Flashing power amp tube and loud pops - PRRI 65

[olliewtf]

Hey all,

my 65 RI princeton possibly bit the dust yesterday. Was playing quietly, and all of a sudden the sound got notably louder and distorted. All of a sudden im hearing loud hum, big pops and crackles, and its all smelling a bit hot. I noticed each pop and cut matched a bright flash in one of the 6v6 power tubes, like a flash gun going off.

Can I safely assume the valve is definitely dead and I need a new pair?

Should I be investigating a possible cause?

Im fairly confident with an iron and a multi meter, I build pedals, but Ive not worked on an amp before (though I understand the risks and precautions needed with valve amps). I would confidently pull the chassis and investigate wtih some guidance.

many thanks!

[jthomas]

Start by replacing the power tubes and see what you have got. The tube likely shorted internally. It may have taken out other components, but I have had the same thing happen and all I had to do was pop in some known good tubes. I was lucky. You also should use your meter to check the fuse as it may have blown. For an extra margin of safety, when you first power it up with new tubes, you might use a lamp limiter, just in case. If you do a youtube search you will find examples of that.

Make sure that you have the right fuse in the fuse holder. Sometimes you find a fuse that is greater than the circuit calls for which can lead to damage to the amp (i.e., the fuse is too large to perform its function to protect the amp by blowing).

[øøøøøøø]

Most likely the 6V6 shorted on its own and you need a new pair.

If it didn’t blow the fuse, the likeliest case is that all else is fine.

Two downstream effects come to mind that might be worth checking:

Sometimes an arcing output tube can take out a screen resistor, and sometimes it can scorch the circuit board or tube socket.

I *think* the PRRI doesn’t have its sockets mounted to the PCB, which is good. But definitely have a peek at that screen resistor

[olliewtf]

Thank you - really helpful input.

Ill order some tubes soon and start reading up on biasing. Ill pull the chassis too and check the screen resistor as suggested.

[øøøøøøø]

Thank you - really helpful input.

Ill order some tubes soon and start reading up on biasing. Ill pull the chassis too and check the screen resistor as suggested.

470 ohm 2W resistors. On the schematic, they say R39 and R40. If they're not mounted directly to the 6V6 sockets themselves, they are likely labeled thusly on the PCB.

Look for a pair of resistors larger than most others in the amp, with yellow, violet, then brown stripes.

If they're not visibly charred or damaged, they're almost certainly fine, and you just need to swap in a new 6V6 pair and check bias.