[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

[brndvnrdn]

Hi all,

I am just finishing my sta level build but am running in to some weird issues.

The B+ AC that comes from the don audio transformer gives me an ac voltage of 355v, which seems a little high.

My B+ DC voltage from the psu is high at 421v DC. Also, the leds on the psu are not lighting up. My power light does come on, so at least it indicates that there is some V+ current flowing.

Also, the tubes do not glow as there is no F+ coming from the PSU.

I am not super experienced in these types of builds, so any help would be greatly appreciated. Any ideas? Maybe the power transformer? I have hooked it up in series for 230v operation as per don-audio's instructions.

 

[bernbrue]

Have you wired the heater underneath ? Pictures would help.
Bernd

 

[brndvnrdn]

Hi Bernd, thanks for your reply.

Yes, I have wired the F- and F+ underneath the board.

Here are some pics of my build https://imgur.com/a/bRW2d

The Lm338 is mounted to the case with insulator.

The power transformer seems to hum a little bit when turned on. Is this normal?

I have isolated the Edcor wires not in use.

Also, when I left the device on for a minute, all of a sudden the meter went all the way to the right and then slowly went back to the left. It is a nos 200mA DC meter.

Thanks for all your help.

 

[bernbrue]

How did you wire the transformer primary? Measure all AC voltages coming from the transformer. Then measure all DC voltages going out from the psu.
Just for testing you could disconnect the bias pcb and check voltages once again.
What tubes have you got in there? Could it be possible that one of the tubes is broken?
Report back what you get.
Cheers
Bernd

 

[brndvnrdn]

The transformer primaries are in series, for 230v operation. Those voltages seem to be right:

V+ AC = 4.9 V
B+ AC =  326 V
F+ AC = 9.3 V

And the PSU outputs with the bias psu pcb attached:

V+ = 3.6 V
B+ = 322 V
F+ = 0.13 V

PSU output without the bias psu pcb attached:

V+ = 3.69 V
B+ = 326 V
F+ ~ 0 V

Possibly a short somewhere in the F+ circuit in the PSU?

Thanks for all the help!

 

[bernbrue]

Yes, there must be an error in the heater psu rail.
Bernd

 

[brndvnrdn]

Allright, put some hours into it and slowly making progress.

I checked the psu since I suspected a short somewhere. Did not really find anything.

I corrected the LED orientation and they now work. When powering the unit on again it had the same issue, but when I pressed the momentary switch the F+ led came on, and the tubes started to glow. The switch is supposed to be a momentary off-switch right?

Also, R3 is getting very hot (solder starting to melt).

Any ideas?

 

[bernbrue]

The heater plus and minus must be connect with the tubes permanently, so no switch or anything else.  Additionally the heater minus must be referenced to ground. This will reduced hum considerably. This connection to ground can be interrupted by a switch, a momentary off switch.
You have placed the switch at the wrong place.

I suggest to mount R3 to chassis. Take one of these 20W resistors that are suitable for external mounting.
Cheers
Bernd

 

[brndvnrdn]

The switch was in the right place, but I figured it out: it was a problem with the lm338 regulator.

My unit works now sounds really great. Still figuring out what threshold/input pad situation I like best, but getting there. Will probably leave both the in and output pad and drop the threshold a bit.

These are the voltages at the test points:

B+ 303V
V+ 3.44 V (seems low but the relays work and so does the power light)
F+ 6.4 V

A 60.3 V
B 0 V
C 1.3 V
D 0.64 V
E 1.25 V
F -0.04V
G 64.4 V
H 110V
I 161.5 V
J 0V
K 2.58V
L 2.54 V
M 0 V
N 163.4 V
O 297 V
P 3mV
Q 5mV
R 296 V
S 210V
T 22.9 V

Nothing out of the ordinary, no?

There is still one small issue: when powering on the unit is quite silent but when the tubes start to heat up and glow there seems to be a hum/buzz coming from the toroidal. Almost like its overloading a bit. Nothing too crazy but not that nice in a studio setting. Anyone know something that I can do against this?

Thanks!

 

[bernbrue]

Good news! You may loosen the power transformer bolt, move the transformer a bit and tighten it again.
Congratulations!
Bernd

 

[brndvnrdn]

Thanks and thank you and Zayance for the design and pcb!

The noise really seems to come from inside the transformer, so tightening it doesnt really help unfortunately. Maybe it's just something with this particular one. Once it's in the rack it will probably be less noticeable.

 

[dmp]

Does the power transformer have a high enough VA rating for the circuit?
What is the brand and model number?

 

[brndvnrdn]

Yes, its the Don-Audio toroidal for this project.

 

[brndvnrdn]

Still enjoying my sta level most issues are fixed now and it sounds lovely.

One thing I want to check: when using the double or triple mode and a fast recovery time, on percussion or drums there is almost a 'gated' like effect. It's almost chopping up the audio. I read the manual and the sound on sound article but wasn't sure if this is normal. Anyone?

 

[garp]

In case of ... maybe someone has a spare gates logo unused ? PM me !

 

[gareth33]

Hi all

Could someone please help me with the wiring for this board? I'm guessing I need to bridge the two '230V' solder points?

Output connector has B 115v and A 115v, where I should I go from for a 230v supply?

Thanks

Gareth

 

[bernbrue]

You'll need this switch.

https://www.mouser.de/ProductDetail/Switchcraft/EPS3SL1/?qs=%2fha2pyFadugO9pDR5d23ezIZQLlI0srb1%252bCN9ThsCGY%3d

Bernd

 

[gareth33]

I have powered her up, but a few things are off. The F+ LED does not light up. Voltages are as follows. Some seems. little off, others are quite close.

A 124 V
B -320 mv
C 1.04
D 0.5
E 1.07
F -112 mv
G 227
H 248
I 178
J 1.1 mv
K 2.5
L 2.47
M -100 mv
N 181
O 315.8
P 13 mv
Q 90 mv
R 315
S 229
T 23.9

All filaments glow it seems. The jewel light does not light up.

Any advice would be much appreciated. 

Gareth

 

[gareth33]

Ok so after some further testing, the voltages are all pretty much spot on now.

The main issue I have is that the 0 v test points (B, F, M etc) are reading small voltages 27mv, and 24 mv for example. Does this mean there's current in the ground plane? Perhaps this will clear up when I calibrate.

I don't have a momentary switch, so I have connected some cable as per the wiring diagram, one of those cables is cut half way, I figure I can break this cable in order to test with the same result.

The other odd thing is test points P and Q on the EL90's. P = 8.6mv and Q = 262 mv

I'm going to swap out the F+ LED, perhaps it's faulty, as the filaments are getting a nice 6.3 v. I also think the jewel light may be blown as V+ voltages are at 3.6v, quite acceptable no?

Going to run some audio through and see how it goes.

Many thanks

Gareth

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[gareth33]

Ok, so she passes audio fine. However, I'm not seeing any movement on the meter, and no compression it seems. Recorded in to PT and could see no change in the waveform and nothing discernible to the ear either.

Also, there is a fair bit of noise. Tried disconnecting the cable to ground (from F-) no hum or no change in signal or noise.

If I tap the 6BA6 tubes I get noticeable microphonics in the output signal.

Not sure where to go from here. Any pointers are welcomed.

Gareth

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