First power up on 124 build. Bias circuit resistor fried.

[martin manning]

Yes you'll need at least 3A to survive the surge on start-up. Ground the red/yellow wire, then your bias circuit won't be doubling as a CT and the first resistor will be happy about that.

[Structo]

Yep

The Red/ Yellow is the center tap for the HT.

Double check all components before powering up again.

Confirm bias voltage before installing power tubes.

[mcshaner2k]

Great! I'll report back

[mcshaner2k]

Well I connected the red with yellow to ground and checked all my wiring and connections again. I installed a 3A fuse and its popping still:(

Here's a few pics. Maybe someone can see something I'm missing here.

[martin manning]

Are you sure that there are no wire ends touching the chassis behind the rectifier boards? You should have G10 backer boards between the PCB's and the chassis.

[CHIP]

Do you have insulated backing boards on your power supply boards?
If not, maybe a a stray wire strand or solder is shorting to your chassis.

[CHIP]

deleted

[mcshaner2k]

I was carful and cut the ends short. I didn't use the insulated boards but instead spaced the boards out away from the chassis using another nut. I even pulled the board away from the chassis while doing all this testing to make sure nothing was shorting out.

[martin manning]

Disconnect the yellow HT lead from the rectifier board or the main filter board (tape it off to be safe) and see if the fuse holds. If it does, that rules out shorted diodes or other problems with the rectifier board. You can check the bias and relay supply voltages then too.

[mcshaner2k]

Ok this worked! No time left tonight to mess with it further. I pulled the yellow HT lead and no popped fuse this time. Measured the voltage there and I'm getting 440dc. Something must wrong on the filter board. I'm glad I made some progress! Thanks

[mcshaner2k]

I think I may have figured out why it was shorting.

I checked my wiring at the filter caps and I had an error. When I assembled the board I had connected the negative leads on both 220uf caps on the bottom side and sent them to ground. I didn't notice this while Looking at the topside of the board as there's a 270k resistor between them.

[martin manning]

If I understand correctly you shorted across one cap and put the full B+ voltage across the other? The cap taking the full voltage may have failed if its voltage rating wasn't sufficiently high.

[mcshaner2k]

Ok I have it straightened out now..

I have both caps positioned on the board so they are parallel to one another with positives on one end and negatives on the other. I have a 270k resistor on either end, one connecting both positives and the other connecting both negatives. I have a jumper on the bottom side of the board connecting one positive to the others negative. I had the ht wire soldered on wrong side, which was the jumper side. I also had the two negatives leads connected on the bottom side and going to ground.

So basically the HT wire was going into the board and going directly to the jumper then across both negatives and straight to ground:( duh.

I didn't catch any of this until I lifted the board. I misinterpreted the schematic when building it. None of the layouts I could find had this board configuration.

Hopefully none of the caps are damaged..

[martin manning]

So you are not using the agb PCB for the filter board?

You may be on your way now, but it's not easy to diagnose an amp with unknown errors built to an unknown mixture of layouts and only a few words to describe the problem!

The good news is if the HT lead went directly to ground then it's unlikely that any damage was done to the caps.

[mcshaner2k]

I'm using the agb standard power supply board not the precision pcb board they offer. I bought the boards thinking it would make my build easier; however, I ended up complicating things by mismatching different elements. Lesson learned...

Thanks again for your help Martin!