Pin 2 to 3 Short

[boots]

I've seen the ole "Pin 2 to 3 Short" many times, but never understood what is going on with that type of failure. In a 6L6 or EL34 or the like, an arc between pins 2 and 3 means the filament shorted to the plate? That seems unlikely, doesn't it?

Is all the action going on inside the tube? Or across the pins on the socket? Or maybe across the wire leads just inside the glass envelope? It always seems to leave a carbon trace on the socket, making me wonder if it happens outside the tube.

I read somewhere that it seems a bad idea to design a tube such that very high voltage and low voltage are fed to the tube on adjacent pins. Some amps are designed with one side of the filament string grounded, which essentially puts B+ a quarter inch or so away from chassis ground.

Does anyone have any insight on the pin 2 to 3 short?

[sluckey]

One common cause of the pin 3 to pin 2 short is caused by wailing away on the guitar with no speaker connected. The open OT secondary winding sends a very high voltage (flyback) back to the primary which is connected to the plate. This high voltage is enough enough to arc across the socket pin 3 to pin 2 which has a low resistance path to ground. Sometimes the arc is bad enough to burn a carbon trail between the pins. If you have a real heater center tap, hopeful you have some proper fuses in place. If you have an artificial heater center tap made up of a couple 100Ω resistors, they will probably explode, which removes the short to ground. In this case you may only need to replace the tube socket/s and correct the open speaker connection. However, be aware that B+ has now been applied to the filaments of every tube on the filament string. They could possibly be damaged too.

Many newer amps have high voltage diodes connected from pin 3 to ground that help eliminate any flyback voltage but they are not foolproof either. Best way to avoid this situation is to ensure that the OT secondary is never left in an open circuit.

[xtian]

+1. I had this type of failure years ago, when I had built a plexi style 50-watt using a 2-ohm Bassman OT, and then accidentally (stupidly) hooked it up to a 16R Marshall cabinet. ZOOM! FLASH! Fire from the chassis! It just burned out an EL34 and my heater balancing resistors, no other damage.

[boots]

I know it is a common failure, and I had the impression it was caused by a bad tube. I have an amp on my bench now with a carbon track on one of the sockets. The B+ fuse was blown, but the filament fuses were OK. The owner told me he was just playing it, minding his own business, when he saw a FLASH from one of the EL34's and the amp went quiet. Of course, I'm aware that owners often augment the truth to make themselves look less stupid, so it could very well be that he fired up the amp without a speaker.

[johnnyreece]

I have one of these sitting on a shelf waiting for me to look at sometime. A long time ago, I bought a JTM45 kit that someone else had given up on (read as: I don't know what they fried before I got it). I tore it down and rebuilt it, and, for some reason, when I'm REALLY cranking it up, I get that same short. I seem to get a bit of a crackle in the sound, then the fuse pops. I've tried replacing the tube sockets, the flyback diode trick, new OT...nothing seems to fix it. Wondering if the PT is somehow compromised...I dunno. Got a two-year-old who seems to think he deserves my attention more than this old amp...Thanks for the reminder that I need to get on this thing again!

[trobbins]

I've seen plastic tube slipped over the pin 3 wire and glued to the socket base in some old guitar amps - as a way of improving the insulation between pins 2-3 where 'creepage' is at its narrowest - a form of bullet-proofing an amp.

Always a good idea to clean any dust or debris off the socket surface between pins 2 and 3, to minimise the chance of tracking, whenever maintaining an amp.

If you are keen, it may be practical to fit a suitable MOV part from each anode to the B+ connection of the OT. Somewhat similar to the flyback diode protection scheme that is often seen, the MOV can constrain the peak voltage on the plate that is the primary cause of the arc/flash-over. Normally the plate voltage may reach say 1kV above ground when the B+ is about 500V, but if the OT has no speaker connection, and sometimes due to gross overload or blocking distortion, the plate can fly a lot higher in voltage, and the MOV can be chosen to start clipping at say 800-1kV above B+, so typically has no interaction. But choosing the MOV and fitting them may not be so straightforward.