6505+ blows output fuse on standby. Does not with no power tubes. (

[The Dude]

You could just have cracked solder joints on those pins.

[g1]

If they are the filament connectors, you will be reading the short across the filament winding of the transformer, which is normal.

[g1]

If the screen voltage is too high, how would I even go about correcting it? By upping the screen grid resistor values? Right now they're the stock value at 100ohm.

My point about the screen voltage was that if there were a fault further down the line (like open dropping resistor R59 for example, or some burnt trace), then the screen voltage would rise (due to reduced load on the supply).
This is a total unknown amp to you, never worked (for you), and you can't trust what the seller said, so you have to start from scratch.
Do you have a variac or a light bulb limiter? If not, I think building the lamp limiter might reduce voltages enough to where you could do some measurements without blowing fuses.

[The Dude]

And, be sure to check the connector that goes on those pins. It could be charred also.

[Biffstar]

I don't have a variac or a lightbulb tester, no, but I did some more narrowing down tonight, and I think I've got it.

So I tested powering on with a single tube at a time. I hadn't done this yet; I had only tried pairs at a time (and kept blowing fuses).



From the left:
-Single tube in socket 1, powers on, stays stable. Flip standby to play, stays stable and I get an idle hiss from the speaker cabinet. Good news!
-Single tube in socket 2, bam... arc after 10 seconds, blows fuse....
-Single tube in socket 3 (after replacing fuse again), powers on, stays stable. Flip standby to play, stays stable and I get an idle hiss from the speaker cabinet. Good news again!
-Single tube in socket 4 powers on, stays stable. Flip standby to play, stays stable and I get an idle hiss from the speaker cabinet. Good news x3!

So in the image above, the socket 2nd from the left is toast. I had light smoke coming from the socket every time I had a tube plugged into any of the other sockets (where I promptly killed the power), but I get NO smoke from the socket if the amp is powered on with no tubes installed at all. I'm no tube expert, but I'm assuming that the tubes are all electrically connected in some way for there to be a short in that socket ONLY if other tubes are present? And would explain why a random tube would arc when multiple tubes are present?

I removed the power board again to inspect that socket, and there are no visible signs of shorting or burning externally:









And none of the pins or solder pads on the underside of the board display any shorts with a multimeter, aside from those that appear to be intentionally electrically connected, and match the other sockets' connections.

So to reiterate, the socket does not smoke if no tubes are present at all in any sockets, but does smoke if there is one or more tubes in the other 3 sockets. Again, I'm not sure why that is.

And for the record, all 8 of the tubes that I have, despite a few of them having arced several times during my testing, are still good and usable. Their life is probably shit now, but they're still usable in any case.

Can these style of sockets be opened up? Do these split in some way to examine the pins & connections and clean/service them? Or is my only option to outright desolder this socket and replace it?

[The Dude]

With no tubes installed, check voltages on that socket and compare to the other sockets. See if something's missing- particularly bias voltage. Also, I'd resolder the socket pins anyway. You can't always see hairline fractures and it's obvious there's a problem there. After doing both of those and verifying voltages are correct, I'd try it again.

[Jazz P Bass]

What's up with the NTE 110 ohm screen resistor?

Shouldn't that be 100 ohms?

[glebert]

Bottom picture, that one trace goes through pin 1 and 8 (if I have my orientation right) and then goes over to something else, does that go to another tube? If you have a lot of current on that net it may cause that socket to heat up even though nothing is in it.

[Biffstar]

I've taken two full-sized shots of the power board, front and back if you guys want to follow the traces.

That resistor is the one I replaced that had blown (as mentioned above). It's a 100ohm; note the second line of writing on the resistor in this picture. It measures 100ohm as well.

The one thing I've noticed, which is clearly intentional, is that some of the sockets have different unused pins compared to others, and all of the 6L6's go in the same orientation. (again, I'm no tube expert on why that is...)





So, does anybody know if I can just split open the dead socket to examine it? Or are these sealed and I have to outright replace it?

[alexradium]

i wouldn't lose time on disassembling a socket,just replace it,its 3 bucks.
The only concern for me is to understand if its just the socket or the board itself that has become conductive between the pins.
Only way to figure out is use an insulation tester.
Did you try with just the 2 outer tubes if its ok?

[Biffstar]

I didn't try with just the two outers, but I did try with 3 tubes, all in the good slots. The amp turned on and kept stable, but smoke appeared from the bad socket.

[52 Bill]

ISo, does anybody know if I can just split open the dead socket to examine it? Or are these sealed and I have to outright replace it?

I don't think that the main body of the socket is made from multiple pieces, it's one molded piece and the pins are inserted into it. Remove it from the board and replace it with a new one. You can test the amp once the bad socket has been removed, as it should work without it.

To satisfy your curiosity, you can cut open the dead socket to see what is going on inside. It probably has a carbon trace that is hidden from view.

Also, what is the burned resistor on the left side of the pc board?

[Justin Thomas]

Also, what is the burned resistor on the left side of the pc board?

^^^^^^THAT!^^^^^
Wow, good catch! I thought that was a diode... is it THE problem? Maybe, maybe not. But whatever it is, it ain't right!

Justin

[Biffstar]

I've ordered a new socket.. should be here in a couple of days.

Also, what is the burned resistor on the left side of the pc board?

What burned resistor? Which photo? I'm not sure I'm seeing what you're seeing.

[Justin Thomas]

Post #24, first picture, between the two leftmost tube sockets. There's a burned carbon film (light brown body) slightly to the left of the larger metal film (light blue body) resistor. There are several other CF resistors on the board to compare the burned one to for reference. Whether or not that one is your problem, it still needs to be replaced, once the reason for it charring is found.

Justin