RCA BA-6A from scratch

[DaveP]

Hi lassoharp,

Yes I guess so, I am just presuming that the glass versions have identical performance.
So its the fact that they are tough, screened as standard and they look the part.

You know I had never given them a thought until EMRR turned me on to them, its all down to Doug!
best
DaveP

 

[lassoharp]

I certainly prefer them for price and the extra shielding is nice just in case.

I'm just always weary of shifts in perception because of what may result - vendor price hikes.

I had to go through 12 6SK7s to find a pair that matched well enough to use in a compressor build.  Would have been out of reach had they been considered 'special' tubes for 'legendary gear'.  I'm thankful for that.

 

[rafafredd]

argh. I can't log in on the groupdiy gmail account anymore. It asks for confirmation as I am logging in from a different place. So, thgere is a confirmation question. It is:

What animal is Jakob associated with?

very clever question. Everybody here should know it. But I've tride many answers, like gyraf, giraf, gyraff, giraff, gyraffe, and giraffe, and none has worked.

Anyone knows the anwer for that question, so that I can log in?

 

[DaveP]

Rafafredd,

I think its all being moved to the technical section at the top of the page, its secure there.

best
DaveP

 

[bobschwenkler]

Thanks for the postings Dave! We've got a pair of BA6As at the studio here and they sound fantastic. If I ever open up my own place I'm definitely going to build a pair for myself.

Did you get a hold of voltage chart? I have an original manual here and can scan anything if needed. Also have an 86A and manual...

 

[DaveP]

Hi Bob,

Yes I got the manual thanks, Nielsk sent it to me.  When I checked out the timings I discovered it was practically identical to the Fairchild #2 setting, so I'm not surprised you think they are good.

Thanks for your support, much appreciated.
best
DaveP

 

[bobschwenkler]

Cool. Ours also have a fast release mod installed. It's a pretty simple resistor in parallel to ground from the CV bus. I forget what value it is, but it lowers the release constant significantly and is really useful.

 

[Jean Clochet]

This is my first posting on here even though I have been reading for a while.

I like your project very much Dave P and would like to build one of these myself.  I'll be watching your project to see how you iron out all the creases to make it much easier for me -  wink ; - )

One thing - you mention getting the voltages from the original BA-6 manual, is there a chance someone could post them here in the thread or point me to the right place for download if it exists?

Thanks for the inspiration!
Jean

 

[DaveP]

Hi Jean,

Thanks, the voltages are as follows:-

6SK7's Vp=135, Vg2=103, Vk=5.15.

6J7's Vp=71, Vg2=42, Vk=1.18  (I used 6SJ7's which are the same except they don't have top electrode)

6V6's Vp=275, Vg2=285, Vk=13.1 (my 6V6's were Vk=15V)

6H6 66V

OD3 155V (my tube was 151V)

5R4GY 315V (I used silicon diodes)  I usually construct separate DC supplies for the heaters so I turn them on first, wait a minute then turn on the HT, so silicon diodes no a problem.

best
DavP

 

[Jean Clochet]

Thank you Dave
I put the voltages on the schematic I have and am posting it here for everyone's benefit.

One thing, it looks like the schematic I have has the no. 2 and no. 3 grids for the 6J7's grids reversed, unless I'm missing something?
Also, I assume that 6J7 grid 3 is connected to its respective kathode.

Thanks again Dave!

 

[DaveP]

Hi Jean,

That circuit is a simplified schematic taken from a magazine article, it is not suitable to use to build anything.  It misses out the 240 ohm cathode resistor for V1/2 for example!
This is the original

best
DaveP

 

[Jean Clochet]

Ah, right!  Makes a lot more sense now... thanks again!
I'll just leave it up as a reminder of what 'not' to build then : - )

Will now print and study the real schematic...

Jean.

 

[lassoharp]

It misses out the 240 ohm cathode resistor for V1/2 for example!

Not to mention the listing of 56K as the 6SK7 cathode resistors . . .

 

[Jean Clochet]

[quote author=lassoharp]
Not to mention the listing of 56K as the 6SK7 cathode resistors . . .
[/quote]

Hahaha!  I missed that one. 
I wonder how many poor readers of the original magazine article - back in 1950 something or whenever- gave it a shot at building one from that schematic?
Thanks again for the correct doc. 

I don't know what ratio of interstage transformer the original BA6A had or what yours is Dave but, looking at the Sowter replacement one, it specifies as 1:1 or 10K:10K.
So, I assume the primary loading of 2 X 4K7 is/was there to provide something for the rising anode impedance of the 6SK7's to work into - to get more reduction from them by acting as the lower leg of a voltage divider.  I'm also assuming that the secondary loading of 2 X 22K is there for maybe a little flattening of frequency response but not sure.  Any thoughts anyone?
I think I'd possibly like to try the Lundahl 1660 interstage myself and would think that terminating the primary as the original is but playing around with the secondary termination would be all I'd need to do.
Hmm ? 

Jean.

 

[DaveP]

The interstage I used was 15K:15K but it dropped the bass from 100Hz, see earlier post, that would not do at all so I took it out and put 470nF caps in instead, that cured the problem.  Other compressors don't have interstages so I figured what the hell, it works with perfect balance.

The gain of the 6SK7 stage is really low because the 4.7k's and the 22k's are a potential divider, I believe this reduces the possible imbalance to a minimum, (Gain traded for balance).

Your Lundahl is a class act and should be fine, I couldn't justify the expense, sooner or later my wife is going to find out how much this has cost!

best
DaveP

 

[Jean Clochet]

The interstage I used was 15K:15K but it dropped the bass from 100Hz

I see.  Yes that won't do. 
If I'm not wrong, the source impedance into the interstage primary is the two 4K7 resistors in parallel with the 22K's plus anode r's so, it's basically the 2 X 4K7 given that the anode r is high and swamps out the 22K's. 
Since the bass loss you measure started at 100Hz, your interstage must have only had inductance in the range of 15 - 20H (maybe because of current imbalance?) by my reckoning. 
With the Lundahl having approx 290H, I "should" get decent response down to 20Hz (-3dB @ 5Hz with 4K7 X 2 source Z)...  Well, I hope so!

I took it out and put 470nF caps in instead, that cured the problem.  Other compressors don't have interstages so I figured what the hell, it works with perfect balance.

Hey, if it works!  I may end up to do that too.

The gain of the 6SK7 stage is really low because the 4.7k's and the 22k's are a potential divider, I believe this reduces the possible imbalance to a minimum, (Gain traded for balance).

Yes I wondered whay RCA had done that.  Although the unit does already have over 50 dB gain and, without the divider, it would be about 15dB even higher!!!  Is that enough gain?  : - )

Your Lundahl is a class act and should be fine, I couldn't justify the expense, sooner or later my wife is going to find out how much this has cost!
best
DaveP

I don't think I want to justify the expense of a Lundahl either so I'll just sneek the delivery of it to work instead : - ) 

Keep up the nice work,
Jean

 

[emrr]

The inductance of DaveP's interstage is listed at the companies website.  I'm remembering 200H for all windings in series, but don't quote me on that. 

The RCA interstage is 1:1, and I imagine a higher Z and winding R than Sowter quotes. 

 

[Jean Clochet]

The inductance of DaveP's interstage is listed at the companies website.  I'm remembering 200H for all windings in series, but don't quote me on that. 

Yes it lists 200H but with caveats:

"With all windings in the same circuit it is also possible to make a choke. Max. possible impedance of all windings in series is 60 kilohms, or 200 Henries, however that would be before any DC is introduced."

So, with the primaries connected to the secondaries (as one coil) and no D.C.  is how I interpret it but? 

I think we need at least 100H - 150H primary (allowing for a little DC current imbalance) to get good bass:
-3dB low frequency = Source Z/(2 X pi X L) 

The RCA interstage is 1:1, and I imagine a higher Z and winding R than Sowter quotes.

Thank you for this : - )  I did not know before now what ratio was the RCA. 

 

[DaveP]

I tested the inductance on my interstage and it was 19H so that accounts for the low frequency loss.
DaveP

 

[Jean Clochet]

I tested the inductance on my interstage and it was 19H...

Thanks Dave.