be according to the voltage chart?

There's one last thing I want to do and then this project is complete and that's installing an output attenuator so I can crank the tubes into distortion.  I have a 600r t attenuator that I bought from Nielsk and I'm wondering how to hook it up.  I looked at the 1176 schematic to see how that attenuator is hooked up on that unit and I've come up with the attached diagram.  How does that look?

Thank you!

 

[lassoharp]

It's still bothersome that you're having residual hum.  This may create issues if you decide to chain in the BA6 downstream as it will amplify the hum level. 

Unfortunately I've tried this to no noticeable affect

So you're saying nothing happens - no increase or decrease?  Apparent immunity?  That would seem to suggest the problem lies elsewhere.

One other relatively easy thing to try, just in case.  Clip in (Parallel) equivalent size filter caps in the PS.  This is kind of covering two things:  1) the odd possibility that one of the new caps isn't up to snuff, and 2) the possibility that the circuit wants to see more filtering.  I realize the theoretical numbers for the stock values are plenty for the amount of current this amp draws but it's easy enough to clip those extra caps in to see.  I did have one SE preamp that seemed to want above and beyond what typical filtering should have provided and I wound up having to use a choke input filter in the PS to get things right. 

 

[letterbeacon]

One other relatively easy thing to try, just in case.  Clip in (Parallel) equivalent size filter caps in the PS.  This is kind of covering two things:  1) the odd possibility that one of the new caps isn't up to snuff, and 2) the possibility that the circuit wants to see more filtering.  I realize the theoretical numbers for the stock values are plenty for the amount of current this amp draws but it's easy enough to clip those extra caps in to see.  I did have one SE preamp that seemed to want above and beyond what typical filtering should have provided and I wound up having to use a choke input filter in the PS to get things right.

That's a good idea.  I'll have to order some more in so won't be able to try it right away, but I'm willing to give it a go.

I might try and draw my grounding scheme out and post that in case I've done something wrong there.  I'm pretty confident (it's basically the Jensen diagram posted earlier in the thread), but it'll be good to see what you guys make if it.

 

[Bren-hur]

Hi, this seems to be the go-to Ba2 thread so I thought I'd post a couple of Q's... I have this devilsh gremlin crackle that makes the unit unusable, it used to work fine but now it crackles... kind of like a bad phone line when it rains... it's totally random, sometimes there sometimes not, which is a total pain in the ass to troubleshoot... The gain pot has no effect on the crackle, it's there no matter what, so to try and lock down the location of the crackle, i ran a signal through the input and traced backwards from the output. The crackle disappears when i get past the output trans... (clipped a coupling cap to the output of V2 to test the tube). This pretty much leaves the only culprit to be the output transformer... or the power transformer maybe...

I have a UTC A25 as the output, an ebay purchase, it tested out ok, but i dunno... Is there any off the shelf brand new output transformers that will work in the BA2 circuit? I want to try a 100% known good in there... I was looking at the cinemag outputs...

http://cinemag.biz/output/output.html

It looks like the CMO-10/600-150 or the CM9589 could be possibilities...

 

[emrr]

Could be current draw in output causing crackle.  Put a larger resistance between v2 plate and transformer to see if reduction of current flow decreases crackle.  Might reveal something.  If it goes away with an extra 10k-20k then it's probable the output is going. Yes, B+ to plate will fall also, but not enough to invalidate test.  Output might still be fine in a parallel feed circuit, and no longer a dc coupled one.

 

[Bren-hur]

Hi, Doug! ... i went to try what you said and of course the crackle decided to disappear, before i tried to add the resistor... guess I'll wait till it starts doing it again...

any comments on those cinemags? or is there any other new transformers you know would work?

 

[emrr]

CMO-30/600 with allowance for DC is the closest Cinemag. 

 

[Bren-hur]

Well... the problem is solved... but not in a good way...

Turned it on this morning and it was buggered.. total distorted crackle... checked the plate voltage and V2 had 0V... voltage is getting to the A-25 and not passing through. I Pulled the tranny and there is no DCR on the primaries.. waaaahH!!! pretty over it... paid 100 bucks for the tranny on rapEbay and barely saw a few hours use... ggrrrrr. It's funny though, coz the other night i was recording and it totally died, like the same thing i heard today, but the next day it was back online, with the crackle i mentioned earlier, anyway looks like i need a new tranny...

 

[lassoharp]

Sorry to hear the bad news.

I suppose at this point it wouldn't hurt to try and open the bottom plate and see if the open has occurred right at the pin connections.  Is there any potting compound showing around the base of the pri pins?

It may have had an excessive amount of current ran through it over a long period of time by previous owner.    Mistakenly running the plate lead to ground could burn one up in a hurry too.

 

[Bren-hur]

I was thinking bout taking it apart... do you know how to get the bottom plate off? I don't want to wreck it trying to dismantle it if it's actually fixable.... don't see any potting compound anywhere on the bottom...

thanks!

 

[lassoharp]

Doug and CJ have taken UTCs apart.  Hopefully they will chime in shortly with some good advice for you.  Good luck.