Voltage drop from B+2 to B+3 question

[markedwardsims]

Yep, I missed the RESISTANCE keyword there. Pin V3 pin 8 to chassis = 15.03k

[markedwardsims]

Yep, I missed the RESISTANCE keyword there. Pin V3 pin 8 to chassis = 15.03k

[sluckey]

I think I've figured it out. Looks like your under-board jumpers are wrong for the 9.1K B+ droppers. Looks like you may have B+2 and B+3 jumpered together! Verify with your ohm meter. Gotta pull the board to fix. I recommend you put these three jumpers on the top side of the board under the resistors. I'd just use bare wire.

[markedwardsims]

That would certainly explain it. I wouldn't be surprised if in my decisioning between the top of the board and bottom, I followed the shape of the jumpers from the top, but on the bottom. I'll get it torn apart and report back.

gktamps, the datasheet on those caps is not incredibly specific. it talks about "markings for polarity". i did find this, which leads me to believe that the big stripe is negative:

Screenshot 2023-04-10 at 1.10.23 PM.png

[markedwardsims]

Sluckey, you are impressive. I was able to get the board turned enough to confirm that the jumpers are in fact as you described. I need a bit of focus time this evening to make the repair and will report back.

[sluckey]

That would certainly explain it. I wouldn't be surprised if in my decisioning between the top of the board and bottom, I followed the shape of the jumpers from the top, but on the bottom. I'll get it torn apart and report back.

You don't need to tear it apart to see if B+2 and B+3 are shorted together. Just use your ohm meter. Only takes 5 seconds. If you don't know how then read the manual.

[gktamps]

Great - glad you verified the cap polarity. I did the same, and the datasheet indicates the negative is marked. My apologies for the confusion - you have enough to figure out. Looks like Steve is getting you to the finish line!

[markedwardsims]

No worries, gktamps! That raised dash was convincing, for sure.

Sluckey, you're amazing. I only had to unsolder a single ground wire and was able to remove the under the board jumpers. I corrected it from the top, and I'm happy to say things look much better. I plugged a guitar in and got sound (couldn't lean on it much at all in the house - this thing is loud). Brite switch works. There is oscillation and quite a bit of noise, but this is a huge milestone!

Here's a new voltage sheet. I adjusted the expected B+ values based on the schematic and noted that I'm using 6V6 power tubes:

IMG_9061.jpg

I'm not sure what the proper tolerance levels are, so a review or critique of these values would be much appreciated.

Thanks all!

[pompeiisneaks]

The oscillations may be actually postive feedback instead of negative on the connection from the tap on the transformer to the presence pot. A quick test is to disconnect the wire and see if it goes away. If it does swap the OT transformer leads from the pin 3 of the power tubes on either side of the power section. This swaps negative feedback to positive and you can then reconnect the presence line.

[markedwardsims]

It sounds like the next step on the noise front is to chopstick a bit and remove tubes to locate the source.

I will give that negative feedback wire trick a try and see if that may fix the oscillation issue.

Thanks!