NTAAD! This should be good to have around

I also had zero experience with load boxes before getting the Suhr RL and as soon as I plugged it in I was satisfied. I had enough experience with the amps through cabs to know what to expect and thatā€™s exactly what I got.

Iā€™m glad I got it because it put the ā€œDo amps and modelers feel the same?ā€ crap to bed for me, permanently and without question and I still end up using my AxeFX for 98% of my recordings.
 
Chocol8 said:
Not at all the same. The guts really matter on a reactive load so while a lot of products can serve the same function, they don't work nearly as well, especially with amps that don't have a lot of NFB, or any at all.
Click to expand...
Whatā€™s NFB, and how is it not the same? Iā€™m asking cuz Iā€™m genuinely curious and Iā€™d like to know!
 
IMG_0535.jpeg
 
Mikael Dez said:
View attachment 42463
Click to expand...
The Suhr Reactive Load has more passive components (2 inductors, ~4 capacitors, ~5 resistors) in the the reactive load portion of the circuit. That contributes a bit to the cost difference.

The load portion of the Julius thatā€™s presented to the amp is the inductor and green wirewound resistor.

The Suhr has a good cooling scheme. Its resistors are mounted on a heatsink, it has a fan, and the enclosure is ventilated. It can take my cranked 50W Plexi for a few hours without getting too warm. The cooling and enclosure is a significant cost difference to the Julius.

The Julius would likely get very hot within minutes if it had to take a full 50W. I built a JohnH Type M2 attenuator and rely on the chassis to dissipate heat rather than a heatsink. It gets hot after about 15 minutes.
 
Mikael Dez said:
Whatā€™s NFB, and how is it not the same? Iā€™m asking cuz Iā€™m genuinely curious and Iā€™d like to know!
Click to expand...

LOL, this is a question for an electrical engineering class, but Negative Feed Back, when talking about tube amp power sections, is taking the signal from the output transformer and feeding its inverse back into the main signal, usually at the phase inverter.

Basically it is done to clean up the signal because any noise not from the original signal is fed back inverted and cancels out. Part of the signal gets cancelled as well, but the designer controls the ratio. Some amps like older Fender Tweeds and Vox based designs don't have NFB, other designs have a significant amount.

The key for attenuation is that NFB increases damping factor and how the power output of the amp is impacted by the impedance of the load. (yes this is getting complicated!) A typical 8 ohm speaker (or speaker in a cab) does not actually have 8 ohms of impedance. There is an impedance curve with a spike in the bass resonance range say 60-100 hz and then it drops and slowly rises as frequencies go up. The frequency response (output level for a given frequency assuming the same input level) of a tube amp will vary with the impedance of the load, more so with low DF/non NFB amps but still true for most.

SO, the frequency response of the amp will change based on the impedance curve it sees. A reactive load or attenuator that does not present the same impedance curve to the amp as the speakers will cause a change in the tone. Some amps more than others, but significant enough for most tube amps that attenuators have a horrible reputation for sucking tone.

The Suhr RL and the JohnH/Frommel designs use a combination of resistors and inductors to fairly accurately match the impedance curve of a typical guitar cab. Because of that, the amp output section behaves like it would into the real cab. Most other products on the market have an impedance curve that is not accurate or gets worse and worse as you increase attenuation. With these devices, the amp output changes and we complain.
 
For anyone interested in an attenuator that works, or learning why, check out the original JohnH thread on the Marshall Forum. There is a lot of really great info if you want or just the designs to build your own. You can also now buy prebuilt from Formal as the lotus.

The Suhr RL is a very similar concept except it is simplified because it only has to work at one level of attenuation. Aiken had a paper on the design concepts that I think helped John and maybe Suhr.

Simple Attenuators - Design And Testing

Background Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve...
marshallforum.com marshallforum.com
 
Chocol8 said:
LOL, this is a question for an electrical engineering class, but Negative Feed Back, when talking about tube amp power sections, is taking the signal from the output transformer and feeding its inverse back into the main signal, usually at the phase inverter.

Basically it is done to clean up the signal because any noise not from the original signal is fed back inverted and cancels out. Part of the signal gets cancelled as well, but the designer controls the ratio. Some amps like older Fender Tweeds and Vox based designs don't have NFB, other designs have a significant amount.

The key for attenuation is that NFB increases damping factor and how the power output of the amp is impacted by the impedance of the load. (yes this is getting complicated!) A typical 8 ohm speaker (or speaker in a cab) does not actually have 8 ohms of impedance. There is an impedance curve with a spike in the bass resonance range say 60-100 hz and then it drops and slowly rises as frequencies go up. The frequency response (output level for a given frequency assuming the same input level) of a tube amp will vary with the impedance of the load, more so with low DF/non NFB amps but still true for most.

SO, the frequency response of the amp will change based on the impedance curve it sees. A reactive load or attenuator that does not present the same impedance curve to the amp as the speakers will cause a change in the tone. Some amps more than others, but significant enough for most tube amps that attenuators have a horrible reputation for sucking tone.

The Suhr RL and the JohnH/Frommel designs use a combination of resistors and inductors to fairly accurately match the impedance curve of a typical guitar cab. Because of that, the amp output section behaves like it would into the real cab. Most other products on the market have an impedance curve that is not accurate or gets worse and worse as you increase attenuation. With these devices, the amp output changes and we complain.
Click to expand...

Must have had them Mushrooms early this morning, eh?
:LOL:


:beer
 
Chocol8 said:
LOL, this is a question for an electrical engineering class, but Negative Feed Back, when talking about tube amp power sections, is taking the signal from the output transformer and feeding its inverse back into the main signal, usually at the phase inverter.

Basically it is done to clean up the signal because any noise not from the original signal is fed back inverted and cancels out. Part of the signal gets cancelled as well, but the designer controls the ratio. Some amps like older Fender Tweeds and Vox based designs don't have NFB, other designs have a significant amount.

The key for attenuation is that NFB increases damping factor and how the power output of the amp is impacted by the impedance of the load. (yes this is getting complicated!) A typical 8 ohm speaker (or speaker in a cab) does not actually have 8 ohms of impedance. There is an impedance curve with a spike in the bass resonance range say 60-100 hz and then it drops and slowly rises as frequencies go up. The frequency response (output level for a given frequency assuming the same input level) of a tube amp will vary with the impedance of the load, more so with low DF/non NFB amps but still true for most.

SO, the frequency response of the amp will change based on the impedance curve it sees. A reactive load or attenuator that does not present the same impedance curve to the amp as the speakers will cause a change in the tone. Some amps more than others, but significant enough for most tube amps that attenuators have a horrible reputation for sucking tone.

The Suhr RL and the JohnH/Frommel designs use a combination of resistors and inductors to fairly accurately match the impedance curve of a typical guitar cab. Because of that, the amp output section behaves like it would into the real cab. Most other products on the market have an impedance curve that is not accurate or gets worse and worse as you increase attenuation. With these devices, the amp output changes and we complain.
Click to expand...
I donā€™t known why it was a funny question, but thanks for the answer!
 
Mikael Dez said:
I donā€™t known why it was a funny question, but thanks for the answer!
Click to expand...

I thought it was funny because it seems like a simple question, but the answer I gave was about as simplistic as I could make it. Guitar players should NOT need to know this stuff, but unfortunately load and attenuator designers have not understood it and most products on the market are needlessly bad. The result is a LOT of people think all attenuators suck tone or they don't know where to begin finding a good one.

My simple advice to anyone who wants to control tube amp volume is:

If you can solder, drill and screw, build a JohnH attenuator or 3. If not, buy the Fromel Lotus. Alternatively, re-amp with a Suhr RL or if you want an all in one box, the PS-2 or PS-100 are pretty good. The Two Notes Reload II also looks promising but I have not had my hands on one yet.
 
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