Lower Gain Tubes to Reduce Compression?

All about tube choices for the JVM series. See workbench for biasing and tube problems you may have.

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Big Al
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Lower Gain Tubes to Reduce Compression?

Post by Big Al » Wed Jul 23, 2014 10:52 pm

What are your thoughts on using lower gain preamp tubes? Would running lower gain tubes reduce some of the compression even if you ended up running the gain knob on the amp higher (OD channels)? I really do not hear any difference from 12:00 and up on the gain knob now so I was wondering if using lower gain tubes and running the gain knob higher would do anything different? If so, what positions would be best to try.
JVM410H: 3H MM Choke, KT77 Tubes, Negative Feedback Mod, Compression Reduction, Channel Four Recto Mod.

Dark Horse
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Re: Lower Gain Tubes to Reduce Compression?

Post by Dark Horse » Wed Jul 23, 2014 11:59 pm

Don't know the answer to the first question because it is a matter of taste. If by lower gain, you mean a 12au7 or similar, most people who've tried it didn't like it as well. If you mean a lower-gain brand of 12ax7, then maybe it might suit your need. To the second question, V1 and V2.

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Tonemeister
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Re: Lower Gain Tubes to Reduce Compression?

Post by Tonemeister » Thu Jul 24, 2014 9:36 am

I assume you are asking about tubes because you do not want to mod the amp?... The Anti-Compression Mod and/or Gain Reduction Mod would be something to consider if modding is not out of the question.
- Marshall JVM410H: Negative Feedback Mod (33k-500k, linear), Compression Mod, One-Wire Cascade Mod, OD2-to-OD1 Mod, P.I. Boost Mod (with 33k in R52), Plexi Cap Mod (1uF), Modified C83 Mod (1- OFF, 2- 0.68uF, 3- 1.0uF, 4- 1.5uF, 5- 2.2uF, 6- 3.2uF), Gain Reduction Mod, Blackface Mod, Mesa Rectifier Mod, 50W (two tubes: 37.5mV), LED Footswitch Mod
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stewarttr
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Re: Lower Gain Tubes to Reduce Compression?

Post by stewarttr » Tue Nov 01, 2016 9:27 am

If my understanding is correct only Guitarists think Gain = distortion. A higher gain tube will actually give your amp more headroom (i.e. less Overdrive/Distortion).

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jimsreynolds
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Re: Lower Gain Tubes to Reduce Compression?

Post by jimsreynolds » Tue Nov 01, 2016 11:24 am

stewarttr wrote:If my understanding is correct only Guitarists think Gain = distortion. A higher gain tube will actually give your amp more headroom (i.e. less Overdrive/Distortion).
Sorry Stewart, that is incorrect. In practice, the more gain you add, the more likely the signal will clip .... hence distortion.

For example, a clean boost pedal in front of the amp (e.g. MXR Micro amp) will serve to add more gain to the clean signal without changing the EQ, therefore making it more likely that the preamp of your amplifier will overdrive. This is a traditional (!) method of forcing your amp to overdrive.

As an example (using fairly wild-ass numbers so don't get hung up on them)

- Your guitar puts out 2V signal.
- The first tube gain stage of your amplifier is designed will not output a distorted signal unless it gets an input signal greater than 3V
- Your clean boost has a gain of '2' and therefore doubles the voltage put out by the guitar to 4V.
- Now, when your guitar volume is up full and you hit the string hard, the first tube stage will start to distort the output signal.

Therefore, gain caused distortion.

In terms of headroom, in the argument above, lets now say that the first tube stage is re-designed so that it will not distort unless it gets a 10V input. NOW we can say that the first tube stage has 'more' headroom because it can handler a higher input voltage without the output clipping.

The simplest way to change the 'design' is to replace the tube in that first stage with one that to gives more or less gain. 12AX7, 12AU7, 12AT7 and 12AY7 tubes all have different levels of gain associated with them. Therefore, if I want that first stage to distort less easily, I can change out the (stock) 12AX7 tube for a lower gain 12AT7. This effectively gives more headroom as the tube stage will not boost the input signal so greatly at the stage output, therefore reducing the risk of the signal clipping..
JimBOB

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namklak
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Re: Lower Gain Tubes to Reduce Compression?

Post by namklak » Tue Nov 08, 2016 1:44 am

eh, maybe.

First stage, the DC gain is roughly the Rplate/Rcathode or 150K/1.5K or 100. Hey, that's about what a 12AX7 gain is. But there is a cathode bypass cap, attempting to raise the gain of the first stage for frequencies above 100Hz. So what happens at freqs above 100Hz if the Rp/Rc (aka gain) is more than the tube can deliver? Compression. So having a higher gain tube in there can potentially reduce compression somewhat. Unless the signal is soooo big that tube stage saturates, like maybe from active pickups or a huge OD gain? BTW, tube saturation is not a bad thing, just sayin...

My oversimplification probably muddied the water more than i should've...
JVM205h. Added 100K from R94/C43 to ground for V1b (OD Channel) gain reduction. Installed 1000pF in series with 47K, in parallel with R58 (82K), to reduce power amp squeal. V3 Valve Wizard diode protection mod. Active power soak for more EL34 goodness.
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jimsreynolds
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Re: Lower Gain Tubes to Reduce Compression?

Post by jimsreynolds » Tue Nov 08, 2016 5:45 am

Apologies, thought you were at a lower level of understanding than you clearly are.

Interesting perspective you put forward - certainly challenged my understanding and lengthened my reading list!

The conventional (received) wisdom seems too be lower gain tube in first stage = less distortion output from the preamp due to less gain into the remaining stages from the first stage. I think my description mostly holds true for a multi gain stage preamp. However I think I see where you are going with that argument so will not bang the drum too loud!

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JimBOB

-------------------------------------------
Amps:

Guitars:
  • 70s hard-tail Strat with EMG 89/SA actives. Gibson "The Paul" with bridge P-Rails, Triple-shot, Roland GK-3 and Torres Tone Mods. Gretsch 6119 Tennessee Rose. PRS SE Semi-Hollow w/Bigsby, Squier Affinity Tele (Modded), Falk "Splitter", Taylor T5, Ibanez RG2610E
    Taylor DN3 Acoustic w/Rare Earth Blend, Ovation Celebrity 6/12 Acoustic, Martin 000X1AE, Rally Resonator, Stretton Payne 12-string.
FX:
  • Joyos galore! T-Rex Moller, TC Flashback X4, MXR '78 Distortion, Crybaby, Line 6 HD500 - 4CM & MIDI switching, (retired), Boss GT8 (retired)

okgb
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Re: Lower Gain Tubes to Reduce Compression?

Post by okgb » Sun Aug 06, 2017 5:45 pm

For me , experimenting with lower gain tubes never seems better, usually a tone / feel shift

Anyone else had good / bad experience with subbing lower gain tubes ?
Amps JVM 410H [too many mods to mention ] '73 50w lead, 70's vibrochamp , mesa mkIII U.S. Vox pacemaker
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Re: Lower Gain Tubes to Reduce Compression?

Post by Spacerocker » Tue Aug 22, 2017 1:06 pm

Since most of the "compression" and "gain" (more correctly clipping) is created by reducing headroom by the use of large plate resistors on the JVM, then reducing the value of some of these is the obvious way to go (and has been done on the JS version) Adding a 220KOhm resistor in parallel with the current value does not effect a permanent mod, and can easily be returned to stock.

This would be a much more effective way of reducing "gain" than changing valves, and I have done this on my amp for the OD channels + one other place (which I can't remember) and am more than happy with the result!
Amps: Marshall JVM410: Hotter Bias, Plexi mod, 10H choke, Stiffness Mod, Neg Feedback Mod to 2203 spec, Tone stack Mods to 2203 spec, "Anti-Compression" mod, dBx 31 band graphic in loop, Marshall AVT20 Valvestate practice amp, Marshall 1980 2203, 2 x 1936 Cabs, 1968 Orignal 4 x 12 Cab with Greenbacks
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