Scratch 2ch Sta-Level Build

[abbey road d enfer]

this is the probe on the output of the 6al5
View attachment 122516

Can't see it.
It looks like the NW pic has a big hum superimposed.
SE would result from weird rectification, so maybe the 6AL5 is duff.
Please re-post picture.

 

[xazrules]

from the audio files it seems is not compressing or compressing too fast, maybe there's an error in the single double switch? very easy to make errors there.
check for capacity and resistance from 6al5 output to gnd in single mode.
or check the bias on 6ba6's. the cathode usually have to sits from 3 to 8 volts, less than thet you have too much gain.
or try replace the 12at7 with a 12au7 and check if the feedback from 6v6 to 12at7 is wired properly.

 

[StarTrucker]

Repost of the 6AL5 plates scope.

I left out the single/double switch. attached side chain schemo
View attachment IMG_7041.MOV

 

[abbey road d enfer]

Can't see the pic.

 

[xazrules]

The input transformer doesn’t have to be ct? And bring the cv there? Is like that in the original schematic too?

 

[abbey road d enfer]

The input transformer doesn’t have to be ct? And bring the cv there? Is like that in the original schematic too?

The dual potentiometer creates a virtual center tap.

 

[StarTrucker]

Updated the last post with pics.

 

[StarTrucker]

The input transformer doesn’t have to be ct? And bring the cv there? Is like that in the original schematic too?

Yes original schemo has a 220k load resistor across the secondary in parallel with the dual pot, no CT

 

[abbey road d enfer]

Repost of the 6AL5 plates scope.

I left out the single/double switch. attached side chain schemo
View attachment 122528
View attachment 122615

I believe you made a mistake.


Still can't see the scope pic (or vid).

 

[StarTrucker]

Fixed that last switch problem.

So, still distortion on compression and freak out on the scope

Grounded the center of the dual pot to test the amp wide open. Here is the sine wave at pin2 of the output xlr. Distortion on the leading edge


Here is the xlr pin 2 with the sidechain engaged. rolling the input knob from 8 to 9 o'clock is when it kicks in.

View attachment ScopeCompression.mp4

And the input of the sidechain at the same point when compression kicks in. The pot in my diagram is wired up as a rheostat. Should that matter? Everything else in the sidechain is the same as the schemo Abbey corrected.

View attachment SidechaintoComp.mp4

 

[abbey road d enfer]

Can you probe the voltage there at various levels.

 

[StarTrucker]

no input signal = 0vdc .085vac
Input signal but no compression (stable sine wave in the last post) - 0vdc, .027vac
Input signal and compression (freaky waveform on the scope in the last post) - Jumps rapidly between .1v, 3v, and 6v DC, jumps around between 1.3 to 3.3vac
max Input signal and compression - Jumps around rapidly between 8,12, and 18v DC, jumps around in the range of 4 to 6.4vac

 

[abbey road d enfer]

no input signal = 0vdc .085vac
Input signal but no compression (stable sine wave in the last post) - 0vdc, .027vac
Input signal and compression (freaky waveform on the scope in the last post) - Jumps rapidly between .1v, 3v, and 6v DC, jumps around between 1.3 to 3.3vac
max Input signal and compression - Jumps around rapidly between 8,12, and 18v DC, jumps around in the range of 4 to 6.4vac

Well, this is not normal. There should not be rapid variations.
Either the 0.47uF is disconnected or defective, or there is something that discharges it.

 

[StarTrucker]

Victory! Bad wiring from the controls on the faceplate to the circuit was responsible for a lot of the voltage bouncing around. It was unstable at the fast setting but connecting both the .47uf and 1uf cap in all positions solved it.

Now the only thing is that the meters don't return to zero but get stuck around -3dB. Just add a trim pot between meter and ground and adjust til it's right?

Thank you to everyone who offered their knowledge and help!!

 

[abbey road d enfer]

Victory! Bad wiring from the controls on the faceplate to the circuit was responsible for a lot of the voltage bouncing around. It was unstable at the fast setting but connecting both the .47uf and 1uf cap in all positions solved it.

I don't doubt it but it would make the attack and release times too long for single operation, which requires only the 0.47uF.

 

[StarTrucker]

Got rid of the 1uf cap, cleaned up the wiring, replaced the 6al5. All is well and the unit sounds great and is solid even at higher gain reduction levels. Thank you for walking me through this!

 

[StarTrucker]

OK, the unit is working fine now. All voltages are right except for the meter does not return to zero. With the 10k pot at zero it still only returns to about -10dB.

Sidechain at -12.6V and the 6ba6 cathode voltages are right, zero signal and the voltages are right.

Can I make R7 bigger to get the meter to come back to zero and then put trim pots in for R8 and R9 so the voltage on the cathodes is correct?

 

[abbey road d enfer]

OK, the unit is working fine now. All voltages are right except for the meter does not return to zero. With the 10k pot at zero it still only returns to about -10dB.

Looks like your meter is not sensitive enough. What is its nominal current for 100% deviation?

 

[StarTrucker]

It's a Triplett 327-T true VU meter with a magnecord face/scale. Should I look into rebuilding it? I have a pair that have the same problem.

 

[abbey road d enfer]

It's a Triplett 327-T true VU meter with a magnecord face/scale. Should I look into rebuilding it? I have a pair that have the same problem.

I believe the recommended meter in the Sta-Level is one without the built-in rectifier. That may be the cause of the problem.