Another BA-6A from scratch

[1000tinyempires]

Like a fool I have decided to take a crack at building a BA6A derived unit. In looking around I found that Triode Electronics has a contender for the interstage.

Looks worth giving a shot.

http://triodeelectronics.com/unintrui.html

 

[vinyvamos]

" I pity da fool". No fool at all, this has been a super fun and rewarding project for me so far. And the sound of it is really great. Can't wait to use it in the studio, when I finish building that! Keep us posted on your build. maybe even start a thread ...

Well in fact this very moment I am researching the interstage and possible alternatives to the pricey Sowter and the lesser pricey Lundahls . Thanks for sharing the link. From looking at the spec sheet though the freq. response is 3.5dB down at 15K, and there is no inductance values quoted. The freq response could be compensated for in the proceeding stage but from what I have read on this forum the inductance of this interstage is critical, or should I say, it needs to have ENOUGH inductance to not fall on it's ass in the low end due to the DC current going to the input tubes. Check out this thread where they go into great detail on this topic:

https://groupdiy.com/index.php?topic=55693.0

 

[1000tinyempires]

Your build is looking pretty stellar. I think when I have enough parts in a pile I should start another thread as this will be where I turn when things aren't quite right.

It's funny having read the other thread I should have just from the start planned on skipping the interstage or adding a tube but I want to take a crack at getting it right. 

Going to have to commence a test bed unit with some extra holes. There is an edcor that specs out 70-18K Hz <1dBu and I wonder if at say <3dBu where you start hearing it what it specs out at.

 

[vinyvamos]

Thanks man! Well this forum is what gave me all the info and encouragement I needed to go ahead with mine.

The Edcor you mention, are you talking about interstage or output? The one I was eyeing up for the output was this: http://www.edcorusa.com/cxpp10-8k

I have emailed them asking for inductance and DC saturation on their 15K:15K line matchers, but I doubt they will be suitable. I have also asked if they would be willing to do a custom job...

The cheapest interstage that was mentioned in that other thread is the Lundahl LL1635, but I don't know if anyone has proved it worthy in a build... It is also PCB mounted which sucks for a non-PCB build like mine...

 

[1000tinyempires]

I am looking at the xpp1-10K for the interstage.

I uses one of those line matchers for a 12au7 push/pull output on a Federal 864a derived build I did for a friend on the cheap and it handled to DC fine and sounded good.  I never properly testes the response though.  I have a 10k matcher in the drawer and will try it in circuit and report back.  Be a little bit before I get to it though, not this week anyway.

 

[vinyvamos]

yes the EDCORs really stand up well against other much dearer trafos I think. I also used a pair in a Federal AM864 build and they sing wonderfully, with no hum induced by the input trafo either .

Here is a first sample of drums through my BA-6A. I have uploaded the original file too for proper comparison. Dropbox link:

https://www.dropbox.com/sh/bondjx0im3j7pqf/AACFTTiKZOtmsXabYGIzo1IZa?dl=0

The one recorded through the 6A is structured like this:

Up until the first diversion from hihats the comp is in bypass. You will hear the click of the relay when I put it active. Then I start with no GR up until the first break. Bringing the gain up more and more until the thing is being slammed to hell. hehe! I'm gonna use that flavour a lot I think...

Then after the break GR is enabled, and she is compressing real heavy at first, then I wind it back. It is on the single setting, I didn't manage to switch to the double release setting before the sample ended, but it is only a subtle difference there.

Please let me know what you think, and those of you who are familiar with the 6A sound I would love to hear your thoughts on how my one is fairing out! I know mine has a a slight boost in the upper end, and I think that may be due to my abnormal input attenuator due to my high ratio input trafo (100K resistors shunting onto a 50K dual pot, giving a 3:1 divider). That's with C5, 6 & R24, 25 not connected. When I connect them I get about 1dB further boost up in the 15-20K region. Not needed for me...

 

[1000tinyempires]

That is sounding nice. It really comes alive when you start compressing.

 

[vinyvamos]

Thanks . But but but... I was doing some frequency sweeps on it last night trying to test another interstage trafo, and there is a 3dB drop off from a few KHz up to 20K when it is compressing. Maybe I have not balanced the input tubes properly?

Anyone have any thoughts on this? HF Attenuation when compressing... It might be something to do with the AC signal coming from the filament winding. Or do I still have to match the 6SK7s as well as doing the balancing?

 

[emrr]

Don't know the answer to this, but I note different vari-mu comps tend to have different response slants, some treble reducing, some treble increasing, that stay the same with changes in transformers.  The slant trend Lassoharp and I noted in out BA-6 interstage tests did not change with the transformer swaps. 

BA-6 is different, but units that use ladders on the input and output side also alter response at greater degrees of input and output attenuation, that's a treble leakage boost. 

 

[shabtek]

Re: triode electronics interstage

Is out of stock.
I bought one over the counter 13 years ago from Ned and Mondo--when it was a cool brick-and-mortar store...
Different owner now, anyway I would not bother for this application.  More for
Instrument/limited
Bandwidth application. I will sell mine but cant be bothered to dig it out right now--IIRC inductance is pretty  low

 

[vinyvamos]

Don't know the answer to this, but I note different vari-mu comps tend to have different response slants, some treble reducing, some treble increasing, that stay the same with changes in transformers.  The slant trend Lassoharp and I noted in out BA-6 interstage tests did not change with the transformer swaps. 

BA-6 is different, but units that use ladders on the input and output side also alter response at greater degrees of input and output attenuation, that's a treble leakage boost.

Well I have tried it at various attenuator settings and it made no difference. Then I started probing the sidechain and saw that the control voltage was rising (well falling? as it's negative) as frequency increased. One answer to that, fit a 470pF cap across the cathodes of the 6H6 diodes. This flattened my response out to with 1dB from 20-20K. Happy days!

Tonight I learnt how to use my ancient General Radio Distortion Meter (all tubes, hehe!) and attached is a screenshot of distortion measurements. It can only go down to 50Hz. They have the following groups:

- Bypass. This is the inherent distortion of my cheap sig gen
- GR disabled, +4 input, and minimum gain level for +4 output. O/P atten fully up
- Heavy GR with gain level maxed and O/P atten adjusted for +4 output

First is interstage trafo #1 which is fitted in the unit. I'm not sure if my measured inductance is correct there at 418H. Then I tested another FTR trafo which reads a lower inductance just to see what difference it might have. Slightly more distortion at the low end, as expected, but not terrible.

Can anyone explain the rising distortion in upper frequencies under heavy GR? Maybe there is an obvious reason for this which I have yet to learn of.