OK, some quick thoughts.
First of all, the LEDs need to be changed to the normal small ones (I use 3mm ones by the way). The most important ones are D2, D3, D7 (as they are used as voltage sources here).
A soon as it is done, you need to check the voltages at the top sides of D3, D7 and probably adjust R9 so that you have around +35V DC there with the input shorted. In my case, I have it with R9 = 470R.
And then we'll see.
Cheers,
Valery
First of all, the LEDs need to be changed to the normal small ones (I use 3mm ones by the way). The most important ones are D2, D3, D7 (as they are used as voltage sources here).
A soon as it is done, you need to check the voltages at the top sides of D3, D7 and probably adjust R9 so that you have around +35V DC there with the input shorted. In my case, I have it with R9 = 470R.
And then we'll see.
Cheers,
Valery
I have R9 at 470R too. New LEDs will be in tomorrow. We'll see what happens then. Thanks for your help.
Sorry I'm a lot slower then most.
Supply is +-70v.
Using 200 ohms resistance between pd-nfb and 200 ohm nd-nfb I am dropping 2.2 volts over each resistance. looks like 11ma of current.
Across r41 and r43 I drop 1.23 volts.
At the direct input I get 3mv. Both boards act the same. Should I adjust the nd pd resistance before going forward? Is it possible that the extra drive simply asked too much from the output stage? Maybe just reducing the bias will make it work together.
Thanks, Evan
Supply is +-70v.
Using 200 ohms resistance between pd-nfb and 200 ohm nd-nfb I am dropping 2.2 volts over each resistance. looks like 11ma of current.
Across r41 and r43 I drop 1.23 volts.
At the direct input I get 3mv. Both boards act the same. Should I adjust the nd pd resistance before going forward? Is it possible that the extra drive simply asked too much from the output stage? Maybe just reducing the bias will make it work together.
Thanks, Evan
Hi Evan,
Looks good. Higher VAS current is good for driving the Lat-FETs directly.
If you want to hook-up a Slewmarter series OPS, it will make sense to reduce the VAS current. You can do it easily, changing R29. Some 270R will give you 4.5-5mA through VAS. In any case, after you get the right VAS current, try to run the signal through the front-end with oscilloscope connected.
If everything is fine, you can hook up the OPS. set the bias trimmer to the lowest bias position before the first power-on with OPS.
Sorry, I'm just trying to make sure some basic safety measures are in place 🙂
Looks good. Higher VAS current is good for driving the Lat-FETs directly.
If you want to hook-up a Slewmarter series OPS, it will make sense to reduce the VAS current. You can do it easily, changing R29. Some 270R will give you 4.5-5mA through VAS. In any case, after you get the right VAS current, try to run the signal through the front-end with oscilloscope connected.
If everything is fine, you can hook up the OPS. set the bias trimmer to the lowest bias position before the first power-on with OPS.
Sorry, I'm just trying to make sure some basic safety measures are in place 🙂
Thank you for the advice and assurance. I like the Idea of being careful. I will change r29 and report back
Evan
Evan
I swapped 274r into r29. Output current is now 5.89mA. I measure around 3mv on the direct input unshorted. If I short the input this drops to near zero. I'm excited to try it out but unfortunately won't have any time until later tonight or tomorrow.
Fingers crossed, Evan
Fingers crossed, Evan
What brand of tube are you using? Are you getting any DC offset on the output or is the servo taking care of it?
Tubes are new tung-sol from tube depot https://www.tubedepot.com/products/tung-sol-12au7-6189-new-production-preamp-vacuum-tube
I ordered them with balanced triodes.
I measure 2.24 volts between pd and nd. Near perfect split 1.11 and -1.13 volts. The two boards are almost identical.
I ordered them with balanced triodes.
I measure 2.24 volts between pd and nd. Near perfect split 1.11 and -1.13 volts. The two boards are almost identical.
My bench usually has 2 square feet open. It looks like a disaster but everything is in reach and exactly where I expect to find it.
