Re: JJ tubes? Anyone tried this tube trick? (*harp-GEAR-related*)

["The Bernadettes" <mdestefano@xxxxxxxx>]

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Anyone had any experience with the JJ tubes, sold by Eurotubes? =
According to the Eurotubes site, he is using JJ 6V6's as a drop in to =
tame down 6L6 amps, they have good tone, and they live even with plate =
voltages as high as 480vdc.=20

Here is something new to me:  they have a preamp tube available that is =
half a 12AX7/ half 12AU7 that would work in the phase inverter position =
to offset the waveform (same thing Gerald Weber does with his "waveform =
symmetry" knob except non-adjustable). This would work in some but not =
all circuits. I incorporated this control when I built my amp, and if =
anyone is wondering about it, it works. This tube sounds like a cheap =
and easy way to help out an amp that is just too clean.=20

kcmojo

Hi KC, I wrote a pretty extensive post on this subject a few years ago.  =
Actually, I think Bob at Eurotubes got the idea from that particular =
post.  I dug it up and am re-posting it below...should answer a lotta =
questions on this subject.

Regards,
Don D.

    Lately I've been experimenting with a 12AX7 type tube swap, that
some of you maybe interested in.  That is a 12DW7 tube.  This tube has
the same pin out as a 12AX7 and is a direct replacement for any 12AX7
type tube.  What is different about the 12DW7 is that it is a
"disimilar" or "unbalanced" dual triode, as compared to the
12AX7/5751/12AT7/12AY7/12AU7 tubes which are balanced or similar dual
triodes.  I realize many of you know this but for those who don't, I'll
explain.  12AX7 type dual triodes are actually two pre-amp tubes in a
single tube.  I've seen a few guys on this list talk about tube swaps as
if a tube performs a single function, but they perform two, depending on
the circuitry of the amp.  All 12AX7 types, except for the 12DW7 are
balanced which means each triode is exactly like the other.  One a 12AX7
for example, both sides or triodes have a gain factor of 100.  The 12DW7
is different in this respect, each side or triode is disimilar.  One
side has a gain factor of 100, the other side has a gain factor of 20.
You can think of it as 1/2 a 12AU7 and 1/2 a 12AX7.
    As you can imagine, this offers some interesting possibilities in
altering gain characteristics of an amp for harp.  Obviously some
knowledge of the circuitry of your amp will help you figure where this
tube might work best.  First the pin out of the 12DW7 is as this: the
first triode is on pins 1,2,&3 and this has the 20 gain factor, the 2nd
triode is on pins 6,7,&8, and this side has a gain factor of 100.
    Some examples: first tube position on a tweed Bassman.  The 1st
triode is the 1st gain stage for the normal channel, the 2nd triode is
the 1st gain stage for the bright channel.  Place a 12DW7 in this slot
and you can A-B between two different gain situations.  It's like having
a 12AU7 in the normal channel and a 12AX7 in the bright channel.  You
can choose between either channel, to see which you like best.  The 2nd
tube position on a tweed Bassman is a cathode follower.  To be truthful,
I'm not sure what effect a disimilar dual triode would have on this
position.  In theory the overall gain of a 12DW7 in a cathode follower
would be kinda equal to the gain of a 12AY7, but there maybe a
difference in the response.  Having a gain factor of 100 on one triode
may help push the amp into breakup but the much lower gain of the the
other triode may help to alleviate the excess gain and lack of play in
the volume control.  I don't currently own a tweed Bassman so I can't
tell you how it would work out in experience.  The 3rd tube is the phase
inverter and I'll get to this later.
    BF/SF Fender, I did quite a bit of experimenting on my BF and SF
Fenders.  The 1st tube position is the first gain stage/tone recovery
for the normal channel, the 2nd tube position is the fist gain
stage/tone recovery for the vibrato channel.  The 1st triode is the
first gain stage after the input and the 2nd triode is the tone recovery
stage, it recovers the gain lost in the tone controls.  In this position
the 12DW7 worked very nicely.  With a gain factor of 20 in the first
position, it kept the mike signal from getting too hot, but having the
the gain factor of 100 coming out of the pre-amp stage gave the amp some
of that 12AX7 type "bark".  In overall gain, again the 12DW7 would
roughly equal a 12AY7, but this assymetric way of applying the gain
seems to have some advantages.  You get a bit of bark but you still get
some play from the volume knob.  Other BF/SF tube positions: socket 3 is
the reverb driver and requires both sides of a dual triode, I would not
recommend a 12DW7 in this position, same goes for socket 5 which is the
vibrato driver.  Socket 4 is the reverb recovery/2nd gain stage.  In
this position the 1st triode is used to reintroduce the signal from the
reverb pan, the 2nd triode is an additional gain stage for the vibrato
channel.  A 12DW7 in this position would lower the gain on the reverb
recovery, or give you more play on the reverb knob, but would still give
you the gain equal to a 12AX7 on the 2nd gain stage.  I personally don't
care for it (12DW7) here.  The extra gain on this position causes
feedback issues and I usually prefer a 12AY7 or 12AU7 in this position,
it lowers the gain on the 2nd gain stage and helps to warm up and remove
the harshness of the reverb.  In fact, on more powerful amps, I
internally modify the amp to bypass this additional gain stage, to help
control feedback.  But in the first two slots of a BF/SF Fender amp, the
use of a 12DW7 is definitely worthwhile experimenting with.  My Super
currently has a 12DW7 in the first gain position of the vibrato channel,
I was happy with the response of the amp with this tube in it.
    The phase inverter position:  this is where it gets interesting.
Until recently common practice has been to do everything possible to
perfectly balance a phase inverter.  That is use a balanced dual triode
with both triodes perfectly matched, both coupling caps matched, matched
tubes, and hoping that both sides of the output transformer were
perfectly matched also (rarely the case with guitar amps.)  Recent talk
amongst amp geeks has been about slightly imbalancing phase inverters in
amps where a "fat", or distorted tone is desired.  Gerry Weber also
mentioned this in his recent article about harp amps where he discovered
that purposely unbalancing the phase inverter fattened up the tone of
his harp amp.  In his case he played with the values of the "balance
resistor" of a paraphase type phase inverter.  Lately I've been doing
the same with the more common "long tailed" phase inverter found in most
later vintage Fender amps, by playing with the values of the "tail"
resistor.  Use of the 12DW7 in the phase inverter slot has a similar
effect.  Use of this tube has the signal of one power tube running much
hotter than the other.  It's no longer a perfect 50/50 push pull
arangement.  This causes the power tubes to push an assymetric wave
form.  It fattens up the tone a bit.  Of course for those looking for a
perfectly clean tone, you won't be happy with this.  Some caveats: since
one power tube will be working harder, it might be a good idea to
occasionally switch power tubes, like rotating tires, so to even the
wear.  Also you may want to visually watch the amp while playing when
you first do this.  On one of my amps, one of the power tubes started to
glow a bit more.  You may wanna check the bias of the power tubes to
make sure one of the tubes doesn't get too hot.  Visually watching the
amp you can actually see one of the power tubes working a bit more.
Also, try playing with speaker polarity, you may find you get better
feedback suppression by switching the polarity of your speakers.  In
actual use, this effect is fairly subtle, next I wanna try both playing
with a 12DW7 in conjunction with altering tail resistor values, and see
how this works out.   Princeton guys, this won't work, the Princeton
uses a single triode cathodyne type phase inverter (the other triode is
used to drive the vibrato effect) and using a 12DW7 in this position
would just alter the vibrato effect.  Not to worry, due to a design flaw
by Fender, the phase inverter of a Princeton is slightly out of balance
anyway.
    The 12DW7 is available as a NOS tube from the usual NOS dealers and
JJ is currently manufacturing a 12DW7 under the European designation:
ECC832.  I've been using the JJ's and are happy with them.  JJ along
with Electro-Harmonix are now producing current manufactured tubes that
rank up there with NOS stuff for a reasonable price. I bought mine
through Eurotubes:   http://www.eurotubes.com    Bob at Eurotubes is
very nice and knowledgeable guy to deal with, has great prices and very
efficient service, highly recommended JJ dealer.  His 12DW7/ECC832 is
$9, not bad.
    For those of you who try this tube swap, check back on the list with
your experiences and opinions.  The usual disclaimers apply to those of
you playing with your amps.  Work safely, make sure you know what your
doing.

Regards,
Don D.