New Builder learned so much from his mistakes he now wants to rebuild

[Deke609]

  The LEDs serve as a precise voltage reference in the C4S circuit.

I suspect that the confusion stems from the fact that elsewhere in the circuit (cathode bias of the 12AU7) the HLMP6000 can be replaced with a jumper and numerous builders who've toasted their LEDs have been so advised [Edit - just to clarify: the jumper doesn't replace the LED, but obviates to need for it by making both cathodes share the same bias from the working LED]. Unless one understood that the same diode used in different parts of the circuit performs different functions, one might assume that the diode can be replaced with a piece of wire in all cases.  I might have made that same mistake just a year ago before I learned a bit about biasing transistors (which, unlike the 12AU7, won't "turn on" at all without proper biasing).

cheers, Derek

[Agweeks]

Thank you Derek for the explanation. I am rebuilding this amp because it was the first amp I ever built. I had a lot of trouble but learned a ton. I think I ended up breaking all of the LED’s because I soldered and resoldered everything over and over trying to fix it. I now seem to recall being told I could use a jumper in place of the LED’s for the regular amp circuit. I am sure I simply assumed it was ok to do the same for the speedball mod. So again, thank you for the explanation. I’m new to amp building and have a great deal of desire to learn.

I saw your edit and yes! That’s it. I was told I could use one diode and solder two jumpers to it.

[Deke609]

Thank you Derek for the explanation.

Don't take anything I say as gospel. On my best days I barely know what I'm talking about - just read any of my posts and the responding corrections from PB and PJ for verification. 

I’m new to amp building and have a great deal of desire to learn.

You and me both, brother!  And if I wasn't so fascinated by it all, I would have given up 2 years ago b/c the physics of electronics is just not intuitive to me. But I am SLOWLY starting to wrap my head around the basics.  Good luck with the amp.

cheers, Derek

[Agweeks]

I received the my leds and replaced the jumper in the big board for the speedball mod.  The amp worked fine using my Mac, streaming Tidal.  Then I attempted to use my new Schiit Bifrost DAC plugged into my iphone (using an apple camera adapter) and PC laptop.  Now the two leds on the 12AU7 don't light up and I get no audio out of it.

I have reviewed all of the connections, reflowed several of the joints and even replaced the leds connected to the tube. 

The voltage to is as follows:

Terminal

1.  180
2.  184
3.  0
4.  184
5.  179
6.  fluctuates at a low level
7.  178
8.  0
9.  177
10.  fluctuates at a low level

I'm going to post a photo of the amp in another post below.

Thank you in advance.

AGW

[Agweeks]

here are the photos

[Paul Birkeland]

If the 12AU7 doesn't light up at all, that is a feed from B7/B8 to A4/A5/A9.

If both halves of the 12AU7 don't glow, that would have to be a bad connection at either B7, B8, or A9.  I can't see A9.

[Agweeks]

Both sides of the tube light up. Could it be anything else?

[Paul Birkeland]

Are your MJE350s in backwards?

[Agweeks]

No. I went back and checked. The text faces the narrow line.

[Paul Birkeland]

Ah, yeah, I see that now.

Typically when I see high voltage on the OA/OB terminals on the small board, that's a solder joint on the middle leg of the MJE-350.

...but you have both sides high, hence my curiosity that potentially the 12AU7 isn't glowing and nothing is conducting.

[Deluk]

From post#18 the Crack was working fine with Mac and Tidal. Changes made were non Crack so logic would say that the fault is external. The RCA's were probably left in place during the change to the Schitt DAC. No, that doesn't explain the apparent led fault. If they were fitted the wrong way round the Mac system wouldn't have worked I guess.

[Agweeks]

This feels odd.  The amp worked.  All the LEDs were on, the volts and resistance was good.  I listened to it through my Mac streaming tidal masters with my HD6xx headphones.  Life was good.  Then I took it down stairs and plugged it into the bifrost and it didn't work.  I tried lots of scenarios down there, changing out the cords, plugging into different outlets, using a PC, plugging up my iphone with--and without--the phone plugged up via the camera adapter.  Eventually I brought the amp back up here and discovered the LED's don't light up.  I went back to the instructions and followed them, reflowing joints and testing voltages.  I went to the forum and read threads on what could be wrong.  I even replaced the LED's.  Finally I come to you because I'm stuck.

I've reflowed the center leg of both MJE-350's.  I've reflowed the joints for B7/B8 and A4/A5/A9.  The Bifrost works with the iphone.  I'm listening to it now through my Stereomour. 

On the outer side of the LED attached to A3, the voltage reads 179.  On the leg soldered to the center tap, it reads 172. The other LED reads the same.

Attached are photos of the LED's not working and both sides of the 12AU7 lit up.

Thanks,

AGW

[Paul Birkeland]

On the outer side of the LED attached to A3, the voltage reads 179.  On the leg soldered to the center tap, it reads 172. The other LED reads the same.

Well, those should be 1.5V above ground.  There's likely a broken ground wire or one that is soldered poorly and causing this issue.

[Agweeks]

I reflowed all of what I understand to be a ground wire.  I am assuming black wire is used to designate ground.  I found where lug 18 is reading 89 volts.  I reflowed it.  It still reads 89 volts.  I checked the direction of the rectifiers and found them to be correct.  Could this be it?

AGW

[Paul Birkeland]

What DC voltage do you get between terminals 12 and 3?

Rectifier orientation is not the cause of this issue.