Archon went Schizzo

[aristotle]

So we have decided to do a few "classic" metal tunes, which the Archon can nail in spades My Archon is going belly up though. After a "rest" of an hour or so, it comes back fine, but playing through it for a few minutes after that and it fizzes out again and then nothing.

Yeah, I know I can send it someplace (and PRS is just a half-hour away) but I like fixing these things myself. Interested if anybody else had similar symptoms and how the issue was resolved. Here are the specific symptoms...

1. All tubes are good. Tested good and replaced by known good tubes just in case.

2. Pulled the head to observe what's going on. Two right most ECC83 tubes (in the front, and right hand side as viewed from the front) don't light after a few minutes, which coincides exactly with when the amp gives up the ghost. And the issue is there for both the clean and lead channels.

3. Power tubes and power section seem fine.

So, what in the amp is time dependent such that it can give up the ghost after a few minutes of playing?

 

[Kred]

I don't have an Archon schematic, but the symptoms indicate a loss of preamp tube filament power. If both channels go through those 2 tubes, it is reasonable that both channels stop working. The ECC83 filament voltage is either 6.3 volts (2 triodes wired in parallel) or 12.6 volts (2 triodes wired in series). Once they cut out, it should be fairly easy to trace backwards with a voltmeter to find where the filament voltage cuts out. The filament voltage could be AC or DC, depends on the design. The PRS web site says the preamp tube filaments are elevated to minimize hum, which means the filament voltage rides on another DC voltage (usually 50-100 volts above ground). The high voltages inside the chassis are lethal, so please be careful.

If the preamp tube filaments cut out after a few minutes, it could be a poor solder joint that heats up with the filament load and increases in resistance. I would look for a poor solder connection on a wire between the preamp tube socket and the PCB, they usually occur at the PCB end. After the solder joint cools down, its resistance decreases and the connection appears to be OK. But heat it up again and it cuts out.

Some amps have a thermal cutout sensor for transformer overheating that trips the main power, but that should cut out the power section, not the preamp section. I have never seen a thermal cutout on a preamp tube filament.

 

[Dusty Chalk]

What power tubes are you using? How's the "glow" on them? I ran into an entirely different problem that I would probably call "browning out" when I tried to use the Groove Tubes russkie 6L6, which is basically a 6П3C., I.E. closer to a 5881 than a 6L6GC They worked, but they sounded like I had put the wrong tube in, so I immediately stopped using them. I would disrecommend using them.

 

[squirrel211]

If the preamp tube filaments cut out after a few minutes, it could be a poor solder joint that heats up with the filament load and increases in resistance. I would look for a poor solder connection on a wire between the preamp tube socket and the PCB, they usually occur at the PCB end. After the solder joint cools down, its resistance decreases and the connection appears to be OK. But heat it up again and it cuts out.

I had an amp in the late 90's that did this. I'm not saying this is your case, but definitely have this area of the amp inspected.

 

[Kred]

Another shot in the dark...

Some tube amps have solid state regulators in their power supplies, to reduce preamp noise. Popular components are the LM317 or LR8. They really need a heat sink to operate in their safe thermal envelope, but most designers think they can get away without one. They have internal thermal limiters which shut them down if they overheat, and they have to cool down and have the power removed to reset themselves. This also matches the symptoms you have.

 

[HANGAR18]

Replacing the 10 amp fuses with 15 amp fuses is a good idea but I like the previous comment about heat. I'm wondering if placing a small room fan behind the amp would help. I've seen performing bands have air blown air through their amps with common household fans like that before.