PRS MT15 Tube Swop

Mart60

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I’m looking at trying different pre amp tubes such as 5751 and 12AY7 in my MT15

Looking at the front right to left can someone suggest if V1 is the best one to swop or V2 or even V3 ?

I’m looking at reducing the gain on the lead channel only

Thanks
 

LSchefman

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V1 brings the guitar signal’s gain up the most, so it’s got the most effect on tone. If you’re going for lower gain, that’d be the first one to address and swap out.

Some 12AX7s also have lower gain than others. For example, JJ makes an ECC83 MG (mild gain) 12AX7, and because it is spec’d for the amp’s 12AX7-based circuit, a tube like it might be worth trying.
 

DreamTheaterRules

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Those who have tried it say V3 is the best one to replace to reduce gain of the lead channel. Assuming your tubes are of similar quality, the 5751 should be better than the AY7.
 

Mart60

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i tried a 5751 in V3 and it’s more to my liking and cleans up with the guitar volume nicely

I’m running it now with gain on 8 o clock approx

Any reason why I couldn’t try a lower output pre amp tube such as 12ay7 or 12at7 in V3 to really drop the gain levels or could this have a negative result with the amp in other areas ?
 

Mart60

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Also why is it best swopping V3 and V1 ??

Interested to know why this is
 

Mart60

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Ok I’ve now tried a JJ AY7 in position V1 and this makes the amp far more usable for other genres .. clean channel is less boomy with HB pickups and sounds good, lead channel with gain on 10 o clock sounds like classic rock and I can now add a tubescreamer to give it a boost into heavy rock / metal
Far more usable in my opinion ..
note I tried the V3 position but I couldn’t hear any improvement compared to V1 which is where you normally swap your first input tube.
Nice improvement and made the amp quieter too
 

DreamTheaterRules

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Ok, so V1 feeds both channels. Originally you wanted to lower the gain in the OD channel. NORMALLY, V2 would be the tube I'd look at first for reducing gain in the OD channel and that has a good reason. In many amps, the first gain stages directly related to the OD channel, but NOT the input gain for everything (V1) is a good place to start. And with most amps, it works best. But multiple Archon and MT15 users have tried the lower gain tube in V2 and V3 and so far all of them said that V3 sounded better. Without a schematic and an analysis of the circuit, I can't say why that would be true, but so far everyone says it is.

#2- I'm a big fan of using a boost in front of a tube amp unless the amp is really cranked up. I've owned a lot of tube amps, and at even medium volumes, a boost seems to make the amp sound better. So unless it's some classic single channel Fender amp that is overdriving when cranked and I"m trying to lower the gain level overall, I never replace V1 with a lower gain tube. Oh, I've tried it. But in every case it defeated the purpose of what I just mentioned. Lowering the input gain made it even more important to use a boost (for me, and for the better tones it gave) and ... to crank the boost up more to compensate. At which point you go, "WHY?"

#3- I don't have this in front of me, but not every tube with a 12 in front is identical in it's electrical properties and just varies in gain from the others. I've read a detailed analysis of why the 5751 is the best tube to replace a AX7 with if you want to lower gain, and it is NOT just because SRV did so. It has to do with voltages it runs, etc. Some manufactures will even void your warranty for running certain AY or AT tubes. In some designs, it throws things off enough to make them a little unstable, but in some it can actually cause damage. IF I find this, I"ll link it later.

I'm glad that what you tried is working for you. I still haven't gotten around to trying my NOS 5751 in V3 of my Archon, but I've never felt the need to do so. I can turn the gain down way low and get classic rock crunch easily without giving up anything. And I while I would never say I wouldn't even try it in V1, I'd be shocked if the amp was better on both channels with it there.
 
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Van Noord

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I've tried this and using too low gain of a preamp tube in V1 V2 or V3 really sucks the life and feel out of the preamp. 12AT7 is as low as I could tolerate.
 

andy474x

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All 12A_7 tubes have different properties that make them more (or less) suited to different tasks. Don’t ask me exactly what they are, I don’t remember! Obviously the 12ax7 is the best at amplifying a signal, with a “gain factor” of 100. Moving down in gain factor, such as the 12at7, the tube amplifies less, but can handle other electrical “stuff” in ways that a 12ax7 can’t, which is what makes them good for tasks like reverb driver tubes or phase inverters.

It’s those other properties that are in addition to gain factor, are likely to change the tone, rather than the onset of distortion, of an amp when put in a preamp stage. I once got to pick a bunch of tubes out of a bunch of old car radios, TV’s, etc., and tried them out in a Valve Jr, which uses a single preamp tube, so it was a great way to really appreciate what a different tube type does to tone. I remember the 12at7’s, 12ay7’s, and other lower gain factor tubes being much thinner and anemic (in that amp), the MT15 has lots of bass, so maybe that could be a good match, if that’s what you’re after. On the other hand, that’s why I always recommend keeping a couple 5751’s on hand, they can cut down the gain of the amp without changing the tone.
 

Mart60

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The 12AT7 would be too low a gain drop. The V1 position works well with the 12AY7 or 5751 .. even the guys at PRS suggest it
 

Mart60

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Yes you’re correct my mistake, still the drop to the 12AY7 gain 40 made the better improvent to me using a guitar with HB

I think the 5751 or 12AT7 do not drop enough to hear any real improvement but the 12AY7 warmed the sound and dropped the gain. Worked for me
 

Choo Choo Boogie

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I use a JJ EC81 (12AT7) in V3 and a Ruby 12AX7AC5 HG+ in V1 and love the change.
To my ears sounds like a cross between a Marshall and a Diezel VH4.
 

Mart60

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I use a JJ EC81 (12AT7) in V3 and a Ruby 12AX7AC5 HG+ in V1 and love the change.
To my ears sounds like a cross between a Marshall and a Diezel VH4.

Isn’t V3 the second tube from the right viewed front of amp ?? ( circuit board shows V3 )
 

Van Noord

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I wanted to relieve some of the stiffness and darkness and add some high end crispness to the preamp voicing so changed V1 to a Tungsol, and put EH 12AX7s in V3 & V4.
V5 & V2 are still JJs.
V6 is a Mesa labeled Sovtek, only due to the stock JJ going microphonic
.
 

DreamTheaterRules

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I wanted to relieve some of the stiffness and darkness and add some high end crispness to the preamp voicing so changed V1 to a Tungsol, and put EH 12AX7s in V3 & V4.
V5 & V2 are still JJs.
V6 is a Mesa labeled Sovtek, only due to the stock JJ going microphonic
.
And the results?
 

Choo Choo Boogie

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my two cents......I got a really nice sound that is a cross between classic rock and a Diezel VH4 channel 3 sounds by using the info above and making these tube changes:
V1 = Tung Sol 12ax7
V2 = Engl ECC83s
V3 = Mullard 4004
 

Goran Lorencin

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You guys are like rocket scientists to me, I`m a newb when it comes to swapping tubes, but have a very sensitive ear and would like to know which tubes (specific model types) are there on offer to be swapped with the factory ones (not to void the warranty and to keep things tidy current wise)? You talk about 12AX7, 12AT7, 5751, ECC83s, 12AX7AC5 HG+, ... god damn where do those names end?! What do they mean? Must I go to Harward tube university to understand all that or? How important is it to use a specific brand/maker tubes?

I just want to find out what options I have when the time comes to swap out some tubes and which tubes I should have at home, ready to be swapped.

What are the alternatives for the JJ EC83S pre-amp tubes?

What are the alternatives for the factory power tubes JJ 6L6GCMS?

I hear many talking that sometimes (Chinese) factory tubes aren`t as good as others made by reputable/other tube makers, how does that equate sound or quality wise?

thx in advance my dear rocket scientists
 
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