BA - 2C (w/6SJ7/6C5) build completed

Just finished a pair of BA-2Cs using 6SJ7s for the 1st stage and 6C5s for the 2nd.  Power supply was built pretty close to the stock supply with a 235-0-235 PT and 6X5GT plus an additional 40uF at C-5.  UTC A-25s were used for outputs. Here's some data at two different bias points on the 6C5 in case someone needs a reference for comparison.  B + for this set = 278VDC

CH 1:  6SJ7  plate      = 58.9 V                        CH 2:  6SJ7  plate  =  65.0 V
                  cathode  = 2.3  V                                            cathode  = 2.26 V
                  plate current  = 1.53 mA                                    plate current = 1.51mA

          6C5 (cathode resistor = 3K)                                6C5 (cathode resistor = 2270)
                  plate    = 256.3V                                              plate        =  252.4 V
                  cathode  = 10.0V                                              cathode      = 9.23
                  pt current = 3.33mA                                          pt current  = 4.07mA

Looking forward to giving these things a test drive in the studio this weekend.  ;D

very nice!

FYI only,

this may, or may not be the output for that beast, only emmr knows for sure:

Relatively speaking, how easy would the wind be for this particular lobotomy?

Hey, I see my inputs!  XT-2747 is the same primary wind as the BA-2 type, but with a 250 sec. 

Doug

You mentioned it being important to squeeze every bit of bass out of this circuit which I assume is why they used the big .50uF coupling cap.  What I'm not entirely clear on is why this circuit is one that might come up a little short on the low end response - original output iron roll off? or, is it more in the circuit design itself (R11 -C8 network)?

I tried a crude bypass of the boost network by shorting around, nothing too noticeable.  Also stuck an extra pad on the input - 50 to 60dB total pad with the +4 outs on CD player gave plenty of room to work with and seemed to take the small distortions out of the sibilance bands.

I'm going to try and get a freq sweep for each one in the next several days and see what they look like.

One other thing worth checking - correct phasing.  I wired: pin 2(XLR) to input pri pin 6, input sec pin 4 to grid.  A-25 sec pin 1 to xlr pin 2,  pin 6 to xlr pin 3 (3-4 CT)

http://vacuumbrain.com/The_Lab/TA/UTC/A-25/utc_a-25.jpg  (link is suspended)

http://www.coutant.org/ba2c/index.html  (bottom of page)   

Lack of loop feedback makes it important to get flat response through other means.  It means you have to use better transformers than were used in later times.  The BA-2 is already not as flat as the other amps in the line because of the lack of NFB. 

I forget what you did for cathode bypass caps; make them much larger and see where the practical point of bottom end improvement lays. 

The important part is input sec and output pri orientation.  Get it backwards, and the highs can be adversely affected.  Not sure on the A-25, follow the RCA plan on the input.  Low-Z only matters for correct polarity through unit. 

Aha,  forgot about manipulation of cathode bypass caps - I used 47uF (stock 40uF).  I'll try a higher value there.

Thanks

A lot of times anything in 40 year old gear rated 40 mfd will actually measure 80-100, since they electrolytics used to have -10/+100% tolerances.  You have to look at these old electrolytic values as minimum values in certain cases.    Try 470 mfd; I will bet that's closer to the practical sweet spot.  You may also find the iron effects swamp any gain you make there.