[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

[dmp]

I'd assume you'd use the 500 ohm winding for the secondary. So blk and red/yellow would go to the 'from-out-tx-secondary' connections, grn / blk on the schematic. The rest of the wires can be capped off

 

[lilpaula87]

I'd assume you'd use the 500 ohm winding for the secondary. So blk and red/yellow would go to the 'from-out-tx-secondary' connections, grn / blk on the schematic. The rest of the wires can be capped off

Thanks a lot! I appreciated.

 

[wrentema]

Thanks Bernbrue and @zayance for creating this PCB. And thanks y'all for all the info here! There was enough info here to finish this build and so far it works great! I build a 2U version to save some space. The 2 EL90s are sticking out a tiny bit. I drilled some holes for airflow. For build #2 (yes going for stereo) I will try to lower the EL90 sockets a tiny bit.

 

[wrentema]

A few technical questions:

For R3 I used 2x 470 Ohm 10 Watt resistors (so 940 Ohm 20 Watt). They're still too hot to touch. It doesn't melt the solder.. Is this a problem? I can replace it with a wirewound of course. Just wondering.

My voltages are here. On the left the voltages from Bernbrue as reference. On the right my voltages. Is there anything that looks worrying? The ones in red are more than 10% off. Mostly wondering about P&Q. The should be the same. I think it changes if I swap the tubes but could test this one more time.

 

[TLRT]

A few technical questions:

For R3 I used 2x 470 Ohm 10 Watt resistors (so 940 Ohm 20 Watt). They're still too hot to touch. It doesn't melt the solder.. Is this a problem? I can replace it with a wirewound of course. Just wondering.

My voltages are here. On the left the voltages from Bernbrue as reference. On the right my voltages. Is there anything that looks worrying? The ones in red are more than 10% off. Mostly wondering about P&Q. The should be the same. I think it changes if I swap the tubes but could test this one more time.

You can use 470R/20-25W wirewounds,then they won't be so hot.

 

[dmp]

A few technical questions:

For R3 I used 2x 470 Ohm 10 Watt resistors (so 940 Ohm 20 Watt). They're still too hot to touch. It doesn't melt the solder.. Is this a problem? I can replace it with a wirewound of course. Just wondering.

My voltages are here. On the left the voltages from Bernbrue as reference. On the right my voltages. Is there anything that looks worrying? The ones in red are more than 10% off. Mostly wondering about P&Q. The should be the same. I think it changes if I swap the tubes but could test this one more time.

Measure the voltage across R3 with a multimeter and then calculate the power with V^2/R
The BOM says 25-50W, so you are undersized, right?
If it's within 80% of 20 W I would increase the resistor rating. Regardless of the resistor rating, you are dissipating a lot of heat, so give them space. I would get one that mounts and move it onto the metal side of the case. Like this
And the voltages you are showing on BA6A show them running hotter. You have 130 ohm for R85/R95? Then they are running 10 mA with 61V, and it's a max 3.4W tube. You can calculate the dissipation with your voltages, 0.6 W I think? Don't think there's a problem.

P&Q are the grid voltages which are pulled to ground by the 150k. Not important. It is a cathode biased stage, so the voltage across the 150R + 100 Trimmer is important. You can calculate the tube current and the dissipation if you are curious.

 

[wrentema]

Measure the voltage across R3 with a multimeter and then calculate the power with V^2/R
The BOM says 25-50W, so you are undersized, right?
If it's within 80% of 20 W I would increase the resistor rating. Regardless of the resistor rating, you are dissipating a lot of heat, so give them space. I would get one that mounts and move it onto the metal side of the case. Like this
And the voltages you are showing on BA6A show them running hotter. You have 130 ohm for R85/R95? Then they are running 10 mA with 61V, and it's a max 3.4W tube. You can calculate the dissipation with your voltages, 0.6 W I think? Don't think there's a problem.

P&Q are the grid voltages which are pulled to ground by the 150k. Not important. It is a cathode biased stage, so the voltage across the 150R + 100 Trimmer is important. You can calculate the tube current and the dissipation if you are curious.

Great info! Much appreciated.

 

[1wildthing]

Hi guys, I’m almost at the end of the build but I’ve experienced a unusual behavior that I want to check. Even when the output gain is completely in CCW position the sound is still going out of the gear. The output pot only increases the volume of a few db. Is this normal? Do you think I need an H-Pad for the output?

 

[wrentema]

Even when the output gain is completely in CCW position the sound is still going out of the gear. The output pot only increases the volume of a few db. Is this normal? Do you think I need an H-Pad for the output?

Yes this is normal. The original does it too. I installed a -15db H-pad to get unity gain (ish). Works for me.

 

[1wildthing]

Guys please I need some help here. I’m testing my unit at the moment. I have some issues:
- when the mode switch is on triple mode the unit seems to produce some kind of distortion glitches even when I reduce the input gain. The glitches are audible even in dual mode but in a smoother way. They are more audible with faster attack. They disappear when I push down the input gain but at that point the gear is not compressing anymore
- I have the same problem that I’ve seen reported by other users: if I tap on pcb or enclosure I can hear the sound amplified coming out of the output (the sound is “glassy”)
- the more I push the input gain the more I hear the ground noise increasing. I’ve tried to remove the 6al5 tube, nothing is changing about the noise but the volume increases and glitches are not there anymore
Could this behaviors be related to a single problem?

 

[1wildthing]

I've recorded a short test using a vocal acapella to let you understand my problem. In this test I'm steadly increasing the input level. I used it in triple mode with fastest recovery time and here is how it distorts.
Do you have any idea?

 

[fallout]

If you tap on the unit and are able to hear it, I'd suspect a microphonic tube somewhere. Tap each tube with the eraser side of a pencil and see if you can locate which one it is.

For the hum, you can try pulling V1 and V2 (or XV1, depending on what input tubes you used) and see if there's any difference. If so look for the usual suspects (cold solder joints, etc) in the circuit before the input tubes for starters. Since you said the hum gets louder as you turn up the input, I'd suspect something in that part of the circuit.

Not sure what chassis you're using but you can check and make sure you have solid ground connections. Sometimes you need to scrape some paint off around screws holes to make a good connection.

I'm not too familiar with this circuit or layout but I do have a set of PCBs waiting to be put together. I think there's also a bias pot in between the output tubes that needs to be adjusted for hum but it doesnt sound like what you're describing.

Not sure about the distortion/clipping you're getting. Are you sure you're not clipping your converters? I'd verify voltages and double check everything in the sidechain again, make sure tubes are good, etc.

 

[scott2000]

I've recorded a short test

Can't open file here for some reason....

 

[fallout]

Can't open file here for some reason....

I had to use Firefox to open the file, Safari didn't like it.

 

[wrentema]

I've recorded a short test using a vocal acapella to let you understand my problem. In this test I'm steadly increasing the input level. I used it in triple mode with fastest recovery time and here is how it distorts.
Do you have any idea?

I actually had similar problems.. And it’s still a mystery. I took out the “mode/recovery time” pcb. Reflowed all the solder points. Checked the connection to the main pcb. Cleaned the pcb.. Installed it again. Did a little prayer to the DIY gods and the problem was solved.. Not sure how helpful this is haha. It’s worth a try..

 

[1wildthing]

If you tap on the unit and are able to hear it, I'd suspect a microphonic tube somewhere. Tap each tube with the eraser side of a pencil and see if you can locate which one it is.

For the hum, you can try pulling V1 and V2 (or XV1, depending on what input tubes you used) and see if there's any difference. If so look for the usual suspects (cold solder joints, etc) in the circuit before the input tubes for starters. Since you said the hum gets louder as you turn up the input, I'd suspect something in that part of the circuit.

Not sure what chassis you're using but you can check and make sure you have solid ground connections. Sometimes you need to scrape some paint off around screws holes to make a good connection.

I'm not too familiar with this circuit or layout but I do have a set of PCBs waiting to be put together. I think there's also a bias pot in between the output tubes that needs to be adjusted for hum but it doesnt sound like what you're describing.

Not sure about the distortion/clipping you're getting. Are you sure you're not clipping your converters? I'd verify voltages and double check everything in the sidechain again, make sure tubes are good, etc.

I actually had similar problems.. And it’s still a mystery. I took out the “mode/recovery time” pcb. Reflowed all the solder points. Checked the connection to the main pcb. Cleaned the pcb.. Installed it again. Did a little prayer to the DIY gods and the problem was solved.. Not sure how helpful this is haha. It’s worth a try..

Thanks for your suggestions, I've reflowed the joints on the control board and scratched off the paint from the star ground area and now the noise has gone. Unfortunately other problems are still there.
I'm not clipping my converters, they are safely in the green area.

Tapping on tubes, one of the 6ba6 seems to amplify much more than the others (I'm using 6BA6 combo).

I've measured test points on the main board again and these are the values:

A= 80.4 V
B= -9.2 mV
C= 1.18 V
D= 0.57 V
E= 1.083 V
F= -0.89 mV
G= 87 V
H= 125.7 V
I= 136 V
J= ?
K= 2.86 V
L= 2.64 V
M= ?
N= 149.8 V
O= 292 V
P= 0.29 V
Q= 0.49 V
R= 3291 V
S= 192.5 V
T= 22.4 V
As for J and M , I'm not getting a steady value. My multimeter dance from a few +0mV to +5 mV.

Do you guys see something strange?

I've noticed that my EL90s get hotter than other tubes; may be this is related to wrong voltages?

 

[fallout]

DC on the grids of the EL90s seems high. I would look at all the components surrounding that part of the circuit for errors/values along with the control circuit for good measure.

 

[TLRT]

Guys please I need some help here. I’m testing my unit at the moment. I have some issues:
- when the mode switch is on triple mode the unit seems to produce some kind of distortion glitches even when I reduce the input gain. The glitches are audible even in dual mode but in a smoother way. They are more audible with faster attack. They disappear when I push down the input gain but at that point the gear is not compressing anymore
- I have the same problem that I’ve seen reported by other users: if I tap on pcb or enclosure I can hear the sound amplified coming out of the output (the sound is “glassy”)
- the more I push the input gain the more I hear the ground noise increasing. I’ve tried to remove the 6al5 tube, nothing is changing about the noise but the volume increases and glitches are not there anymore
Could this behaviors be related to a single problem?

This is called "thump" your symmetrical amp has inbalance so the DC control signal is audible. You need curve tested 6ba6's,(they swing between 60-260v on the anodes with the musical program) and matched 12at7 triodes,/ el90 pairs.(but match to your working voltages not standard tube tester defaults (Maybe check the 6al5,but that is not really needed, I match them too for 1%)Then apply sinewave to the control circuit and set the maximum rejection with the el90 cathode trimmers.
6ba6's are microphonic generally, but still cheaper to buy more items and try to find the best ones than 6386 where the new JJ's are not really have good curves...

 

[1wildthing]

DC on the grids of the EL90s seems high. I would look at all the components surrounding that part of the circuit for errors/values along with the control circuit for good measure.

This is called "thump" your symmetrical amp has inbalance so the DC control signal is audible. You need curve tested 6ba6's,(they swing between 60-260v on the anodes with the musical program) and matched 12at7 triodes,/ el90 pairs.(but match to your working voltages not standard tube tester defaults (Maybe check the 6al5,but that is not really needed, I match them too for 1%)Then apply sinewave to the control circuit and set the maximum rejection with the el90 cathode trimmers.
6ba6's are microphonic generally, but still cheaper to buy more items and try to find the best ones than 6386 where the new JJ's are not really have good curves...

Unfortunately I don't have a tube tester. I bought on ebay a matched 6ba6 and el90 nos couple. There's any trick I can try without having that stuff nor a great experience tube wise?

 

[TLRT]

Unfortunately I don't have a tube tester. I bought on ebay a matched 6ba6 and el90 nos couple. There's any trick I can try without having that stuff nor a great experience tube wise?

You can find me in the black market I sell curve matched tubes. The original schematics shows the grid voltages per every 5dB compression, I adapted this for ef93/6ba6,you cannot do this measurments easily with standard tube testers, only the expensive ones knows this feature.However you still need equipment to properly calibrate your unit(signal generator,and some kind of spectrum analyzer,I use the RME digicheck for this)