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Author
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Topic: Tweek a Twin, swap 12AX7 to12AT7?
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Ken Fox
Member From: Ray City, GA USA
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posted 02 August 2003 06:41 PM
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I was reading a post on the Fender Forum about tweeks for the Twin for jazz players. The Twin and other Fender amps were optimized for the single coil guitar pickup> The humbucking pickup and especially the new steel humbucking pickups are often too much gain for the preamp stages. The gain of a 12AX7 is a factor of 100. Swapping V2 and V4 to a 12AT7 can reduce preamp gain a bit and help. The gain factor for it is only 70 in the 12AT7. The plate resistance is different, so the frequency response is a bit different as well. Another tube with the same plate resistance of the 12AX7 and a gain of only 70 is the 5751.
I tried the 12AT7 tubes in my Twin with my Tele that has the newer Bill Lawrence pikups (stacked humbuckers in a single coil case, I believe). The volume did not seem so much less at all. The tone was greatly improved, in my opinion. It took a lot of the sharp edge off the amp and had more compression and sustain.
Has anyone else got any experience to share with this type of a tweek? Combined witht he mid-shift tweek in the tone stack that Brad Sarno and others have recommended, I think this might be a good thing for a Twin used for steel. Oddly enough, the 90's Vibrasonic (Twin with a 15" speaker) used an unbypassed cathode resistor on the the first half of the very first preamp tube to reduce gain in the "Steel" channel. Maybe this is an easier way to do the same, without a wiring change.
[This message was edited by Ken Fox on 03 August 2003 at 12:03 PM.] |
Alvin Blaine
Member From: Sandy Valley, Nevada, USA
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posted 03 August 2003 03:05 AM
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The 12AT7 and 5751 will clean up your headroom a little. To me its kind of like turning the volume knob down(just a little) on your guitar. The big advantage, to me, is that if you aren't overdriving the pre-amp then you can turn the amp up a little more and get the power amp tubes work a little harder. I think that by doing that you get a fatter sound.
I like the sound of an old tube amp(Fender) when the power tubes are just starting to sing. I use single coil pickups, low gain pre-amp tubes, JBL speakers, and very strong power-amp tubes, then turn the amp up and let those 6L6 tubes work.
I always carry a few pre amp tubes in my gig bag. It just depends on what I'm playing and at what volume I'm playing it at.
Stevie Ray used 5751's in his old Vibroverb to keep it from distorting to early(he also had a twin transformer and SS rectifier in it).
A lot of blues harp guys use 12AY7's or even 12AU7's in the pre amp stage. They run those old shure green bullet or Astatic mics into old amps and all you get is feedback and distortion with a 12AX7.
This is what I was told on the gain factor,
if a 12AX7 is 100% then:
5751 - 70%
12AT7 - 60%
12Ay7 - 40%
12AU7 - 20% |
Cartwright Thompson
Member From: Portland, Maine, USA
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posted 03 August 2003 03:33 AM
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I like a 12AY7 or 5751 in the first hole (I have the reverb connected to my "Normal" channel- it seems a little beefier than the Vibrato channel) of my '66 Twin Reverb. You can still find good NOS examples of both of these tubes for reasonable $$$...unlike most 12AX7's.
Also, the number 2 input on the Twin is "lower gain".
JBL's are also important. |
Dana Duplan
Member From: Ramona, CA
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posted 03 August 2003 06:27 AM
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I was going to mention the harp amp thing. I found when setting up my Pro Jr. for harp with lower gain tubes, it worked great for lap steel!
Question: When tube amp folks talk about the "first preamp tube", or "first gain stage", which are they speaking of? The one closest to the power tubes, or vise-versa?
D |
Ken Fox
Member From: Ray City, GA USA
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posted 03 August 2003 07:06 AM
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On a two channel Fender amp with reverb:Remember each tube is two triodes (two tubes in one package) V1 (closest to the right) is channel 1 preamp tube, 1/2 pre and 1/2 post volume/tones controls V2 is channel 2 preamp (vibrato channel), 1/2 pre and 1/2 post volume/tone controls V3 is the reverb tank drive (both halves of tubes are in parallel to add more drive to the reverb tranformer) V4 is reverb tank signal recovery. The other half of the tube combines the channel 2 dry mix and the reverb mix. Due to this third amplification of the channel 2 signal, it is out of phase with the channel 1 signal. As above, a lot of guys mod the amp to get reverb on channel 1, this also puts both channels in phase. V5 tremolo V6 is the inverter or power tube driving stage.
Alvin, appreciate the input there. You confirmed what I thought I was hearing with my Twin when I tried this out! I think I will start carrying a handful of extra preamp tubes and try this on a job soon. Truthfully, I liked the sound I heard much better with my Tele using the 12AT7 tubes. It would most likely be a big improvement for steel as well.[This message was edited by Ken Fox on 03 August 2003 at 07:07 AM.] [This message was edited by Ken Fox on 03 August 2003 at 07:22 AM.] [This message was edited by Ken Fox on 05 August 2003 at 12:15 PM.]
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Chris DeBarge
Member From: Boston, Mass
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posted 03 August 2003 09:13 PM
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I use a 12AY7 in the first preamp tube slot in my 4x10 bassman, it does make a small difference. It's like Alvin said - it allows you to crank the power tubes a little more. |
Dana Duplan
Member From: Ramona, CA
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posted 04 August 2003 07:15 AM
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Ken:
When you say "V1" is closest to the right, do you mean closet to the power tubes? Is that the first gain stage?
Thanks |
Ken Fox
Member From: Ray City, GA USA
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posted 04 August 2003 08:53 AM
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Valve 1 (V1) is to the far right, facing the rear of the amp. [This message was edited by Ken Fox on 04 August 2003 at 08:53 AM.] |
Jerry Erickson
Member From: Atlanta,IL 61723
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posted 05 August 2003 10:52 AM
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Hey Ken,
You forgot that V5 is the tremolo tube
and V6 is the phase inverter/ driver tube.
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Ken Fox
Member From: Ray City, GA USA
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posted 05 August 2003 12:16 PM
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Right you are, I fixed it! |
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