Long skinny builders thread

[donhughes111]

@ meanie and manniraj

Did you trim your boards or are they built differently than mine?

Don

 

[manniraj]

@ meanie and manniraj

Did you trim your boards or are they built differently than mine?

Don

Yes I trimmed my J2 boards near the LED and cut off the portion and then made sure that the traces do not touch any of the chassis portion as I had used the 300mm depth chassis.

 

[Reima]

Finally finished the PSU, I am getting +-23.8V. It has been quite a while since I started this project, the end is now in sight.
I have been working from home since Covid started, the problem is that it was not my home I was working from.

 

[codyt]

Reima - double check your thermistor hookups at the primaries against the schematic. I would also consider twisting the primary pairs together to avoid any potential noise issues.

 

[Reima]

According to the schematic (F6 PSU) there is a thermistor in series with each transformer primary coil (120V). Does it matter if the thermistor is on the live leg or neutral leg as long as there is one in series with each coil?

 

[codyt]

It will absolutely work the way you have it, but I believe splitting the thermistors between Line and Neutral is preferred.

 

[Reima]

Made some changes as per codyt recommendations.

 

[Reima]

Next steps are to hook up the inputs, outputs and test. I will work on them next time I am home.

 

[Reima]

I decided to bring the J2 to where I am spending most of my time. Today I finished the wiring and checked the offset. I got 0.001V on both channels. Tomorrow I will let it soak for a while and then recheck the offset.

 

[6sX7]

Suggested to have the thermistors "laying" on the top side of the primary wires with the bodies bent up into the air. They will get warm with operation and shouldn't be in contact with anything.
Here's a picture from the F4 6L6 build: IMG_1504_zps2330b348.jpg

 

[Reima]

6sX7 thanks, I will make the changes to the thermistors tomorrow.

 

[JeffYoung]

Nice catch 6sX7; I didn't even see it hiding under there.

 

[Reima]

My J2 is singing now. I first tested with it my pair of cheap test speakers. She is now hooked up to my Frugel-Horns being feed by my DAP through my Pre Iron SE. Sounds cleaner than my Sony Vfet, so far I have only listened with a glass of red wine in my hand.
The only problem that I have is that the blue leds are too bright.

 

[Zen Mod]

glue some blue tack on LED

 

[Reima]

^ and silly me was thinking about changing the led resistors.

 

[william2001]

Evil blue LEDs burn a hole straight into my retinas. With my diy, everything left channel gets a nice toned down orange LED, and everything right channel get a nice toned down red LED. Maybe strange, but I like it..

 

[JeffYoung]

Red fingernail polish works well too....

 

[Bud Barnez]

The J2 design doesn't use the SJ74/SK170s in a balanced circuit. The SK170's are just implementing a constant current source for the SJ74s (which form a long-tailed pair).

As it turns out, you're actually better off with GR parts for the current source (the SK170s). According to my analysis (with all the usual caveats), you want the total to be 6 - 10mA (so, for instance, a 4mA and a 4.5mA would be ideal).

Due to difficulties in sourcing parts, been periodically scouting for the harder to find bits before starting assembly of this amp.
Like others, I've struggled to find genuine SK170 in GR grade. There are some BL grades being offered currently in the swap meet section in case these are of interest to others.

Would you advise picking up the lowest Idss versions there rather than holding out for a GR grade set? Could get <6.4mA quads.

For reference:
GR = 2.6 to 6.5mA
BL = 6 to 12mA

So a 6.4mA BL grade would technically fall within the bounds of the GR grade, but higher than Jeff's recommended sweet spot.

 

[JeffYoung]

I'd be very surprised if they sounded any different. My analysis pointed to the sweet spot being both shallow and wide.