6505+ 112 Red Plating power tube?

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cyb

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So I turned on my 6505 + 112 for the first time in a few years. Last time I used it, it worked for a while then randomly blew a fuse and I ended up keeping it at my parent's house in storage.

Anyway, I replaced the fuse and turned it on and immediately noticed that it sounded terrible. I put the post gain on 3 which should be pretty damn loud but it was barely over TV volume. I then noticed a strange burning smell and looked at the power tubes and this is what I saw:

upload_2019-4-19_19-55-9.png


I immediately turned off the amp after this.

I assume the tube is hosed and I should replace them both, but could it be an issue with a component inside of amp itself? I'd hate to buy another set of tubes just to have it happen again somewhere down the road. I've barely used this amp since I bought it way back in like 2012 so it's hard to imagine a power tube already failing but I'm certainly no expert in tube amps, lol.
 

DudeManBrother

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If these are the same tubes you were using when the fuse blew then yes definitely replace them. Blown fuses usually indicates a bad power tube. Red playing usually indicates a bad power tube.

Swap the tubes and see if the problem follows the tube or if the problem is at the socket. Then you’ll know if it’s likely just a bad tube, or something else causing the tube to redplate.
 
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cyb

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I swapped the tube slots and turned it on. The tube that is in the same socket as the one that originally red plated started getting brighter and acting like it was going to red plate too. Then the amp made a popping noise and the tube went back to "normal" glowing levels. I plugged my guitar in and the amp was back to sounding like it should. I played for about 10 minutes with no issues.

I have a bad feeling this isn't just tube related, but some sort of intermittent issue with something on the amp PCB itself...
 

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DudeManBrother

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Sounds like a grid resistor could have a bad solder joint. I think there is a 700 ohm 5 watt resistor on pin 4, and a 2.2k resistor on pin 5 (not positive on those values though). If you have a DMM you can measure those resistors. Otherwise you could probably poke around with a chopstick and find the bad joint, if you’re comfortable working on the amp.
 

DudeManBrother

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191FA14F-84B8-406F-BD06-46A0F974E8BA.jpeg

Here’s where I’d focus first, but I’d definitely be poking around all the resistors and capacitors in the output section.
 

cyb

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Thanks for the info! I just blew another fuse. I do have a DMM so I'll check it out sometime this weekend.
 

cyb

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I checked the resistors and caps in the output section. Measured the correct resistance to pins 4 and 5 of each power tube socket. Didn't find any shorts. solder joints look fine. Anything else I should be checking?
 
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KailM

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You might as well replace the flyback diodes (D25, 26, 27, and 28 --I think; kind of hard to read the schematic on the picture that was posted above -- in any case, they're the four diodes near your power section.

The 6505+ 112 seemed to be equipped with a lot of sub-par diodes. I ended up replacing those flyback diodes in mine, after a power-tube failure. I also had to replace all the rectifier bridge diodes at one point, because they failed. Never had a problem again after replacing them.
 

cyb

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On second glance, there are a few solder joints that could be suspect. I tested all the diodes on the PCB and they all seem to be good. Should I replace them anyway?
 

vick1000

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On second glance, there are a few solder joints that could be suspect. I tested all the diodes on the PCB and they all seem to be good. Should I replace them anyway?
No. Have you tried new tubes yet?
 

cyb

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Not yet, don't have any spares on hand. One of the tubes is obviously cooked, but the other one looks like it may still be okay. Should I try powering it up with one tube, or even no tubes to see if it blows a fuse again?
 

vick1000

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Not yet, don't have any spares on hand. One of the tubes is obviously cooked, but the other one looks like it may still be okay. Should I try powering it up with one tube, or even no tubes to see if it blows a fuse again?
NO!!!

You have to have a pair of power tubes in that amp. No tubes would be OK, and would expose any shorts, but it's proabably just a bad tube or two, or grid resistors failing. Could be crappy rectifier diodes as well. I just suggested trying known good tubes to eliminate them.
 
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cyb

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I'm going to have to buy new tubes anyway, so I might as well try it...just hope the new tubes don't blow up too.
 

DudeManBrother

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Buy a pair of 6L6’s and hit all the questionable solder joints. The fact that something popped and voltages returned to normal, before blowing another fuse, says that it’s an intermittent connection. You’ll want that repaired so you don’t blow the next pair of tubes.
 

cyb

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I'll do that! Thanks for everyone's input so far, I really appreciate it. I'll report back after I order some more tubes.
 

cyb

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Unfortunately bad news. I reflowed all the suspect solder joints and bought some new JJ tubes. When I turned on the amp I noticed the power tube in the same socket as the original tube that was red plating immediately started red plating so I shut the amp off right away. not really sure what else to try, I may have to just bite the bullet and take it to a shop...
 

oneblackened

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Redplating usually means the tube isn't getting any bias voltage. Check the resistance of the grid stopper resistors and the bias feed resistors (for V6, you'll want to look at R73 and R95, and for V7 R74 and R96 - R73/74 should be 220k and R95/96 should be 2.2k). The flyback diodes (D25-28) are also worth checking - they're all 1N4007s.

Check the continuity of the traces to the grid pin (pin 5) too - it could be a cracked trace, in which case you'll need to run a jumper wire.
 

cyb

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I checked R95 and somehow overlooked this earlier...

Added some solder and now the amp is working flawlessly with no red plating. Hopefully this saga is over. Thanks again to everyone!
 

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