Plexi 50 testing, getting weird pin values

[neikeel]

First recheck your connections.

Take the HT fuse out.

Switch the amp on - mains fuse ok?

and then follow the ac into the mains switch, check the heater voltages from V5 to V1.

Check the voltage at the apex of your rectifier (400-450v dc). Follow that voltage to where it is hooked up to the HT fuse.

If that all checks out then you have a short in the HT down stream of the HT fuse.
-it could be the choke (unlikely unless you have trapped the wire between board and chassis bolt)
-check the wire to the OT centre tap

- check the wire that exits the choke back to the board which feeds the HT onto the first of the dropping resistors (the screen filter cap and the screens are fed from these, follow the wire to the cap and check the cap is earthed, then do the same for the wire to the output socket pin 6 and the resistor that jumps to pin 4, again no shorts or wiring errors.

You need to follow this method front to back.

As I cannot see your bus wires or the wires to each respective turret and am not familiar with the classic tone layout it is hard to say more (I am better with Marshalls or the Metros as they are very accurate copies for wiring colours and layout).

Sometimes you have to disconnect wires to isolate parts of the circuit, but that can come later.

Do the above first and report back. Someone will be able to guide (I have to go out later!)

PS preamp wiring looks a lot neater!

[Major Sparky]

I checked my connections to make sure they were good and I couldn't find anything off so I took out the HT fuse, plugged in the amp and turned it on. As soon as I flipped on the power switch the mains fuse blew. So, what's going on? This wasn't happening before I made the those connections I missed on the board. I went back and double checked and made sure I made the right connections.

To clarify the connections I missed were the connections between the two pairs of diodes, and the connections at the positive side of the two 10 uf caps that connect the turrets together, the connection that connects those two turrets to the turret to the left of them, and then the connections between pins 1 and 2 of the bias pot. And one last connection between one side of the 47k resistor to the turret that connects to pin 3 of the bias pot.

Does anyone know what's going on? I'm hesitant to do anything else because I only have one more mains fuse left so I need to know what's going on before I risk loosing that one.

[neikeel]

If it blew the mains fuse with the HT fuse the problem is up stream of the HT fuse, ie in the mains wiring, or the switch wiring or more likely the rectifier and mains cap wiring.

Do you have a link to the schematic and layout you are using?

[Major Sparky]

Here's the layout, I don't think triode has the actual schematic on their website, just a link to a site that has marshall schematics.

http://site.triodestore.com/50WPlexiLayout.pdf" onclick="window.open(this.href);return false;

Some of the wire colors aren't the same. For instance, one of the caps has a red and blue wire going from it, I switched the colors accidentally, but they are wired up correctly.

[neikeel]

Here's the layout, I don't think triode has the actual schematic on their website, just a link to a site that has marshall schematics.

http://site.triodestore.com/50WPlexiLayout.pdf" onclick="window.open(this.href);return false;

Some of the wire colors aren't the same. For instance, one of the caps has a red and blue wire going from it, I switched the colors accidentally, but they are wired up correctly.

Ok just to check that the bias feed is in place (on your layout it is a short dotted yellow jumper wire from the turret with ac (one of the red wires for the PT) to the bias feed resistor (220k)?

Oh and BTW if you have a spare lamp (normal bulb) at mains voltage you can wire this in across your mains fuse holder - if you have a short the lamp will glow, if all ok it will not. That way you will not keep on blowing fuses. Of course once it glows you power off - do not leave it glowing!

[Major Sparky]

I checked the diodes to see if any of them were fried. Two of them read about 548mv for forward voltage drop, the other two read about 8mv. I'm guessing those are the one that are fried. I'm gonna replace them and see if that helps things.

[Major Sparky]

I changed the fried diodes and now my amp switches on without blowing fuses. However, I'm still getting pin voltages that are off.

Here's what I'm getting:



And here's what metro says it's supposed to be around:


Anyone know what could be wrong? The measurements were taken with both switches on and no tubes in.

[neikeel]

Those voltages are probably not far off depending on you meter.

The one that is off is pin 8 on V2 which I wold expect to be around 200v based on your other voltages, check the resistor (100k) on top of V2 socket.

[toner]

...The measurements were taken with both switches on and no tubes in.

V1-V3 will have very high plate (pin 1 and 6) voltages and the cathodes (pin 3 and 8 ) won't be right without tubes.

Put V1-V3 tubes in and check voltages again. Some will still be higher than the chart until you install power tubes but they should be fairly close. Post any voltages that may be way off.

[gldtp99]

Yes, without tubes installed it is about useless to compare measured voltages to a chart---------measure voltages with tubes installed to compare to voltage charts/schematics.
The only voltage I'll measure without power tubes installed (on a new build) is the bias voltage------ i measure bias voltage and make sure the bias adjust pot works properly before installing output tubes........................gldtp99