[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

[j.frad]

Hi,
thanks I remember doing something similar in my daoc, bringing the AC lines down helped also.
I already bumped R3 to 1K, will try something bigger.
The distortion I hear is present also with vocals. I will do as you suggest and see what happens.

How effective is the bias pot, I mean, when doing the calibration procedure, by how much should the hum drop?
I can barely hear it changing.

Jules

 

[bernbrue]

Hi,
I don't know if you followed the wiring guide and if your grounding is properly done. So I can only guess. The bias pot is only for balancing the cathodes of the output tubes. As soon as you disconnect the heater supply from ground (use a switch) the unit should start to hum. Then you turn the trim pot until hum is reduced to a minimum. Reconnect heater supply to ground. That's it.

Bernd

 

[j.frad]

Wel i Will double check but im pretty sure i followed it accurately.
Disconnecting the heated supply does make the unit hum but the trim doesn't change the hum level, does it reduce it by a lot?

 

[bernbrue]

Hi,
It's not the entire heater supply that needs to be disconnected, it's just the connection to ground. The wiring guide shows it quite clearly. See picture attached in the second post of this thread. The pot reduces hum, but not that much. When reconnected to ground, hum should completely disappear.
Bernd

 

[j.frad]

Yes absolutely, This was very clear 

 

[ilfungo]

Hi Bernd

Can work as output transformer for this project?

http://www.audiomaintenance.com/acatalog/CA-18-VTB2290_extended_info.html

Thanks!

 

[bernbrue]

Not really shure. It's not said how much watt this transformer can handle. Ratio is different as well. Please ask audio maintenance for further details.
Bernd

 

[ilfungo]

Thanks Bernd!
I've just buyed from a member this:

From sowter:

1232 Altec 436C Line output

23kCT/150+150 ohm output transformer for driving a balanced line from push-pull 6CG7 etc. 100% M6 core for high level capability and low distortion. Twin secondaries for 600/150 ohm outputs. Voltage ratio 12.4CT:1+1 Max dc per side of primary 20 mA. Similar to Peerless 16402. Colour coded leads. Supplied with end frames for vertical or horizontal mounting style o. Fully shrouded style "s" option. Rectangular Mumetal Can with threaded grommet style "e" option.


Could be a good options for the Sta Level?
THANKS!!!

 

[bernbrue]

Honestly, I'm not really a transformer guy. Specifications sound good.
Bernd

 

[Paperweight]

I finally got around to slapping together a uTracer vacuum tube curve tracer. Going through about 100 JAN 5749W/6BA6 tubes and trying to match them wired as triodes. Well guess what?  The old fashioned matching by current draw method works just as well.

I've gone through 10 of the tubes so far. To simplify the testing process, I set it up to measure at -1 on the grids and vary the plate voltage from 2 to 150 volts over 20 intervals. I kept track of the measurement at the top end on a sheet of paper so I could circle close matches. Tube #2 read 146.58 volts at 30.785mA. Tube #10 read 146.58 volts at 30.714mA. I did 10 tests on a single tube and the current reading can vary at times by about .300mA up and down. Not too bad. So these looked like a decent match.

Here the #2 tube is the dotted line and the solid line is #10. This is the curves from -1 through -15. Not bad.

This is the curves from -16 through -30.

Finally, the curves from -31 to -50. I probably shouldn't have tried to squeeze so many into the last plot.

So, anyway, if you can devise a way to measure the current draw of a 6BA6 wired as a triode, you can test and match them and get some decent pairs for $5-10 depending on price and availability. It should save quite a bit over the cost of a NOS or new production 6386. Surplus Sales of Nebraska has 28,000 GE made mil spec 6BA6/5749W at $2.75 each.

 

[bernbrue]

Very interesting tests. I´ve tested only a few 6BA6 in my build and had always the impression that they perform extremely well. In direct comparison with a 6386 I can´t really hear any difference. The only thing you have to do is to tweak the bias voltage.

Have you finished your build yet? Please show us your Sta level.
cheers
Bernd

 

[Paperweight]

I wanted to build a dual version from scratch on turret boards and house it in a wooden case. It would probably cost over $200 just for two board sets otherwise. I have a pair of meters and a possible chassis but no faceplate. I'll probably save this until I get some other things finished

The boards are stuffed and soldered but I still have to wire up a Dual LA-2A, dual 1176 Rev D, Gyraf G9 mic pre and a quad of SC-1 mic pres. All of these have cases except for the SC-1 pres which need to have the faceplate drilled. I hate metal work.

 

[bernbrue]

Just a test! I get notifications via email that people have posted in my thread but infact there is no post.  I just wanted to test, that I can see all the latest posts.
Cheers
Bernd

 

[ilfungo]

Hi Bernd!
My Sta level is Alive!!!

2  test point are bit off ( 82v Your refernce):
A 58.1v
G 56v
could be a problem?

I'm tring to use a meter that is specified to be 0 - 30  full scale 100 mV.
It's working but I can't go to 0  ( full scale)
could I change some resistor to go up?

Last :
I'm using Edocor CXPP 10 600 8k
The 2 cables marked as screen must be connected somewhere?

The output it's too hot could I use a Tpad between transformer and xlr out?
THANKS!!!

 

[bernbrue]

Hi,
the wires marked as screen must be isolated. Cut them at the end and use some sort of shrinking hose.
Are you using a 6386 tube or two 6BA6? What is your voltage at test point H and your overall B+ voltage?
Congratulations for your successful build.
Cheers
Bernd

Edit: use this pad calculator
http://www.nu9n.com/tpad-calculator.html
and insert the H Pad right between molex connector "XLR out" and the wires going to the XLR out.

 

[ilfungo]

Now with meter calibrated ( I don't know if it have influence:
A  60v ( as sta level schematics)
G 58V
H  108v
Main B+ 302 v

T pad it's not usable?
Thanks!!!

 

[bernbrue]

Your voltage are alright, nothing to worry about. Of course you can use a variable t pad attenuator. There is a drawing how to wire it a few post before. In case you want to use a fixed pad I would use a 15dB H-pad. Just take a few 220 Ohm resistor and you're done. Pictures?
Cheers
Bernd

 

[ilfungo]

Thanks Bernd
n the next days I will post some Images, just the time to finish all  mod...
I've tried the t pad and it's much better!
Could be a problem if I remove the pot and insert 1k resistor for have the same ammount of pad but with addition of T pad between output transformer and xlr?
Thanks!!!

 

[ilfungo]

Sta Level !
Thanks Bernd for this great project!!!
Incredible sound!!!
for now,some crappy images...

 

[ilfungo]

inside