Ba-6a owners can you provide some info ?

[Rob Flinn]

Hi all you BA-6a owners (you know who you are)

I`ve been trying to plan the building a replica Ba-6a & have been studying the 2 diagrams that are floating around & notice a few differences. Can someone help try to clear these up, because I`m trying to redraw a correct version of the diagram (I know you`ve heard this all before with the Fairchild).

1. The 2 cathode resistors on the 6sk7. Are they 56ohm (as in diagram with handwritten component values) or 16 ohms (on printed value version) ?

2. On the handwritten diagram there is an 18K resisitor in the area of the bal A bal B section which is a 240ohm resistor on the other version. The 18k is in a different position going from 100ohm bal pot wiper to the 10k pot 4k7 resistor network.

3. On the cathodes of the 6j7. On the handwritten diagfram the resistors are 1k7 & 56R in series, whereas on the printed value version they are 1k1 with 56r.

4. In the PSU part of the handwritten diagram the 10W resistor is 6K & on the printed one it is 6K8.

5. Is there any info on the meter ?


Also is there any chance someone could take some voltage readings. More specifically the cathode & anode readings on all the valves & also the HT readings for the different levels in the PSU.

Sorry to ask so much in one go, & I`m sure this will not be the only set of questions asked on this unit !

 

[NewYorkDave]

I know a guy who owns two BA-6As. I'll forward your questions to him tomorrow.

In the meantime, perhaps this will help:

400kB PDF

I don't remember for sure where I got that document... It was probably from the same friend.

 

[Rob Flinn]

Thanks Dave

That diagram is the one of the diagrams that I`m working from. I believe that this is probably the correct diagram out of the 2 that I have (the other one has the values hand written in) Its a shame its not slightly clearer to read. The good thing is that since most of the circuit is balanced if you can`t read the component value on one side you can look at the other !

Some test voltages would be useful if your friend would be so kind. I want to work out the HT current, so I know what to look for in my mains transformer. Looking at it it looks like it would draw about the same as the sta levels that I`ve built (very roughly a similar valve layout). but since the HT is nearly a third higher it must draw more current.

Anyhow its late & I`m off to sleep.

 

[audiocraft]

Hi Rob ,

I have an original BA-6A VU meter ........
What specific meter info do you need ....???

Luis

 

[Rob Flinn]

I have an original BA-6A VU meter ........
What specific meter info do you need ....???

It can`t be a VU with built in rectification, because they have to use a bridge rectifier on the output of the limiter to feed it. Therefore it must be a dc meter. The full scale deflection of the meter would be useful to know & perhaps a close up pic of the scale on its front so it is possible to get the valve tester prt of it acurate to some degree.

Thanks

 

[audiocraft]

Here goes a pic of the meter scale ....

http://groupdiy.twin-x.com/albums/userpics/10008/thumb_VU%

I have made one with a bigger resolution , if you need it let me know..

Full scale deflection value is 300mV and DC resistance of the coil is 54R ..... assuming this meter is in good shape ...

The serial numbers on the back are B-147578-1 and 57218 but there is no indication of manufacturer ....

Luis

 

[Rob Flinn]

Luis

Thanks for doing this but the link appears to not be working.

 

[Bryson]

[quote author="Rob Flinn"]Luis

Thanks for doing this but the link appears to not be working.[/quote]

Here's a Larger Photo

 

[audiocraft]

Just keep forgeting how to post links in a correct way .........

Luis

 

[Rob Flinn]

Bryson

Thanks for this

 

[rafafredd]

About the BA6A schem... Does anyone can clear my mind about R14 and R15? Why they are there? As I understand the B+in being fed trough the interstage transformer, so why RCA used those resistors? Sorry, I don´t have a link to an online RCA BA6A schem (if someone can point to it, it would be nice), but most of you may have this on your HD... :grin:

 

[Rob Flinn]

I think R14 & R15 would change the Z of the transformer primary, but I don`t know why this is done

 

[CJ]

Those were added later I believe to reduce thumping. When the tubes are doing a lot of GR, imbalance can occur and the transformer can get whacked with some dc. The resistors limit the dc into the transformer. Just my theory.

 

[PRR]

> R14 and R15? Why they are there?

The gain of a tube with a hi-Z load does not change very much as current is reduced. While Gm falls, Rp rises, and the net result is a slight change of gain where we want a significant change of gain.

The fix is to load the tube with resistance similar to the idle Rp, so as current and Gm is reduced and Rp rises, gain falls significantly.

It also sets the impedance that the transformer sees, and transformers can be fussy about impedance.

 

[Rob Flinn]

Anyone have any idea of the HT current for the Ba-6a ?

I`m trying to get a mains transformer for it !!!