[Ian Bell]

I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?


Cheers

Ian

[Patrick Turner]

Ian Bell wrote:

I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?

Cheers

Ian

Iv'e often seen it used in amps from the 1950s instead of EF86,
but its a 7 pin mini tube.
Its supposedly more noisy ( hum) than EF86.

But with dc heaters it should be fine.

It also has 3 times the gm of the EF86, and can therefore give 3 times
the voltage gain;
so one used as a line stage amp can have a lot of shunt NFB applied to
place gain where
its wanted, say 10 instead of the open loop gain of 300.
But this means THD at a volt output is difficult to measure because its
so low.

There are millions of this unloved tube around.

Its a nice triode when strapped in triode.

Patrick Turner.

[Phil Allison]

"Patrick Turner"



It also has 3 times the gm of the EF86, and can therefore give 3 times
the voltage gain;


** Not true for a resistance coupled stage.

The best voltage gain for an EF86 in pentode mode is around 300 times -
with anode series resistance of 300 - 400 kohms and DC supply of 400 volts
or so.

A 6BA6 cannot match this, let alone give 1000 times gain.





....... Phil

[Phil Allison]

"Phil Allison"

**Correction:


" A 6AU6 cannot match this, let alone give 1000 times gain."




........ Phil

[Ian Bell]

Bret Ludwig wrote:

On Jun 4, 4:45 pm, Ian Bell wrote:
I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?


A couple of boo-teek pro audio vendors are peddling mic preès
straight out of RDH 4 with good results. So I'd say yes.

Can you give me a link to any of these please Bret?

Cheers

Ian

[Ian Bell]

Patrick Turner wrote:



Ian Bell wrote:

I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?

Cheers

Ian

Iv'e often seen it used in amps from the 1950s instead of EF86,
but its a 7 pin mini tube.
Its supposedly more noisy ( hum) than EF86.

But with dc heaters it should be fine.

It also has 3 times the gm of the EF86, and can therefore give 3 times
the voltage gain;
so one used as a line stage amp can have a lot of shunt NFB applied to
place gain where
its wanted, say 10 instead of the open loop gain of 300.
But this means THD at a volt output is difficult to measure because its
so low.

There are millions of this unloved tube around.

Its a nice triode when strapped in triode.

Patrick Turner.

Thanks for that Patrick. Useful info.

Cheers

Ian

[Phil Allison]

"Ian Bell"


Thanks for that Patrick. Useful info.



** It was total ********

- you ****WIT audiophool, pommy ****head.





......... Phil

[Patrick Turner]

Phil Allison wrote:

"Patrick Turner"


It also has 3 times the gm of the EF86, and can therefore give 3 times
the voltage gain;

** Not true for a resistance coupled stage.

OK, I exagerated, but gm for EF86 is about 1/2 that of 6AU6
for the same Ia.



The best voltage gain for an EF86 in pentode mode is around 300 times -
with anode series resistance of 300 - 400 kohms and DC supply of 400 volts
or so.


Most EF86 are in circuits with up to gain = between 100 and 200.

see typical apps descriptions
http://www.mif.pg.gda.pl/homepages/f...010/e/EF86.pdf

typical gm for where Ia = only typical 1mA is around 1.0 mA/V.
Look at how EF86 are used in Quad-II amps and 22 control units,
and in Leak gear et all. Almost nobody ran them at their potential
higher gm point at Ia = 3mA, because B+ would need to be too high
to get a 300V drop across a 100k load R.

So EF86 gain for Ia = 1mA approx = 0.001 x 100k load = 100 on a good
day.
With Ea = 100V and dc RL = 150k, (with following grid bias R bringing
down
total acdc RL to 100k), then B+ is around 250V.



6AU6 data is at

http://www.pmillett.com/tubedata/HB-...t_1/6AU6-A.PDF

See where gm varies from 3.9mA/V to 5.2mA/V for Ia from 5mA to 10.6mA
respectively.

So for Ea = 100V, and Ia = 5mA, and dc RL = 50k, then B+ = 350V.
But if the total acdc load = 40k, gain = 160, but if the
load was made 100k total and Ia = 2.5mA, gm would still be
twice EF86 and twice the gain is possible.

With a µ-follower circuit, the active load of the pentode can be perhaps
500k,
and gain can be up around 1,000.
This can also be the case for EF86, if carefully set up, say with
a paralleled 12AX7 as the top follower tube. If the R between anode of
pentode and cathode
of follower is say 22k and triode µ =100, then active load = 2.2M.
biasing R will usually mean the total active load is rarely more than
1M.
At such high loadings, the gain formula A = gm x RL no longer is
accurate,
and gain has to take in Ra, so A = gm x Ra x RL / ( RL + Ra ) and if gm
= 1.5mA/V at 1.4mA for Ia,
the A possible = 0.0015 x 2,500,000 x 1,000,000 / ( 2,500,000 +
1,000,000 ) = 1,071 = 60.6dB.


In preamps, such high pentode gain of more than 100 is always
accompanied with a loop of feedback to reduce
the gain for the stage for line level preamping to maybe 10.
If there is no follower buffer the pentode gain is then also determined
by the dc load,
and floowing biasing R, and the loading because of the shunt feedback
resistance.

If the follower is used to buffer the pentode from loading effects and
ideally with a µ-follower, then gain reduction can be from 1,000 to 10,
and there is 40dB of NFB, so that 5% THD quoted for say 50Vrms output is
reduced
100 times to 0.05% and at 1Vrms its a lot less than 0.01%.

Shunt FB was done without the expensive buffer in many phono amps, Quad
22 included.

At Ia = 2mA, EF86 has Ra = maybe 2.5M, and it gm = 1.2mA/V, µ = 3,000.

see the curves for Mullard EF86,
http://www.triodeel.com/ef86_p4.gif

At Ia = 2mA, 6AU6 has Ra = also maybe 2.5M, and gm = 1.8mA/V, so µ =
4,500.

amp factor, µ = gm x Ra ( foa all tubes ).

You will never see these figures of gain unless RL was a true CCS.

Tubes like 6BX6/EF80, and 6EJ7/EF184 ( all not well loved by audiophiles
),
have even higher gm at similar Ia,
and thus have what is called a higher figure of merit, important in RF
apps,
but also if you ever want to make a low thd SE amp with FB.

In mass made consumer gear, µ-follower circuits were NEVER used and only
perhaps used in scientific gear
requiring THD at a volt output to be less than 0.01%, which was regarded
as
absolutely fabulously low THD in 1955, and very difficult to measure.

The simplistic original answer I gave was overly simplistic, and
misleading,
and I hope people wanting to use the 6AU6 or other pentode try them to
find out what is possible.
The gain you get is very variable with the circuitry chosen.

A 6BA6 cannot match this, let alone give 1000 times gain.

I think you mean 6AU6, and a gain of 1,000 is possible with the
µ-follower trickery described above.
6AU6 is a "sharp cut off pentode" like the EF86.

6BA6 is a the close cousin to 6AU6, same pin out, but its variable µ,
or "remote cut off" and gm depends on biasing much more than with 6AU6,
and with high Ia the 6BA6 can have higher gm than 6AU6.
6BA6 is magic in radios where -ve AVC is applied from the AF detector
to drastically alter the gain, so that a 200 watt radio station sounds
as loud as a 5,000 watt station.

The use of vari-µ tubes is interesting for ppl building
voltage controlled gain amps. At high voltage output levels, the vari-µ
tube
will give much more 2H before it clips, and can colour guitar sound very
nicely in a guitar amp
where of course pentode signal stages are never used, and 12AX7 are
king.
where you have an oscillator producing varying levels of output at
different F the
6BA6 can be used to make the response virtually flat by
a dc feedback signal fed to bias the tube gain.

Its a wonder more ppl don't
use all the remote cut off tubes laying around in their guitar amps.






...... Phil

[Patrick Turner]

Phil Allison wrote:

"Phil Allison"

**Correction:

" A 6AU6 cannot match this, let alone give 1000 times gain."

There must be thousands of 6AU6 and cousin 6BA6 laying around.
gee they were so common, small compact tubes they are.

Patrick Turner.

....... Phil

[Patrick Turner]

Ian Bell wrote:

Bret Ludwig wrote:

On Jun 4, 4:45 pm, Ian Bell wrote:
I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?


A couple of boo-teek pro audio vendors are peddling mic preès
straight out of RDH 4 with good results. So I'd say yes.

Can you give me a link to any of these please Bret?

Cheers

Ian

The RDH4 mic pres have a pentode with its shields taken to 0V,
and also anode to 0V. But the screen is taken to B+ via a load R
so it acts as a triode with the same µ & Ra as the fully trioded
pentode.
All is well if the Ig2 x Eg2 product
is well below the Pd rating for the screen.

The idea is to get good gain and avoid the partition noise caused by
screens in pentodes giving them more
hiss type noise than when triode connection is used. The anode acts as
additional screening if grounded,
and presumably the shot noise of electrons being absorbed by the screen
is less than that of them hitting the anode.

But when tubes was all there was, mic signals were kept high to get good
SNR.
low output mics had transformers to step up the voltage.
Now we might use a fet input opamp...

Patrick Turner.

[Patrick Turner]

Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?

Cheers

Ian

Iv'e often seen it used in amps from the 1950s instead of EF86,
but its a 7 pin mini tube.
Its supposedly more noisy ( hum) than EF86.

But with dc heaters it should be fine.

It also has 3 times the gm of the EF86, and can therefore give 3 times
the voltage gain;
so one used as a line stage amp can have a lot of shunt NFB applied to
place gain where
its wanted, say 10 instead of the open loop gain of 300.
But this means THD at a volt output is difficult to measure because its
so low.

There are millions of this unloved tube around.

Its a nice triode when strapped in triode.

Patrick Turner.

Thanks for that Patrick. Useful info.

Cheers

Ian

I guessed a little high for 6AU6 gain being 3 times that of EF86,
but the '86 is a rather gutless wonder compared to most other
pentodes which are commonly used with higher Ia.

In another post I made I explain it further, but at least try 6AU6
on a bread board to know it.

In Mullard 520, the input tube was direct coupled to phase inverter LTP
with Ea = 90V for V1 EF86.
Since such a small voltage signal of only maybe 2vrms was needed from
the tube,
it was seen as enough to drive the following 12AX7, and power into the
Miller C.

Gain was a typical 90, and distortion was low, and anyway there was 20db
of global NFB.

I prefer using the EF86 in triode in the same location, and using a 6CG7
instead of 12AX7, and having more Ia flow. It makes music dynamics sound
fresher.
The amp is less sensitive, but it matters not; Mullard amps with
clipping brought on with 0.2vrms input were too sensitive.

A massive number of wonderful recordings were made in the 1960s using
EF86
or 6AU6 in the gear, so they can't be all bad.

Patrick Turner.

[John Byrns]

In article ,
Patrick Turner wrote:

Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

I notice RDH has several examples of using a 6AU6 in preamplifiers. These
seem readily available for reasonable prices (unlike the EF86 which seems
to be getting more expansive every day). Has anyone any experience using
the 6AU6 in preamps and is it any good?

Cheers

Ian

Iv'e often seen it used in amps from the 1950s instead of EF86,
but its a 7 pin mini tube.
Its supposedly more noisy ( hum) than EF86.

But with dc heaters it should be fine.

It also has 3 times the gm of the EF86, and can therefore give 3 times
the voltage gain;
so one used as a line stage amp can have a lot of shunt NFB applied to
place gain where
its wanted, say 10 instead of the open loop gain of 300.
But this means THD at a volt output is difficult to measure because its
so low.

There are millions of this unloved tube around.

Its a nice triode when strapped in triode.

Patrick Turner.

Thanks for that Patrick. Useful info.

Cheers

Ian

I guessed a little high for 6AU6 gain being 3 times that of EF86,
but the '86 is a rather gutless wonder compared to most other
pentodes which are commonly used with higher Ia.

In another post I made I explain it further, but at least try 6AU6
on a bread board to know it.

In Mullard 520, the input tube was direct coupled to phase inverter LTP
with Ea = 90V for V1 EF86.
Since such a small voltage signal of only maybe 2vrms was needed from
the tube,
it was seen as enough to drive the following 12AX7, and power into the
Miller C.

Gain was a typical 90, and distortion was low, and anyway there was 20db
of global NFB.

I prefer using the EF86 in triode in the same location, and using a 6CG7
instead of 12AX7, and having more Ia flow. It makes music dynamics sound
fresher.
The amp is less sensitive, but it matters not; Mullard amps with
clipping brought on with 0.2vrms input were too sensitive.

A massive number of wonderful recordings were made in the 1960s using
EF86
or 6AU6 in the gear, so they can't be all bad.

Don't forget a the great recordings that were made in the 1950s using
the 5879.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Iain Churches]

"Patrick Turner" wrote in message
...

But when tubes was all there was, mic signals were kept high to get good
SNR.
low output mics had transformers to step up the voltage.
Now we might use a fet input opamp...

In the 70's many tube mics were converted to FET.
Now there are firms in Germany and the UK which
specialise in converting them back again:-)

Iain