Cruelty to a 12AU7 ?


** Have an Ampeg SVT-2 Pro ( bass guitar head ) on the bench - it uses
six 6500s in parallel push pull ( class AB) with grid bias to generate about
300 watts. Drive signal to the 6550 grids comes from a pair of 12AU7s with
half of each wired as a direct coupled cathode follower.

The 12AU7's operating conditions are:

Plate voltage = + 395

Grid voltage = - 78

Cathode voltage = - 56

Cathode current = 3 mA

Plate dissipation = 1.35 watts.

Heater / cathode voltage during warm up = -195 volts.

QUESTIONS:

How long will a cheap, Chinese short plate 12AU7 survive ?

What happens to the 6550s when the heater cathode insulation fails during
warm up ?

( The above IS what happened with this one. )

Also - what 12AU7 is the most likely to be OK here ?


...... Phil




.

Relevant Pages

  • Re: Cruelty to a 12AU7 ?
    ... half of each wired as a direct coupled cathode follower. ... Heater / cathode voltage during warm up = -195 volts. ... Specs say it has a -200 volt cathode / heater rating too. ... Plenty of NOS 12BH7 around because it is not the darling tube of the ...
    (rec.audio.tubes)
  • Re: Cruelty to a 12AU7 ?
    ... half of each wired as a direct coupled cathode follower. ... Heater / cathode voltage during warm up = -195 volts. ... gain triode halves, and gain should be similar or slightly less than the ... Specs say it has a -200 volt cathode / heater rating too. ...
    (rec.audio.tubes)
  • Re: how long...
    ... Suppose the cathode has got to warm up, but with ~1kW fed into it, that won't take long, electrons should be quite whizzy after that. ...
    (uk.rec.sheds)