Hello, r.a.t,

Any you folks familiar with any of the new 12AX7s (Russian / Czech / Chinese
etc) being tightly spec'ed on heater current. I want to put a new 12AX7 in
an old phono amp which had series string heaters for two 50EH5's and a 7025
(and 33 ohm dropping resistor). The old 7025/12AX7A shows 12.6 volts right
on the dot when in this configuration. I tried two of the new tubes that
seem to be more highly reputed on the web connoisseur (wannabe?) sites, the
JJ ECC83S and the Sovtek 7025/12AX7WB, and they never came anywhere near
12.6 vac in the same string, the cathodes having a dull red glow that is
scarcely visible. Measurements I made show that both these new tubes
require about 170 mA to come up to 12.6 volts at which point the cathodes
look a normal orange. I suppose I could hack the series string by putting a
suitable (~5600 ohm, 3 watt) resistor from the top of the AC line down to
the 12AX7 to furnish the extra 20 mA, but then any in-spec tube would see an
overvoltage. Is there a clean answer to this, other than a filament
transformer or a $$$$ NOS tube.

Thankx
HTRN

I don't know about the differences between 7025's/12ax7's, but at least
in the 12au7/5814 world, there's a difference in that the 12au7 takes
150mA filament current while the 5814 takes 175mA filament current.

Now, all my data sheets say that both the 7025 and 12ax7 take 150mA
current. But maybe Sovtek/Tesla/JJ (whatever they call themselves) put
the wrong heater in and they're doing it consistently across all their
lines. I do note that the Sovtek part number has a "W"... sometimes the
ruggedized versions of tubes had different filaments than normal, but I
don't think this was the case for the 12ax7.

Are the Sovtek and JJ obviously different construction or is it
possible that they're just different labels on identical tubes from the
same plant?

Tim.

HiTech RedNeck wrote:
Hello, r.a.t,

Any you folks familiar with any of the new 12AX7s (Russian / Czech / Chinese
etc) being tightly spec'ed on heater current. I want to put a new 12AX7 in
an old phono amp which had series string heaters for two 50EH5's and a 7025
(and 33 ohm dropping resistor). The old 7025/12AX7A shows 12.6 volts right
on the dot when in this configuration. I tried two of the new tubes that
seem to be more highly reputed on the web connoisseur (wannabe?) sites, the
JJ ECC83S and the Sovtek 7025/12AX7WB, and they never came anywhere near
12.6 vac in the same string, the cathodes having a dull red glow that is
scarcely visible. Measurements I made show that both these new tubes
require about 170 mA to come up to 12.6 volts at which point the cathodes
look a normal orange. I suppose I could hack the series string by putting a
suitable (~5600 ohm, 3 watt) resistor from the top of the AC line down to
the 12AX7 to furnish the extra 20 mA, but then any in-spec tube would see an
overvoltage. Is there a clean answer to this, other than a filament
transformer or a $$$$ NOS tube.

Thankx
HTRN


The heaters of 12AX7 / ECC83 are not meant for series operation, that
is: they are not adjusted for the current like the european P-series
tubes are (300mA). Because of the allowed variations (tolerance) in
resistance the heaters of 12AX7 and the E-series may or may not work
if connected in series. This is what you are experiencing.
You do not state if the heaters are DC or AC operated. I would shun AC
operation in series in a phono amp. A redesign with DC heating might
be worth the effort if the phono amp is worth anything.

Kind regards, Eike
--
Lions go to absurd lengths to retain the posession of their skins.
- Stewart Edward White 1913

AFAIK a 10% (or more) variation in any measured parameter is normal in ANY
batch of tubes of ANY brand, but maybe special-grade TFK or Mullards. Once
upon a time tubes were quite normal stuff and nobody cared for a
zero-dot-something offset. Even if You bought a NOS tube You could have got
the same result. If the tube does not sound right, lower the 33R series
resistance and it's done.


ha scritto nel messaggio
oups.com...
I don't know about the differences between 7025's/12ax7's, but at least
in the 12au7/5814 world, there's a difference in that the 12au7 takes
150mA filament current while the 5814 takes 175mA filament current.

Now, all my data sheets say that both the 7025 and 12ax7 take 150mA
current. But maybe Sovtek/Tesla/JJ (whatever they call themselves) put
the wrong heater in and they're doing it consistently across all their
lines. I do note that the Sovtek part number has a "W"... sometimes the
ruggedized versions of tubes had different filaments than normal, but I
don't think this was the case for the 12ax7.

Are the Sovtek and JJ obviously different construction or is it
possible that they're just different labels on identical tubes from the
same plant?

Tim.


BTW: JJ tubes are made in the former Tesla factory in the Slovak republic
while Sovtek tubes come from the Reflektor plant in Saratov, Russia. I
never had anything to complain about the quality of both (others may
disagree) but for sure they're different tubes from different manufacturers.
Just look at the mica spacers cutting or at the envelope sealing: it is
clearly done by different machines.

Ciao

Fabio

"HiTech RedNeck"

Measurements I made show that both these new tubes
require about 170 mA to come up to 12.6 volts at which point the cathodes
look a normal orange.


** A Sovtek " 12AX7WA " I just tried draws 182 mA at 12.6 volts.

Other assorted Chinese and Soviet X7s drew 161 to 172 mA.

Specs are very rubbery these days .......




.......... Phil

"Eike Lantzsch, ZP6CGE" wrote in message
...
HiTech RedNeck wrote:
Hello, r.a.t,

Any you folks familiar with any of the new 12AX7s (Russian / Czech /
Chinese
etc) being tightly spec'ed on heater current. I want to put a new
12AX7 in
an old phono amp which had series string heaters for two 50EH5's and a
7025
(and 33 ohm dropping resistor). The old 7025/12AX7A shows 12.6 volts
right
on the dot when in this configuration. I tried two of the new tubes
that
seem to be more highly reputed on the web connoisseur (wannabe?) sites,
the
JJ ECC83S and the Sovtek 7025/12AX7WB, and they never came anywhere
near
12.6 vac in the same string, the cathodes having a dull red glow that
is
scarcely visible. Measurements I made show that both these new tubes
require about 170 mA to come up to 12.6 volts at which point the
cathodes
look a normal orange. I suppose I could hack the series string by
putting a
suitable (~5600 ohm, 3 watt) resistor from the top of the AC line down
to
the 12AX7 to furnish the extra 20 mA, but then any in-spec tube would
see an
overvoltage. Is there a clean answer to this, other than a filament
transformer or a $$$$ NOS tube.

Thankx
HTRN


The heaters of 12AX7 / ECC83 are not meant for series operation, that
is: they are not adjusted for the current like the european P-series
tubes are (300mA). Because of the allowed variations (tolerance) in
resistance the heaters of 12AX7 and the E-series may or may not work
if connected in series. This is what you are experiencing.
You do not state if the heaters are DC or AC operated. I would shun AC
operation in series in a phono amp. A redesign with DC heating might
be worth the effort if the phono amp is worth anything.

I'm toying with the idea of DC but the same problem remains. Current enough
to bring the new 12AX7 up to 12.6 volts will overheat the 50EH5's (pentodes
in 7 pin mini envelope). I have one possible dodge with DC. 13 volt 5 watt
zener across the heater of the 12AX7, and a helper resistor from the "A-"
supply skipping over the 50EH5's to the 12AX7. Zener checked with socket
empty to be sure it is 13.2 or less. This will clamp the heater voltage at
safe level ( 5% over) no matter what tube is there.

HTRN

HiTech RedNeck wrote:

"Fabio Berutti" wrote in message
...
AFAIK a 10% (or more) variation in any measured parameter is normal in ANY
batch of tubes of ANY brand, but maybe special-grade TFK or Mullards.
Once
upon a time tubes were quite normal stuff and nobody cared for a
zero-dot-something offset. Even if You bought a NOS tube You could have
got
the same result. If the tube does not sound right, lower the 33R series
resistance and it's done.

Well, haven't gotten it back to the point of sound yet, but I checked
emission which seems to be 1/3 at that level. This concept of all tubes
being able to work normally at the same heater current is as old as the AA5.
Nobody sold "special grade" tubes for those, and yet the lower volt tubes
(mixer, if, detector/amp) all worked nicely with good orange cathodes no
matter what brand you stuck in. The same went for the later series string
TV sets.

Also the RCA manual says that a 7025 is a "better" 12AX7A which in turn is
okay in series heater configurations. Well someone should tell that to
Sovtek concerning 7025/12AX7WB. There are Chinese tubes which are
explicitly stamped 12AX7A but who knows how they measure current in China.
My cynical guess now is that if the tube works at rated heater voltage they
don't care about the current, and to heck with the manuals....

HTRN

One has to wonder about exactly what the cathode temperature might be if there
isn't any adherence to making the current what it should be.
12AX7 is supposed to have 150mA x 12.6V, and all the samples I have
measured go very close, but most are olduns.
The cathode is constructed to give the right amount of emission with 1.89 watts.

at this wattage, it should be possible to get the plate resistance curves
seen in the data sheets for 12AX7, or very close.
The 12AX7 will run with 14V or 10V. Its not ideal though,
but the heater has a hot resistance of 84 ohms, so the current change is minimal
with
a voltage range of 4V.

I really hate series heaters in any gear when 50V heater tubes are combined with

input tubes, or when they use the input tube heaters as a set of series
resistances
to make one common bias resistor for the 150mA of plate current in a stereo amp
using 4 x EL84
output tubes.
Even silly Fisher amps had this BS.
The only proper way to cathode bias any amp with multiple output tubes is to
have separate
Rk and Ck networks for each cathode.
The series heaters and or using heaters for Rk is all bean counter design,
rubbish,
and should all be re-engineered out and a separate rectified 12.6V for any phono
stages used;
its important to keep the 12.6V as the heater voltage, and not worry about the
current.
A heater supply need have only a 12.6Vac winding rectified to about 16V into
4,700 uF,
then a suitable dropping R to the heaters, maybe there are 4, so I = 0.6A, and
so to get from 16V to 12.6V R = 5.6 ohms, 10 watt rated.
The final 12.6V should be filtered with another cap of say 10,000 uF
and ripple voltage at the filaments should less than 10mV.
A 12V x 5W zener diode at each socket will slightly limit the heater voltage but

it will shunt reg it, and stop the voltage soaring if you pull out a couple of
tubes during
fiddling around / tweaking episodes.
But if you always keep all the tubes in then no need for zeners.

A shunt reg can easily be made using almost any npn TO220 bjt and 12V zener
to the base of the bjt collector to +12,6V, and the emitter to 0V.
If the V tries to rise above 12.6V, the bjt is turned on by the vener I flow
which will be low, and its a reliable regulator / limiter.

The suggested supply will also make a slightly soft start to the heaters.

If there is a 6.3Vac winding handy, it can be used to make a voltage doubler
for about 15Vdc which can also be RC filtered down to say 12Vdc for a phono
stage.

Patrick Turner.

In article , "HiTech
RedNeck" wrote:

Hello, r.a.t,

Any you folks familiar with any of the new 12AX7s (Russian / Czech / Chinese
etc) being tightly spec'ed on heater current. I want to put a new 12AX7 in
an old phono amp which had series string heaters for two 50EH5's and a 7025
(and 33 ohm dropping resistor). The old 7025/12AX7A shows 12.6 volts right
on the dot when in this configuration. I tried two of the new tubes that
seem to be more highly reputed on the web connoisseur (wannabe?) sites, the
JJ ECC83S and the Sovtek 7025/12AX7WB, and they never came anywhere near
12.6 vac in the same string, the cathodes having a dull red glow that is
scarcely visible. Measurements I made show that both these new tubes
require about 170 mA to come up to 12.6 volts at which point the cathodes
look a normal orange. I suppose I could hack the series string by putting a
suitable (~5600 ohm, 3 watt) resistor from the top of the AC line down to
the 12AX7 to furnish the extra 20 mA, but then any in-spec tube would see an
overvoltage. Is there a clean answer to this, other than a filament
transformer or a $$$$ NOS tube.

Why not try a low $ used 7025/12AX7A, they live a long time? What went
wrong with the original 7025/12AX7A?


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

The standard 6.3/12.6 volt tubes are not rated for series string
operation and may or may not be satisfactory. The only solution is a
variable series resistor in the heater string and measurement with each
tube change.

My recommendation: rewire the beast with parallel heaters from a
transformer, or better yet a regulated DC supply.