Re: Cruelty to a 12AU7 ?



Jon Yaeger wrote:

in article 6eicqsF79285U1@xxxxxxxxxxxxxxxxxx, Phil Allison at
philallison@xxxxxxxxxx wrote on 7/20/08 11:07 PM:


** 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



A qualitative answer:

I have an Allen mono amplifier consisting of (4) 6L6GTs wired together
through an odd output transformer arrangement.

The output stage is direct cathode coupled using 6SN7s. With this
configuration the driver selection seemed to be critical.

In my limited experience, I found that the 6SN7s had to be matched and in
perfect working order, or else the plates of the output tubes would glow
orange or red. Some Russian tubes I tried caused problems. Emissivity and
gain seemed to have a lot of impact.

The use of 6SN7 would be ideal but matching triode sections is not
critical.

But Ea should not be in excess of +250Vdc even though 6SN7 is rated for
a higher Ea.
Don't tempt fate with a too high Ea.

But what is critical is the bias current in the output tubes, and if the
output works in class
AB2 with too low an RL to extract every ugly morsel of power available,
then expect red/orange anodes, especially if the idle dc is too high.
AB2 is rough PA sound. Pda at idle for Ab2 should not be more than 10
watts for 6L6.

In many amplifiers designed by optimistic fuctards, there is no way to
easily check idle bias current
in EACH output tube and no means to adjust it properly; ie, each tube
individually.
And the same OFs seem to think nobody will ever over drive the amp with
a load that's too low,
and there isn't any active protection against Ik going way too high
and causing tube thermal runaway.

Its possible to get 80 watts from a pair of 6L6 in AB2 with Ea =
+600Vdc, and Eg2 = +300Vdc
and some high RL value to allow a wide Ea swing of about 500V peak at
each anode.

The maximum anode efficiency for tubes in AB2 with low idle bias Ia is
about 67%.
If you have 80W of output, there must be 120W of input power, so there
is 40W dissipated in each output tube.
So 20W each tube, 2W less than the rated max, but if somebody has a load
too low, say a 4 ohm speaker instead of 8 ohms,
then Pda easily exceeds the 6L6 22 watt Pda rating, and at suswtained
levels the tubes will fail.

On the same basis, you can get 140W from a pair of 6550/KT88 in AB2.
Or 100W from a pair of EL34.

As long as you have plenty of spare tubes its OK and you don't mind them
blowing rather often.




The best results I had were with NOS American made 6SN7s (RCAs in my case).
There are a whole lot of 12AU7s that should work for you, e.g. Telefunken,
Sylvania, etc. I would recommend Telfunkens because they seem to be stable
and their gain is relatively stable over time.

12AU7 is only what it is and I wouldn't trust one with Ea at over 400V
as in Phil's Ampeg repair.
12BH7 is better, but Ea should be reduced.
And 6SN7 is OK, but is damned octal, and can't be used in Ampeg unless
the
socket is changed which isn't necessary because other suitable tubes are
available.
6CG7 long plate are also OK as these were made sometimes with the *same*
anode/cathode/grids
of 6SN7 but crammed into a nine pinner tube, and slightly de-rated for
Pda.
But 6CG7 wouldn't suit the Ampeg because its heater pin out is different
to 12AU7.

BTW, The NOS Oz made 6CG7 is a really beautiful sounding tube. Don't
tell anyone.

Patrick Turner.






jon
.

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