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Power Amp -> Attenuator -> Power Amp in series

lukas099

lukas099

Jan 3, 2020
18
20
#1
This is more of a theoretical question than something that I actually want to do.

Say you have an underpowered tube amplifier that you really like the tone of, but it's not powerful enough to gig with. As I understand, you can't run a power amp output into another power amp or you will "release the magic smoke". But what if you attenuated the signal? Could you do it then?
 
#2
No. The amplifier requires a certain impedance load (typically 4 or 8 ohms). As I understand, tube amps are especially sensitive to that (don't have one myself, just what I've picked up here on TB). Feeding the output to another amp input will not deliver that impedance.

Regards,
Wayne A. Pflughaupt

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#3
There, I believe, are such things as amplifiers that take a signal after the output transformer and use it for a line out, so theoretically I suppose such a thing could be constructed. I can visualise practical problems though. In practice I suggest reducing the rest of the band's on stage volume and using the PA and monitors to do the heavy lifting is a better option.
 
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#6
I have a P&P Power Attenuator 100 that I use with my 50W tube amp to record silently, or practice at home with at a lower volume. I haven't used it that way yet, but I could also plug it into a power amp or the FX return of a class D amp for more than 50W. I like the fact that it is a passive device (no need for a power supply), it's tiny and lightweight.
It's funny because there's a fan, even though it's a passive device.
 
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#7
gtirard said:
I have a P&P Power Attenuator 100 that I use with my 50W tube amp to record silently, or practice at home with at a lower volume. I haven't used it that way yet, but I could also plug it into a power amp or the FX return of a class D amp for more than 50W. I like the fact that it is a passive device (no need for a power supply), it's tiny and lightweight.
It's funny because there's a fan, even though it's a passive device.
Click to expand...
There's a fan because even though it's a passive device, it's dissipating a lot of power.
 
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#9
it occurs to me the thread title might be a bit misleading, because what you're asking about is a tube amp into an attenuator, which makes way more sense than what looks like you're asking if a solid state power amp into an attenuator is a good idea (it's not)
 
#11
What you are considering could make sense with a tube amp. Probably not with a solid state amp.

The Ampeg PF50T and PF20T does exactly what you describe. They have an out that is derived from the output transformer. They also have a built in load so they can be run with out a speaker.

I believe the Ashdown CTM30 2.n is also configured with a transformer derived out and has a built in load box. The older LB30, CTM 30, and Little Stubby do not have the built in load, so they must be used with a cab.

I have an Aracom PRX-150 DAG attenuator that can be configured to function as a load box. Aracom Amplifiers Crank it up! Power Rox PRX150-DAG Power Attenuator (aracom-amps.com) It has a line out, so it will do what you want with a low powered tube amp. No idea regarding the resulting tone. As has been said, do not use this sort of device with a Class D or any other amp that is operating in bridge mode.

One final option is a silent speaker chamber/isolation cab. This is basically a sealed chamber with a mic inside. I believe the Rivera Silent Sister is one of the better sounding isocabs but it's big, expensive, and made for guitar. AxeTrak makes a small isocab for bass. Again, no idea on the resulting tone. AxeTrakBass -
 
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#12
I'll bite.
The troubles with the notion are varied. The "sound" that one likes comes from not only matching the dynamic properties of a speaker in it's cab to the plates of that tube amp's output tubes electronically but you have the acoustic properties of that particular driver and the way the speaker cab loads that driver to the "room".
This could get quite involved and glazed over "deer in the headlights" comes to mind. Yes a load "box" which likely is a resistor either non-inductive or with inductive properties is contained within. The problem, drivers are not just a DC resistance but have AC impedance which we use most for amp "loading. Driver's AC impedance varies with frequency, they also vary inductance seen by the amp. Typical dual output tube amp 40-50 watt, loads the tube plates 5,000 ohms that gets "matched" to a driver's 16, 8, 4 ohm impedance "center values".

Operating the "amp and speaker" to the desired level it sounds best to the user you could through an isolation circuit then measure the voltage range of that output voltage point, adjust at that late point to match the input voltage requirement of the power amp for the desired "loudness". Change the room drastically and the "base sound" has changed, the reason we have more than a volume control on our amps. Start over with what is the voltage output from the tube amp again. There may be some exaggeration in this above, but not much I would imagine.
 
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#13
Since it hasn't come up yet, this sounds like the fryette power station though in reverse relative to intended use. It's a dummy load/attenuator for amps up to 200 watts and also has its own 2x6L6 power amp built in. You can send the output of the first tube amp into the attenuator/DI and then use the internal amp for a stage monitor.

Link Removed
 
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#14
B-string said:
I'll bite.
The troubles with the notion are varied. The "sound" that one likes comes from not only matching the dynamic properties of a speaker in it's cab to the plates of that tube amp's output tubes electronically but you have the acoustic properties of that particular driver and the way the speaker cab loads that driver to the "room".
This could get quite involved and glazed over "deer in the headlights" comes to mind. Yes a load "box" which likely is a resistor either non-inductive or with inductive properties is contained within. The problem, drivers are not just a DC resistance but have AC impedance which we use most for amp "loading. Driver's AC impedance varies with frequency, they also vary inductance seen by the amp. Typical dual output tube amp 40-50 watt, loads the tube plates 5,000 ohms that gets "matched" to a driver's 16, 8, 4 ohm impedance "center values".

Operating the "amp and speaker" to the desired level it sounds best to the user you could through an isolation circuit then measure the voltage range of that output voltage point, adjust at that late point to match the input voltage requirement of the power amp for the desired "loudness". Change the room drastically and the "base sound" has changed, the reason we have more than a volume control on our amps. Start over with what is the voltage output from the tube amp again. There may be some exaggeration in this above, but not much I would imagine.
Click to expand...

Huh? Not following this.
 
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#15
lukas099 said:
This is more of a theoretical question than something that I actually want to do.

Say you have an underpowered tube amplifier that you really like the tone of, but it's not powerful enough to gig with. As I understand, you can't run a power amp output into another power amp or you will "release the magic smoke". But what if you attenuated the signal? Could you do it then?
Click to expand...
So while the rest of the world is going ampless, you want to figure out a way to use more amps? I like your style.
 
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#17
Done it many times (while playing guitar) Big 4-ohm ceramic resistor with an adjustable slider to set the level fed to the larger (clean) power amp. Small single-6V6 practice amp (dimed) into my elec piano amp/speaker. A tube amp MUST see it's designed load, can't see why anyone would want to do this from a solid state amp, or anything for bass. Good hard overdrive tone. Understand that's how the Guess Who got that No Time / American Woman guitar tone.
 
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#18
If it's not a tube amp- no.

If it is a tube amp (with a tube power section and transformers), then yes, you would be reamping. Common in guitar universe. Not sure how good it would sound on bass.

How to reamp:

1) Connect first amp's amp output into an attenuator with a power rating sufficient for the output of the amp AND A DEDICATED LINE OUT that is not just a speaker out.
2) Connect second amp into cabinet
3) Connect line out from attenuator (NOT SPEAKER OUT) into 2nd power amp.
4) Turn it all on, dial in the sweet spot on amp 1, use the controls on the 2nd power amp to control volume

The attenuated signal in the load box is STILL TOO HOT to run directly into a power amp so it is imperative that the attenuator/load box has a line out. You cannot simply plug amp 1 into an attenuator, roll down the volume, and then plug it into power amp 2. It has to be a line out, and the attenuator/load box has to be strong enough to handle the output from the first amp. There aren't many attenuators that can handle more than 200 watts.

Also, depending on the attenuator, you may want to connect an additional cabinet to the attenuator's speaker output just to take some of the work off of the attenuator and protect against failures.

Overall: I wouldn't advise it for bass.
 
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#19
Some modern tube amps have outputs for sound reinforcement so you won't need all that attenuator stuff. The modern fender acb50 has both an unbalanced 1/4" out at max +4dbu and an xlr at max -20db that are post power amp. I've run it into a powered cab for more volume. Both the combo and my powered cab have 15" eminence speakers so it's sort of similar in sound, and more so if I have a lpf set pretty low.
 
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#20
lukas099 said:
This is more of a theoretical question than something that I actually want to do.

Say you have an underpowered tube amplifier that you really like the tone of, but it's not powerful enough to gig with. As I understand, you can't run a power amp output into another power amp or you will "release the magic smoke". But what if you attenuated the signal? Could you do it then?
Click to expand...
Ok let me explain what is called a final circuit in an amp real simple like. The output of the amp finals requires a load ( speaker ) or something to dump the power into. In electronics testing a dummy load is used in place of the speaker so the amp has something to unload into. Failure to have a load attached to an open amplification circuit will cause heat to rapidly build up in the amplifier and poof the smoke out. The issue you are having, and cannot resolve by stealing some of that signal, is retaining that warmth and tone you so desire. The only legit way to do so is by microphone in front of the cabinet that is partially responsible for delivering that magical tone. Another way is to use a Kemper amp and attempt to take a digital image of the tube amp. While not the same, subjectively its a pretty close representation and is 100 percent recall able with the push of a button. Many studios have gone this route rather than heating up hard or impossible to replace tubes in rare heads.
 

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