MT15 FX Loop

[Ajaz]

Does anyone know why master volume does not works if you only use FX return on MT15. You end up controlling volume externally. I have not seen this on other amps. Also, according to link below tube diagram, it seems fx loop uses 12AX7 tube, not 6L6.

https://www.eurotubes.com/store/pc/PRS-MT-15-High-Gain-Option-p3094.htm

 

[andy474x]

The master volume is before the effects loop in the circuit, so if you go only to the effects return, it’s out of the signal chain.

Also, yes, the effects loop driver is a 12ax7. If you’re using the effects return, the 6L6’s are still in the signal path, as they are after the effects return and phase inverter.

I find the amp to be more controllable and have better clarity with 12at7’s for the effects loop and phase inverter positions, and also a lower gain V1, like the new production Mullard 12ax7/ecc83.

 

[Ajaz]

The master volume is before the effects loop in the circuit, so if you go only to the effects return, it’s out of the signal chain.

Also, yes, the effects loop driver is a 12ax7. If you’re using the effects return, the 6L6’s are still in the signal path, as they are after the effects return and phase inverter.

I find the amp to be more controllable and have better clarity with 12at7’s for the effects loop and phase inverter positions, and also a lower gain V1, like the new production Mullard 12ax7/ecc83.

Thank you. I always thought master volume was for power section only. So is it correct to say that on MT 15, there is no master section control for volume, which means the 6L6 are always maxed out.

 

[andy474x]

Yes, I believe so. The MV is just changing the amplitude of the incoming signal to the power amp. In that arrangement it also changes the amplitude of the signal going through the effects loop.

 

[Ajaz]

Yes, I believe so. The MV is just changing the amplitude of the incoming signal to the power amp. In that arrangement it also changes the amplitude of the signal going through the effects loop.

Interesting. So you are saying that MV has no impact on tube saturation? I assume that would true since pre-amp tubes are being saturated by gain knob and power amp tubes have no knob, so they are fully saturated all the time.

 

[Stephen J.]

Start about 24:30 of this video. He talks about the power tube and phase inverter relationship in amps. I don’t know if this is helpful or not.

 

[andy474x]

Interesting. So you are saying that MV has no impact on tube saturation? I assume that would true since pre-amp tubes are being saturated by gain knob and power amp tubes have no knob, so they are fully saturated all the time.

Not saturated, exactly - in a circuit like this, they’re tuned to provide their maximum amplification factor all the time, yes, but whether they actually reach their limit and saturate depends on what they get from the preamp, via adjusting the MV, the power section isn’t going to distort and saturate until it actually receives a signal with enough amplitude to make it exceed its limit of clean headroom.

Imagine it like this, the power section in an amp like this is always going to amplify the input signal by a certain amount, until it reaches its limit. For example, the purely arbitrary numbers here, but say the power section is amplifying everything that comes into it by a factor of 10, and its maximum clean, undistorted output is 100dB. With the master volume at half, say the preamp signal is 10dB going into the power section, 10dB multiplied by an amplification factor of 10 from the power section is 100dB, the maximum “clean headroom” of the amp. Increase the master and let more than 10dB signal into the power section and the amp will be trying to amplify the signal past its actual power limit, and then the power section will distort and saturate, because there’s nowhere else for it to go, say 15dBx10= amp trying to create 150dB but can’t and the sound waveform will clip. Decrease the master volume, and thus the signal into the power section, and you will be below the output limit of the amp, say 5dB coming from the preamp, and that’s 5dBx10 = 50dB, within the limit of what the amp can actually produce and not trying to push the power section past its limit.

Again, those are totally arbitrary numbers, and that explanation oversimplifies and glosses over a lot of important physics, but it is the fundamental concept. I think what you’re thinking of are the other operating parameters of the tubes, which yes are basically set to maximum all the time in this amp, but whether clipping and saturation happen are totally dependent on how big the signal being amplified is.

 

[Deleted member 5962]

Start about 24:30 of this video. He talks about the power tube and phase inverter relationship in amps. I don’t know if this is helpful or not.

OMG, I’m SO glad he (John) said that. I’ve said that for years but people have heard the opposite for so long that most people look at you like you’re an idiot when you say it. So, now that an expert has said it, I’ll repeat it... MOST of what we hear and love about the back end of an amp getting pushed is PI overdrive, not power tube overdrive. You heard him say those old fenders and Marshalls weren’t overdriving the power tubes at all. Those stages are designed to run clean. It’s the PI that’s providing that overdrive when the amps are cranks. The most overlooked tube by most people, is one of the most important ones you buy, IF you push it.

Also, the Kmaster does a little bit more than John just said. It has another GREAT trick up it’s sleeve. He doesn’t want to give away trade secrets, but it is trademarked... the Kmaster when turned down, reduces voltage to the PI tube... meaning it can give you that killer overdrive you love in the PI stage, while actually pushing the power tubes EASIER. So, retain the “cranked” tone, at lower volumes. BRILLIANT! (That is the patented part, I think). This is such a killer idea. When the patent expires it should be on all amps. There may be more too it than I understand, but just the verified parts are enough to make it the best, most effective and most versatile master volume ever.