Putting a 12ax7 in the PI then a 12at7

ken374

Member
Messages
6,950
Wonder if this is safe? would it give my pedals a tad more gain and bite? It may be my new speakers but seems like my pedals have a little less gain with the 68 twin. Might be the tone stack.
 

Silent Sound

Member
Messages
5,344
In most cases it's safe. It depends on the amp. I assume you're talking about a '68 Fender Twin Reverb. If that's the case, then yeah, a 12ax7 is safe in the PI slot. Pretty much any of the 12a_7 tubes will work in any of those preamp tubes in that amp. Now having said that, I want to warn you the reverb send tube (the first 12at7) is under a good bit of stress already so I probably wouldn't put anything other than a 12at7 in there. I know of several people who have substituted a 12ax7 in there (including myself) without having any problems, but that's the one tube in that amp that is being pushed closer to it limits than any of the others, which is why it gets a lot hotter than the others. Plus I think the reverb sounds best with a 12at7.

Generally, a 12ax7 in the PI of a Twin Reverb will usually give you a bit quicker breakup and sometimes change the tonality of the amp a bit. However, it being a Twin Reverb, you probably still won't be turning it up loud enough to take advantage of that quicker breakup. I personally like the 12at7 better there, but if you're out of 12at7's or just want to experiment, there's no harm in playing around with it.
 

ken374

Member
Messages
6,950
I cant to seem to find the tube layout and what they drive, fenders email wont open!
 

harpinon

Member
Messages
8,975
An AT will result in less amp volume and gain.
If its more gain you want then you'll want to stay with ax7 tubes. There are high gain ax7's available. I have a couple groove tubes labeled hg.
 

schecter

Member
Messages
105
12AT7's have a gain factor of around 70% vs an AX which is around 90-100mu
More important an AT7 also has a higher current rating (which makes it an ideal reverb tank driver). The lower gain is a factor but the larger factors are:

• We have almost 10 times the current available to drive the output tube grids before the phase inverter starts to break down and clip (in the same PI/amp)
• We have a transconductance of 5500 vs 1600 of the 12AX7. Keeping this simple, it means it takes a lot less input signal for a given output signal.
With this said an AT7 in the PI of a push pull AB amp w/ LTP PI will drive the grids harder and give you more clean headroom than an AX will which will clip sooner and generate more harmonic character (crunch).. PI distortion is different (not as smooth as in a regular gain stage) so some people like that sound and others do not!..Also going from a 12AX7 to a 12AT7 in the PI (phase inverter) will yield a change in output tube distortion, touch, and output dynamics in most amps. (The AX being slightly more dynamic!!) due to added harmonic content....

BTW.. IMO a more ideal (dynamic) setup for a 12AX is altering the PI values by changing the tail from a 22k to a 10k and 820 cathode to 470..Typical Marshall/5f6a Bassman design..Fender designed theirs for the cleaner/headroom of an AT not an AX...Just sayin

Tony Albany
 
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ken374

Member
Messages
6,950
12AT7's have a gain factor of around 70% vs an AX which is around 90-100mu
More important an AT7 also has a higher current rating (which makes it an ideal reverb tank driver). The lower gain is a factor but the larger factors are:

• We have almost 10 times the current available to drive the output tube grids before the phase inverter starts to break down and clip (in the same PI/amp)
• We have a transconductance of 5500 vs 1600 of the 12AX7. Keeping this simple, it means it takes a lot less input signal for a given output signal.
With this said an AT7 in the PI of a push pull AB amp w/ LTP PI will drive the grids harder and give you more clean headroom than an AX will which will clip sooner and generate more harmonic character (crunch).. PI distortion is different (not as smooth as in a regular gain stage) so some people like that sound and others do not!..Also going from a 12AX7 to a 12AT7 in the PI (phase inverter) will yield a change in output tube distortion, touch, and output dynamics in most amps. (The AX being slightly more dynamic!!) due to added harmonic content....

BTW.. IMO a more ideal (dynamic) setup for a 12AX is altering the PI values by changing the tail from a 22k to a 10k and 820 cathode to 470..Typical Marshall/5f6a Bassman design..Fender designed theirs for the cleaner/headroom of an AT not an AX...Just sayin

Tony Albany
Think its safe then? I have all the head room I need lol got to experiment loud though. I do have a At7 in the v3 reverb and one in the PI
 

Guinness Lad

Senior Member
Messages
15,860
Putting a 12ax7 where a 12at7 will kill the tube faster, it's safe otherwise. I had a Twin reverb ages ago before I knew anything about tubes, the 12ax7 I put in there lasted for 4 or 5 years of use.
 

ken374

Member
Messages
6,950
I'll wait to see how it responds loud, I changed the v1 to a old stock GE that smoothed the top and lowered the pedal volume that made a nice improvement
 

schecter

Member
Messages
105
I don't think it's an issue as far as doing any damage to the tube or amp by going to a 12AX there!..They just operate at different parts of the curve..One breaks down faster and clips harder than the other..12AX's can handle up to 300v on the plates with no problem.. Purely a taste thing!!..Have at it..

As far as killing the tube faster..Look at how many Marshalls and hundreds of other amps out there use 12AX7's in the LTP PI with no issues or show any signs of wearing out any faster...

Tony Albany
 

Guinness Lad

Senior Member
Messages
15,860
I don't think it's an issue as far as doing any damage to the tube or amp by going to a 12AX there!..They just operate at different parts of the curve..One breaks down faster and clips harder than the other..12AX's can handle up to 300v on the plates with no problem.. Purely a taste thing!!..Have at it..

As far as killing the tube faster..Look at how many Marshalls and hundreds of other amps out there use 12AX7's in the LTP PI with no issues or show any signs of wearing out any faster...

Tony Albany
Wrong, some amps have higher current levels at certain points, if the amp is designed in a manner where it's pushing current the tube will fail quicker. Read up on it a bit more. For example reverb on a Fender one is the driver, if you don't have current capacity it won't be correct.
 

Guinness Lad

Senior Member
Messages
15,860
The 12AX7 / ECC83
12AX7 tube


Specification and max ratings(*)
Filament Voltage
6.3-12
V
Filament Current
300-150
mA
Plate Voltage (max)
330
V
Plate Current (max)
6

mA
Plate Dissipation (max)
1.1
W
Screen Voltage (max)
---
V
Screen Current (max)
---
A
Screen Dissipation (max)
---
W



The 12AT7 / ECC81
12AT7 tube


Specification and max ratings(*)
Filament Voltage
6.3-12
V
Filament Current
300-150
mA
Plate Voltage (max)
300
V
Plate Current (max)
15
mA

Plate Dissipation (max)
2.5
W
Screen Voltage (max)
---
V
Screen Current (max)
---
A
Screen Dissipation (max)
---
W

12at7 is double the current capacity, this can be huge depending.
 

schecter

Member
Messages
105
Wrong, some amps have higher current levels at certain points, if the amp is designed in a manner where it's pushing current the tube will fail quicker. Read up on it a bit more. For example reverb on a Fender one is the driver, if you don't have current capacity it won't be correct.
If we take a look at Merlins (Valve Wizards) Respected web site
http://www.freewebs.com/valvewizard/acltp.html

He mentions in his AC coupled long tail pair design specs (under biasing) you will notice that he points out the quiescent anode current of about 1.3mA. per triode (well under 6ma)Since both triodes share the same tail resistor, the total current through the tail is twice this value, giving 2.6mA total for both triodes.This may rise a bit when operating but gets nowhere near it's 6ma rating that you posted below..

The 12AX7 / ECC83
12AX7 tube


Specification and max ratings(*)
Filament Voltage
6.3-12
V
Filament Current
300-150
mA
Plate Voltage (max)
330
V
Plate Current (max)
6
mA

If you have any hard technical data or point me to a respected amp design article proving that in a AC coupled LTP PI draws more than 6 ma or that an AX needs to changed more frequently than an AT..Please post it as I am always interested in these findings

Thanks
Tony Albany
 
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tmac

Goldmember
Gold Supporting Member
Messages
2,236
I like using a 12AX7 PI in my Fender blackface type circuits, well, sometimes anyway. That's what I have in there now. It will definitely overdrive quicker as you turn it up... but some people don't care for a 12AX7 there, especially if they want to stay on the cleaner side of the tracks.
 




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