Tom Schlangen wrote:

Hi Patrick,

The Ra = 2.5k at Ia = 30 ma, Ea = +300v,
u = 14.3, so gm = 5.72 mA/V.

thank you for adding this data. May I include
it as an example in the sheet?

( 6BQ5 triode u = 20, Ra slightly less.)

I agree on 6BQ5 Ra (say, 2k), but I have a hard
time to get mu in excess of, say, 15 from trioded
6BQ5. But I don't nitpick: Be it 15, 17,5 or
20 under optimal circumstances, it doesn't matter
much under real world circuit conditions.

The u of all the EL84/6BQ5 I have tested had u = 20.
Circuit gain was about 18,19 with high value RL.

See the 2.5 Mb of data from STC at

http://www.retrovox.com.au/STC6BQ5.pdf

The triode curves there show the gain with CCS load is 18.2 at 20 mA Ia,
and 20 at 40 mA.
Samples I tested were Sovtek and other Oz made tubes and they went u=20
at 14 mA.

These EL84 are extremely linear with a CCS load.





1/2 12AX7 are not regarded as good triodes any more
these days

Probably it just ain't "fashionable" nowadays :-)

OK in preamps. But the " in brigade " likes more in everything,
so all triodes are expected to have high current, high gm and low Ra,
regardless of where
they are used, but it does not necessarily follow that the sound will be
better
and the measurements are better.

The 12AX7 was a cheap high gain signal triode invented at a time
when the need for such a creature came with demand for electronic
equipments to do more than
just put a radio voice on the mantle piece.
I think they were used even in V2 rockets in WW2.



On the other hand, the absurd prices (of certain
brands) on Ebay tell another story.

Personally, I never used 12AX7 in a DIY project myself,
I admit. No hard reasons, but my finger just automagically
grab an octal based 6SL7 out of the drawer when highish mu
is needed.

I have used 12AX7 often, and they are great for the first stage of a
phono amp for
MM. Hopeless for a low output MC; you need the step up tranny, or
a 6C45pi, but then ppl argue about what sounds best....

I have only used 6SL7 in the front end of SEUL amps, with
6SL7 or ECC35 for V1, ECC32 or ECC33 for V2, and then 13E1 for output.
I had to pick through the SLs though to get a quiet one.

I'll have to work more at training my finger to be automagical.



Tom

P.S: Next on the bench probably will be triode strapped
EF184/6EJ7 frame grid pentode.

There must be 100 tonnes of EF80 or 6BX6 lingering forgotten and
unwanted around the world.
Many were made for the TV IF stages of tube TVs.
But they make fabulous triodes, imho.

Any chance of doing a test on those?

Patrick Turner.



--
The opposite of a correct statement is a false statement. But the
opposite of a profound truth may well be another profound truth.
- Niels Bohr