Re: Cruelty to a 12AU7 ?


"Patrick Turner"
Phil Allison wrote:

** 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 ?

Not very long with Ea = 460V and more during warm up.


** During a normal warm up ( ie amp on standby) plate voltage = zero, but
cathode supply = -195.


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


But if you wish to keep the direct coupling, then use a much more rugged
tube such as a 12BH7 with the same pin out as a 12AU7.

It should just drop straight in without adjusting anode loading on the
gain triode halves, and gain should be similar or slightly less than the
12AU7.


** Thought of that, just after making the post.

Original 1970s Ampeg SVTs used a 12BH7 to do the job.

Tried a ( Soviet) 12BH7A already, worked like a charm.

Specs say it has a -200 volt cathode / heater rating too.



....... Phil





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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)
  • Cruelty to a 12AU7 ?
    ... half of each wired as a direct coupled cathode follower. ... Plate dissipation = 1.35 watts. ... Heater / cathode voltage during warm up = -195 volts. ... What happens to the 6550s when the heater cathode insulation fails during ...
    (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)