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Topic: 12AT7 brand ? |
Bob Hoffnar
From:
Austin, Tx
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Posted 5 Nov 2012 10:34 pm |
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What brand tube do you guys like for the reverb spot in an old Twin ? I need to buy a couple and wonder if NOS is worth it and what to watch out for.
Thanks, Bob |
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Matthew Dawson
From:
Portland Oregon, USA
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Posted 5 Nov 2012 11:18 pm
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| I think you just have to try some out. If you buy them used they are really cheap and usually last forever anyway. I get all the old US and European brands used for $1.50 each at the local surplus store. Most are pulls that have seen light use in test equipment. |
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Ivan Funk
From:
Hamburg Pennsylvania, USA
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Posted 5 Nov 2012 11:39 pm
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| I like the NOS JAN Phillips |
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Ken Metcalf
From:
San Antonio Texas USA
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Posted 6 Nov 2012 5:40 am
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Bob
I replaced mine (2v) with one from the BIG store... Groove Tube / Sovtek and a set of JJ power tubes.
from Tubestore.com
Even though my old power tubes tested okay... What a diff in the sparkle and tone.
I have a Tube Bias Meter if you ever want to use it.
I also have a tube thumper I can show you.. 
_________________
MSA 12 String E9th/B6th Universal.
Little Walter PF-89.
Bunch of stomp boxes |
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Ken Metcalf
From:
San Antonio Texas USA
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Posted 6 Nov 2012 6:03 am
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Taken from http://fenderguru.com/amps/twin-reverb
12AY7 or 12AT7 as preamp tubes – Less preamp gain.
If you want cleaner and spankier preamp characteristics, you may replace the V1 or V2 12AX7 preamp tube with 12AT7 and 12AY7. These tubes have different frequency responses than 12AX7, particularly when distorting. People describe these tubes to have less harsh and buzzy distortion. This mod does not alter the tone significantly when amp is played clean or when only the power amp section distorts. You’ll have to increase the volume setting to achieve a similar volume as before. The reason is that 12AX7 tube has a voltage gain factor = 100, while 12AT7 = 60 and 12AY7 = 45.
Pull out the V1 normal channel preamp tube – More preamp gain in vibrato channel.
We’ll start with saying that this is a must have mod. It is so easy to enable and disable that it can hardly be called a mod. If you are like most players and only use the Vibrato channel (reverb, tremolo, the brigth cap and the extra gain stage), you should pull out the V1 tube. This is the preamp tube for the normal channel which you are not using. Vice versa; If you’re using the Normal channel, you can pull out the V2 tube. All AB763-similar circuits (Deluxe Reverb, Super Reverb, Pro Reverb, Twin Reverb, Vibroverb, Vibrolux) are designed so that signal is leaking between the two channels. The amp will play louder at the same volume knob setting when pulling the V1/V2 tube that you’re not using. The stronger signal will push the second gain stage (V4 tube) harder and give you increased sustain, compression and harmomics. This mod does not change the amp’s clean headroom.
This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always
Replace the 12AT7 reverb driver with 12AU7 – Better reverb control.
Reverb is an important character with vintage amps, yet so individual and mysterious. We all know that speakers change their tonal character during age. So does the reverb. The reverb function sounds and behaves differently between “identical” vintage amps. Some amps have long, lush and soft reverb while others are mushy and overwhelming. We often find the reverb sweet spot around 2.5 on the reverb pot, varying from 2 to 4. Some amps are sensitive and difficult to control the reverb on. The whole dynamic area can be within a narrow interval, i.e. 2 and 2.5. These amps require a careful touch when dialling in the reverb, which irritates us.
The reverb circuitry consists of two tube sections (reverb driver V3 and reverb recovery V4) and the physical reverb tank. All these components will drift during age and minor differences in component values are noticeable to man’s ear.
If you replace the V3 12AT7 reverb driver with a 12AU7, you will reduce the effect of the reverb and it will be much easier to control with the reverb knob. So simple as that.
Use normal channel for reverb control – Adjust EQ and depth of reverb.
This mod is relevant only for two-channel amps with normal and vibrato channel. This trick is great for the reverb enthusiasts among us, and who is not? Plug your guitar into the vibrato channel, then unplug the reverb return cable on the back of the amp (the one that comes from the reverb tank output) and plug it into the normal channel input. You will need a converter to go from male phono/RCA jack to a 1/4″ male jack. You may now use the normal channel as a reverb control where you can adjust the depth and tone using the volume, bright switch, treble and bass knobs (and mid if you have a Twin Reverb). The reverb knob on the vibrato channel will have no effect any longer.
This mod is not applicable together with the Pull V1 mod, as you need the normal channel preamp tube.
Tremolo disconnect mod - More preamp gain in vibrato channel.
The effect of this mod is similar to pulling the V1 normal channel preamp tube when playing the vibrato channel. This is also a very popular mod in AB763-similar circuits (Super, Twin, Virboverb, Pro Reverb, Deluxe, Vibrolux).
By original design the tremolo circuit will absorbe current/signal even when one turns the tremolo off with the footswitch. This mod suggests to entirly disconnect the tremolo circuit from the signal path by replacing the tremolo intensity pot with a switchabe pot (spst). One side effect with this mod is a noticable click and a volume difference between tremolo on and off using the new spst pot. If one uses the tremolo regularly one should still use the tremolo pedal to enable/disable the tremolo and leave the intensity spst pot at your desired level. The spst pot is not good for enabling and disabling the tremolo very often (because of the click and the volume difference) but is a nice when you play without tremolo. A good thing with this mod is that you have both the tremolo pedal and spst pot to use.
With a new switchable/spst pot set at intensity=0 the mod will kick in and raise the signal level in the preamp section, right before the phase inverter. Once you’re turning up the tremolo the circuit is connected again and the tone will remain original. This mod does not increase the amp’s clean headroom. We would describe the effect as making the tone richer, fuller and more powerful with stronger mids that pushes the power amp section harder (phase inverter + power tubes). We like this mod a lot for stratocasters wi the Fender AB763-similar circuits since they boost a relatively scooped and thin sounding guitar and amp. In addition to pulling the V1 tube, this mod is one of those must have mods which we never undo once having implemented it.
This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps. This mod can be implemented in two ways, either by unsoldering the tremolo circuit or replacing the tremolo intensity pot with a swicthable pot, so-called SPST pot. The switch is enabled when you turn down intensity to zero. There are also switchable push/pull pots where you pull the knob to enable the switch.
Order a 50k SPST from one of our recommended amp part dealers.
Remove the original 50k trem intensity potmeter by unsoldering the wires to the potmeter lugs and unount the pot from the chassis. The black plastic wheel is to be used for the new pot.
Install the new 50k SPST pot to the chassis. This is a switchable pot offering a mid positioned switch in addition to the variable resistanse 0-50KOhm between the lugs. At level=0 you’ll completly disconnect the tremolo circuit.
See the wiring diagram below.
You’ll have to solder the yellow wire to one side of the mid positioned switch in stead of to the right pot lug as before.
Then solder a new (red) wire between the other side of switch to the right lug.
If you’re permanemtly disconnecting the tremolo circuit you can just clip of the brown and yellow wires and insulate the ends with tape. Note that the figure below shows the Super Reverb. The Twin Reverb would be the same.
_________________
MSA 12 String E9th/B6th Universal.
Little Walter PF-89.
Bunch of stomp boxes |
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Tim Marcus
From:
San Francisco, CA
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Posted 6 Nov 2012 8:04 am
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you can find JAN phillips for about $10 - cheaper than new production tubes in many cases. They do a great job driving the reverb.
Or you can spend $25 and get an RCA from Gregg Levy at hi test
12AU7 is not a great choice unless you are confident that your power transformer can handle that much extra plate current draw. Also, new production 12AU7's are unusable, in my experience. The chinese and russians seem to have a hard time balancing the triodes and it makes a mmmmmmm sound
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Milkmansound.com |
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Tim Marcus
From:
San Francisco, CA
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Brett Lanier
From:
Hermitage, TN
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Posted 6 Nov 2012 8:28 am
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| Bob, I recently bought two RCA 12at7's on eBay for $20 (buy it now). Much cheaper than 12ax7's or 5751's... I used one for the reverb driver and one for the phase inverter in my twin and they sound great. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 6 Nov 2012 5:05 pm
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+1 for NOS for 12AT7... this workhorse tube needs to deliver current... new tubes still don't have the cathode chemistry that the old ones had. JAN Philips or RCA... easiest to find. Use the best one in your PI and the next best in reverb driver.
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Too much junk to list... always getting more. |
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Ken Metcalf
From:
San Antonio Texas USA
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Posted 7 Nov 2012 1:18 pm
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I use the first channel for the reverb sometimes... It is easy and works very good !
No mod necessary just a $2 thing from the store.
_________________
MSA 12 String E9th/B6th Universal.
Little Walter PF-89.
Bunch of stomp boxes |
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Ken Fox
From:
Nashville GA USA
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