Metal 6V6- Safe in old fender amp?

[vibrasonic]

I found a pair of NOS metal 6V6's (GE) , are they safe to use in my Tweed Harvard or 6G2 princeton's ?

[hank57]

I found some tubes at an estate sale. Took 18 tubes for $10 and if half them work for a while, I made out good! Now there was a GE metal 6v6 that I put in an Oahu Valco amp and it sounded to great, just like the glass.

My own experience but I'd wait for an expert.

[19181911]

I would be safe and have a tech put them on a tube checker... I hate putting a bad tube in a good amp. The term NOS has become so loose that I don't trust just anyone. Can you detect any wear on the pins... that's about your only visual clue to any potential issues with the old metal tubes.

[stratman_el84]

I'd be cautious in general regarding using metal-envelope power tubes in guitar amps. Not saying "don't", but do some checking, have a tech test the metal power tube(s) in-circuit while monitoring dissipation. Metal-envelope versions of glass-envelope tubes are de-rated for plate dissipation, as a metal envelope doesn't conduct heat away from the tube internals as efficiently as glass. Metal-envelope tubes were developed as a "hardening" effort to increase durability for military electronics.

Military and commercial equipment that was designed for metal-envelope tubes kept the tubes well within published specs. Guitar amps, on the other hand, routinely exceed tube ratings. That's why it's a good idea to have a tech check & test to see if the guitar amp is going to exceed the metal-envelope tube's ratings to a dangerous degree.

Personally I don't care much for the tone of most metal-envelope tubes in a guitar amp, whether preamp or power tubes. Metal preamp tubes (and even many power tubes) are often very microphonic, as they can't secure the tube guts as tightly inside the metal envelope as they can to a glass envelope because of the differing amounts and rates of thermal expansion and contraction between glass and metal.

That's just my preferences, though. Many swear by metal tubes. Each to his own. My main point is that metal power tubes can't dissipate the same amount of plate wattage as a glass tube, and caution should be used as guitar amps run tubes over their published specs almost as a matter of course.

Strat

[Blue Strat]

General requirements for metal tubes:
1) The plate voltage and dissipation specs are appropriate for the amp in question
2) The tube is good (as shown on a tester)
3) The socket isn't wired with a connection to pin 1 (which connects to the case of metal tubes) as is often the case in Fender amp, and there's no metal tube "grabber" that contacts the metal of the tube (and shorts it to ground).

[stratman_el84]

Oh yeah, the pin-1-to-case! Good catch, Mike! Forgot about that one. I haven't done much with metal tubes since my military-surplus ham-radio days back in the '70s. I remember getting caught by that. Fried a screen circuit (I think it was?...30-plus years of dust on those memories) that way, as the piece of equipment I was working on used pin 1 as a tie-point for the screen resistor on the tube socket.

Strat