My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.

"NGS" wrote in message
...
My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.

In simple terms, the phase splitter of phase inverter (PI) divides the
signal so that half goes to each power tube in a push-pull design. (If
you've got 4 or 6 or 8 power tubes, there is a pair or group on each side
that acts as "one section".) I don't think it makes sense to use something
other than what's specified. For the other pre-amp tubes, use whatever
12A?7 tubes that give a sound you like. 7025 and 5751 are variants of the
12A?7 family. I think using another PI tube will give you signal loss at
the wrong place in the circuit.

Phil S wrote:
"NGS" wrote in message
...

My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.


In simple terms, the phase splitter of phase inverter (PI) divides the
signal so that half goes to each power tube in a push-pull design. (If
you've got 4 or 6 or 8 power tubes, there is a pair or group on each side
that acts as "one section".) I don't think it makes sense to use something
other than what's specified. For the other pre-amp tubes, use whatever
12A?7 tubes that give a sound you like. 7025 and 5751 are variants of the
12A?7 family. I think using another PI tube will give you signal loss at
the wrong place in the circuit.


Makes sense, thanks. Follow up question. Telefunken, Mullard, Amperex,
etc. obviously can be useful in "preamp" locations BUT for the phase
splitter does the TONAL QUALITY of the tube still matter. I'm guessing
it still does since it amplifies the signal going to the two power tubes
in PP but maybe not as much as say V1 or V2. Thanks.

"NGS" wrote in message
...
Phil S wrote:
"NGS" wrote in message
...

My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.


In simple terms, the phase splitter of phase inverter (PI) divides the
signal so that half goes to each power tube in a push-pull design. (If
you've got 4 or 6 or 8 power tubes, there is a pair or group on each
side
that acts as "one section".) I don't think it makes sense to use
something
other than what's specified. For the other pre-amp tubes, use whatever
12A?7 tubes that give a sound you like. 7025 and 5751 are variants of
the
12A?7 family. I think using another PI tube will give you signal loss
at
the wrong place in the circuit.


Makes sense, thanks. Follow up question. Telefunken, Mullard, Amperex,
etc. obviously can be useful in "preamp" locations BUT for the phase
splitter does the TONAL QUALITY of the tube still matter. I'm guessing
it still does since it amplifies the signal going to the two power tubes
in PP but maybe not as much as say V1 or V2. Thanks.

I've been told you can put a microphonic tube in this spot and generally it
won't be a problem. I'd put a decent tube in there, but wouldn't waste a
Tele or Mullard. The tone gets made before the PI.

I've been told you can put a microphonic tube in this spot and generally it
won't be a problem. I'd put a decent tube in there, but wouldn't waste a
Tele or Mullard. The tone gets made before the PI.


Not true unless your amp is strictly a preamp overdrive type with a
class AB2 or class B output stage. Every tube in the signal path has an
effect on sound quality. A poor quality tube used as the phase inverter
will cause the amp to lack life and punch. It is the number one reason I
have seen that Marshall and Fender amps sound weak. I have seen so many
people change out output tubes and preamp tubes thinking the phase
splitter tube is there just for the ride. It is the heart of a push pull
stage. In fact, a funky phase splitter circuit can cause your amp to
loose as much as 50% of it's power. Bad resistors, a bad tube and bad
coupling capacitors all contribute to phase splitter imbalance and the
result is a lousy sounding amp. I rebuild guitar and hifi amps for a
living and I don't know who started the 'urban legend' of putting your
junk tube in the phase splitter socket. If anything, this should be your
best tube. Also you should always use the correct tube in your phase
splitter socket. Do not try different types of tubes. If it calls for a
12AT7 use a 12AT7.
Bill B.

"WDB" wrote in message
...

I've been told you can put a microphonic tube in this spot and generally
it
won't be a problem. I'd put a decent tube in there, but wouldn't waste
a
Tele or Mullard. The tone gets made before the PI.


Not true unless your amp is strictly a preamp overdrive type with a
class AB2 or class B output stage. Every tube in the signal path has an
effect on sound quality. A poor quality tube used as the phase inverter
will cause the amp to lack life and punch. It is the number one reason I
have seen that Marshall and Fender amps sound weak. I have seen so many
people change out output tubes and preamp tubes thinking the phase
splitter tube is there just for the ride. It is the heart of a push pull
stage. In fact, a funky phase splitter circuit can cause your amp to
loose as much as 50% of it's power. Bad resistors, a bad tube and bad
coupling capacitors all contribute to phase splitter imbalance and the
result is a lousy sounding amp. I rebuild guitar and hifi amps for a
living and I don't know who started the 'urban legend' of putting your
junk tube in the phase splitter socket. If anything, this should be your
best tube. Also you should always use the correct tube in your phase
splitter socket. Do not try different types of tubes. If it calls for a
12AT7 use a 12AT7.
Bill B.

Goes to show ya. Thanks for setting the record straight. Sorry for the
mis-info.

"WDB" wrote in message
...

I've been told you can put a microphonic tube in this spot and generally
it
won't be a problem. I'd put a decent tube in there, but wouldn't waste
a
Tele or Mullard. The tone gets made before the PI.


Not true unless your amp is strictly a preamp overdrive type with a
class AB2 or class B output stage. Every tube in the signal path has an
effect on sound quality. A poor quality tube used as the phase inverter
will cause the amp to lack life and punch. It is the number one reason I
have seen that Marshall and Fender amps sound weak. I have seen so many
people change out output tubes and preamp tubes thinking the phase
splitter tube is there just for the ride. It is the heart of a push pull
stage. In fact, a funky phase splitter circuit can cause your amp to
loose as much as 50% of it's power. Bad resistors, a bad tube and bad
coupling capacitors all contribute to phase splitter imbalance and the
result is a lousy sounding amp. I rebuild guitar and hifi amps for a
living and I don't know who started the 'urban legend' of putting your
junk tube in the phase splitter socket. If anything, this should be your
best tube. Also you should always use the correct tube in your phase
splitter socket. Do not try different types of tubes. If it calls for a
12AT7 use a 12AT7.
Bill B.

I agree, and I tried replacing a Sovtek 12AX7 PI with an EH 12AT7 and it
didn't work well. Now I use a JJ ECC83 as the PI and it's great.

"NGS" wrote in message
...
My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.

There's a little gain... it also provides both sides of the push-pull
final section with drive.

Marshalls use 12AX7 here... Fenders use 12AT7. Try both and
see what you like, can't hurt.
__
Steve
..

NGS wrote:
My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.
Ok,
Here's the problem with using different tubes in the phase splitter spot.
A phase splitter or inverter has to not only supply the upper half
of the signal wave to one output tube and the lower half to the other
output tube, it also has to deliver sufficient voltage to cause the
output tubes to produce full power. For tubes like the EL34 and 6L6, we
are talking a phase inverter producing between 40 and 70 volts.
To get that kind of voltage swing out of a twin triode, you must use
the correct plate resistors and cathode resistors relative to the
impedance of the output tube's control grid circuit. Since the
specifications for cathode resistors relative to plate resistors and
next stage impedance are dramatically different when comparing a 12AX7
to a 12AT7 and 12AU7, using anything other then what the circuit was
designed for will net you less power from the amp. The 12AX7, 12AU7 and
12AT7 all have the same pin out connections, but that is where the
similarity ends. All three have different gain, different plate
impedance and different Gmu. To give an example. A 12AX7 in a standard
circuit has a plate impedance of around 60,000 ohms while a 12AU7 would
exhibit a plate impedance of around 7000 ohms. This also has an effect
on the frequency response of the amplifier and can net you loss of bass
or treble.
The 5751 is very similar to the 12Ax7. It is a kind of in between
tube between the 12AT7 and 12AX7. This tube will generally work anywhere
a 12AX7 will.
Hope this helps,
Bill B.

Snip
The 5751 is very similar to the 12Ax7. It is a kind of in between
tube between the 12AT7 and 12AX7. This tube will generally work anywhere
a 12AX7 will.
Hope this helps,
Bill B.

It does, thank you and to all the responses.

Along these lines, this Mann/Garnet bass amp I'm messing with at the
moment came with a 12AU7 PI tube. The tube is correct because the
sockets are labelled. This is a cathodyne-type PI where only one triode
is used, feeding one power tube with the plate and the other with the
cathode thru a resistor network. Similar to late 50's Tweed deluxe 5E3,
i think.

Anyways, what happens when I put in a 12AX7? The tone perks up, and the
amp is noticably louder. Obviously, the PI tube is providing more drive
to the output stage, and the amp has more punch. I will look at R
values and see if they need to be changed, butt...I'll be definitley
switching to the 12AX7.

Porky

NGS wrote:
My guitar combo has five preamp tubes. They are all supposed to be
12AX7. For the effects send and return I use a 12AT7. For the first
preamp tube V1 I use a 5751. The last position V5 is the phase
splitter. What happens if I were to use a 5751 or 12AT7 or even 12AU7
in this position? Is it just the last boost stage prior to the power
tubes? Thanks.

Very interesting responses, very educational and some very entertaining.

ANYWAY. I'm going ahead with all 5751 in all positions now including
the efx send and return except the last gain stage V3 where I'm going to
use a Mullard 12AT7 to soften up the boogie a bit.