Scratch 2ch Sta-Level Build

[StarTrucker]

Finishing up a scratch build 2 ch Sta-Level. Having a couple problems that I hope someone here can divine the answer to.

Used the Drip schematic with the mode and recovery knobs. Swapped the rotary Recovery switch for a 5meg pot. Voltages everywhere in the amp are really close to spec (300V coming in from the PSU), but the output tubes seem to be drawing more current than the schematic. 9.5V on the cathodes and 180v on the screens.

1. Voltages are flying around when set to single or triple mode. Bounce around +-20% everywhere in the side chain and input section and about +-2% everywhere else in the amp. This happens on both channels. Double or bypass mode is rock solid. When set to single mode and minimum recovery (220k between 6AL5 Plates and GND) the threshold sounds like it's at 0 and no signal passes except for the quietest parts.

2. Blowing out the inputs on my digital interface, even when it (Antelope Discrete 4) is set to max headroom. The schematic says there should be up to 16Dbu of attenuation on the output but the pads seem to be more like 3db max attenuation. Also, this is primarily above about 1kHz, not much attenuation down low. With the input all the way up and the output rolled all the way off, I'm getting about 20v P-P of output.

3. VU Meters are not going up to 0, even with the 10k calibration pot set to 0ohm (just the 68ohm resistor to ground). The meters do work though, showing compression at the right times but not returning to zero.

Ch1 Input - Edcor 600:15k
Ch1 Output - Edcor 8k:600

Ch2 Input - Jensen 10k:10k
Ch2 Output - UTC LS55 8K:500
Strapped 500Ohm resistor across Output XLR jack to simulate 500ohm load before hitting the interface inputs. Still way too hot for them. Is there a better kind of pad to use?

Thank you to everyone here who helped bring this to life!

 

[abbey road d enfer]

Used the Drip schematic with the mode and recovery knobs. Swapped the rotary Recovery switch for a 5meg pot. Voltages everywhere in the amp are really close to spec (300V coming in from the PSU), but the output tubes seem to be drawing more current than the schematic. 9.5V on the cathodes and 180v on the screens.

Whatever modification of the release circuitry has nothing to do with the output stage's operation.
9.5V on cathodes indicates either a lower current or a wrong value of the cathode resistor (including balance potentiometer), or a defective tube.
Measure these voltages with only the output tubes in place, it's the first step.

1. Voltages are flying around when set to single or triple mode. Bounce around +-20% everywhere in the side chain and input section and about +-2% everywhere else in the amp. This happens on both channels. Double or bypass mode is rock solid.

Double and triple modes disconnect the control voltage issued from side-chain, thus putting the unit in bypass, until connected to another unit. There is nothing to learn from these positions with a single unit.

When set to single mode and minimum recovery (220k between 6AL5 Plates and GND) the threshold sounds like it's at 0 and no signal passes except for the quietest parts.

Seems to indicate a wrong control voltage. You need to check the DC voltage at pins 1 & 5, should be about 25V. Also check the voltages on the cathodes of the 6386.

3. VU Meters are not going up to 0, even with the 10k calibration pot set to 0ohm (just the 68ohm resistor to ground). The meters do work though, showing compression at the right times but not returning to zero.

Seems to indicate an abnormally low idle current for the input pair, which corroborates with a too low control voltage.
Could be many things, including aforementioned 25V reference, but also duff 6AL5, or anything disconnected in this area.

 

[StarTrucker]

Ok,

1. Measured voltages with just output tubes in place.
B+ = 319V
Screen = 201V
Cathodes = 11.4V


2. So can I delete the whole Sw1A part of the schematic and go straight from the 56k and 4k7 resistors to the volume pot a-la 436C?

3. Pins 1&5 are 24.6V
Using 2 6ba6 instead of 6386. Cathodes are
1.26V
1.24V
Wired them up using the Manley T-Bar schematic. Did not include the 68ohm resistors. Are they necessary?

4.Fixing other issues and then checking on this.

Thank you!

 

[abbey road d enfer]

Ok,

1. Measured voltages with just output tubes in place.
B+ = 319V
Screen = 201V
Cathodes = 11.4V

Which is correct.

2. So can I delete the whole Sw1A part of the schematic and go straight from the 56k and 4k7

Forget about the 56k, just connect the 4.7k.

resistors to the volume pot a-la 436C?

You mean the release pot...?

3. Pins 1&5 are 24.6V

That is good.

Using 2 6ba6 instead of 6386. Cathodes are
1.26V
1.24V

Probably OK.

Wired them up using the Manley T-Bar schematic.

Show us...

Did not include the 68ohm resistors. Are they necessary?

Can't tell without a schemo.

 

[StarTrucker]

This is the T-bar scheme. Din't bother with the 6386 socket. Just put in two 6ba6 tubes and connected the input pot leads directly to the grids, no 68ohm resistors.


For the Release pot 436C question (3rd quote in your post) I meant can I wire directly from the 4K7 to the dual input pot and leave out the 10meg and 1uf cap, and leave the release pot and .5uf cap to ground.

 

[abbey road d enfer]

This is the T-bar scheme. Din't bother with the 6386 socket. Just put in two 6ba6 tubes and connected the input pot leads directly to the grids, no 68ohm resistors.

I doubt the 68 ohms resistor have any use, so you're probably good without them.
Grid-stop resistors are often in the 10's of kiloohms. Maybe they meant 68k?
There's no hard rule for grid-stop resistors. They're needed only in case of HF oscillations.

For the Release pot 436C question (3rd quote in your post) I meant can I wire directly from the 4K7 to the dual input pot and leave out the 10meg and 1uf cap, and leave the release pot and .5uf cap to ground.

You can leave out the 10 Meg, for the 1uF cap, it's debatable. You may start without it, it would give faster attack and release times, which may result in unpleasant LF distortion.

 

[xazrules]

Voltages all around the place to me seems oscillation or motorboating.
Have you tried disconnect completely the sidechain? Just remove the attack resistor after the 6al5. It will become a normal amplifier. If you used the sta level output pad it adjust only 6db. Remove the 1k to have more range.
Cheers

 

[StarTrucker]

Wired up the side chain like Abbey said and updated the output pad and all is well as far as speed controls and run away voltages are concerned.

2 last things : fast transients on vocals and electric guitar are distorting even when set to moderate speeds. Voltages all look right but there is a pretty good sizzle on the big hits and plosives. Put a 200ohm balance pot on the 6ba6 instead of the pair of 130Ohm resistors?

The unit is microphonic. Tapping the chassis or even singing into it gets picked up and amplified. I've heard microphonic tubes ringing when you tap them but never being able to sing into the inside of the amp and hear it through the speakers.

Thank you for your expertise!

 

[xazrules]

Yeah the 6ba6 are very microphonic no worries about that. Mine go in feedback if near a speaker!
Which type of distortion is it? Send some sample, it could be the tubes not well matched.

 

[abbey road d enfer]

fast transients on vocals and electric guitar are distorting even when set to moderate speeds.

That's the limit of the design. The weak point is the limited current capability of the side-chain for feeding the time-constant capacitor.
It's what Narma Reims took up to fix when he created the Fairchild 660/670. Added a 20W amp for the task.

 

[StarTrucker]

Replacing the 6ba6 pair with a NOS GE pair. Other than this, what is it that causes the distortion when it compresses? Right now it is unusable when in comp mode.

 

[abbey road d enfer]

Any chance to visualize waveforms? Or at least listen...

 

[StarTrucker]

Probed the output XLR and sidechain output and nothing useful to look at there. Sine Waves were clean and bass through the unit was too fuzzy to see anything. I attached a couple clips of the snare drum and bass and then bypassed on each maybe you can hear the problem.

 

[abbey road d enfer]

I only get an error message.


You say bass is too fuzzy.
Can you post a waveform of a low frequency sinewave?

 

[StarTrucker]

Attached two pics and a video.

First pic of the clean 55hZ sine wave is the grid of the 6v6 with the comp in bypass. Second is the gird of the 6v6 with compression engaged. The video is of the output with the probe on the output XLR with the same 55hZ wave and compression engaged.

First clip : Raw Bass
Second Clip : Gates Bass
Third Clip : Gates Snare
Fourth Clip : Raw Snare
View attachment bassbypass.mp3
View attachment gatesbassdist.mp3
View attachment gatessnaredist.mp3
View attachment snarebypass.mp3
View attachment IMG_7015.MOV

 

[abbey road d enfer]

Looks like there's too much gain.
Can you probe the 12AT7 grids?

 

[StarTrucker]

These are the grids without compression and with. More gain with compression engaged.




Probed the plates here. They go in phase when compression is engaged.

 

[abbey road d enfer]

Nothwest looks very weird. Looks like an oscilation. Are you positive it is "without"? Is it the same scale betwee Nothwest and northeast?

 

[StarTrucker]

Yes nothing is changed on the scope or input. 10k:10k input transformer and 1.23v input from the signal generator. Can the 6al5 in the side chain oscillate being a diode and not an amplifier?

Since it's a pretty clean trace on the sine wave without any hair on it does that mean it's the sidechain or timing network modulating the otherwise clean wave getting passed through the 6ba6?

 

[StarTrucker]

this is the probe on the output of the 6al5
View attachment IMG_7041.MOV