Power Amp -> Attenuator -> Power Amp in series

[el murdoque]

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.

I can't fathom that level of engineering, but can offer some empiric data, because I have chased that particular rabbit.
I've tried several low powered tube amps (H&K Bass Master, Reußenzehn Studio Bass Deluxe, Ampeg PF 20-T, Ampeg PF 50-T) with the intention of getting their output signal, with the power tube sound and compression sent to a high powered class D amp and essentially build myself a very loud tube amp sounding device where a class D amp does the heavy lifting so my back does not have to.
All my tries were in vain.
The output signal of any of those amps connected directly to a cabinet was vastly different to the signal of the amp running into a load box and sending the transformer out signal into a poweramp (often needing something in the signal chain to make the output signal able to drive a power amp) and then to a cab.
In layman's terms, someone explained to me that the output transformer and the speaker(s) have a conversation that goes forth and back instead of the output of the amp shouting into the input of the cab with no response.
The impedance of a cabinet is a dynamic thing that changes with the load that is applied, which changes the behavior of the output transformer.

In all my experiments, the output I got from the speaker was significantly lower quality and less enjoyable than using an output from the preamp, omitting power tubes and output transformer entirely.

What I did not try is a device like the Two Notes Torpedo Reload, which acts as a speaker and cabinet sim. In my book, getting another expensive device that has considerable bulk, to make the combination of power amp and small tube amp kinda defeats the purpose and one could just step up and bring a big tube amp then.

 

[Wasnex]

In layman's terms, someone explained to me that the output transformer and the speaker(s) have a conversation that goes forth and back instead of the output of the amp shouting into the input of the cab with no response.

IMHO the same problem is apparent when you use a power attenuator. As you crank up the attenuation, the interaction between the amp and cab is weakened. As result, the sound and feel degrades. As player it's pretty noticeable IMHO. Probably less noticeable for the listener.

You could try using the low powered amp with a speaker to preserve the interaction between the tubes, transformer, and speaker.

 

[Wasnex]

What I did not try is a device like the Two Notes Torpedo Reload, which acts as a speaker and cabinet sim.

The Aracom PRX 150 manual makes a similar claim:

Despite this language, the sound and feel still becomes progressively degraded as the attenuation is cranked. IMHO 4-6dB is bearable. I could tolerate more, but it takes away from the playing experience.

I suspect you would notice the same problem with the Torpedo Reload.

 

[lukas099]

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)

I did not know that tube amps were not considered power amps.

 

[el murdoque]

You could try using the low powered amp with a speaker to preserve the interaction between the tubes, transformer, and speaker.

Yeah, that's what I thought, too.
But when you do that, you could use a mic and mini mixer to feed the power amp, creating something of a mini PA setup. I mean, why grabbing the hot signal and feeding it through artificial cab emulation when you have a cab right there and can do the original thing - for less money, too .. ?

 

[Stylee Mousse]

Reactive load attenuators, like the Weber one I use that uses a speaker motor to create a reactive load, still are not too pleasing when doing a lot of attenuation.

Exactly as WASNEX says, it works really well at 6 db or so and progressivly less well after that.

I use mine to tame down a 90 watt non-mv Traynor/410 guitar rig to reasonable stage levels. Works great for that. That particular unit's line out sounds unpleasant to me, I definitely don't want to record it or amplify it otherwise.

I do play bass through that rig in the studio when I want real dirt. Mic on cab, attenuator making the speakers operate within their survivable range. It's a nice flavor we all know: fender tone stack and tube output, driving speakers that are feeling a lil sweaty. Or just being a plexi type amp doing Lemmy when a recording wants that.

 

[agedhorse]

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.

Except that "real simple like", heat is NOT the issue with running a tube amp unloaded.

The issue is that when a tube amp with an output transformer is driven into an open load, the almost pure inductance of the output transformer generates back EMF (especially when the signal is clipped or has very fast transitions) and high voltages are generated which can break down the insulation between parts.

This is true for solid state amps driving heavy inductive loads also, but they generally have more effective protection from this type of damage.

 

[tech21nyc]

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?

What you are referring to is called slaving. This has been around for decades. Some Mesa Boogie bass and guitar amps have slave outs which allows you to take a padded down signal from the speaker output and send that to another power amp, effects or both. Slaving is more common in the lead guitar community these days. I did this at our gig yesterday. I run my 50 watt Marshall style tube amp into a 100 watt Fryette Power Station. The Link Removed has an internal reactive load (Power Handling Capacity of Reactive Load (Max continuous): 200W @ 2/4 Ohms; 150W @ 8 Ohms; 200W @16 Ohms) that reduces your amps output to line level and amplifies that signal with its internal 100 watt 6550 power section.

There are a few ways to implement this. One method is to use your amp as you normally do and take a line out of that amp (you need a voltage divider or line out box) and send that signal to a more powerful amplifier. You can do the aforementioned load method (which silences your amp) and amplify the signal any way you like. Tube or SS will work but the idea of the tube amp is that it will more accurately respond to your speaker system in the same way due to the damping factor/tube difference. Depending upon the wattage of the "tube" bass amp you have something like the Fryette will work. One stop shopping.

Suhr makes a the ISO Line Out box which has no load. That gives you the equivalent of a Slave out. The Suhr box has the advantage of a phase and ground lift.

 

[walterw]

I did not know that tube amps were not considered power amps.

technically they are but that misses the entire point of the thread i think

a thread about "tube amps into attenuators into solid state amps" would likely elicit more answers from folks who immediately know exactly what you're talking about (although i see people who know what's what have already chimed in so that's good)

 

[Ox Han]

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?

I am aware of a few ways to do this;

1. If the low powered amp has a "line out" (NOT speaker out) you could run this into the input, or "power amp in", of another amp. Not all amps have these features though, so onto number next one.

2. Attenuator w/ line out - if the speaker on the low powered amp can be unplugged, you can insert an attenuator between amp and speaker. If you have an attenuator that has a line out, or di out, then you run this into the input or "power amp in" of another amp. Not all combos allow the speaker to be unplugged, so on we go…

3. Fryette power station - I am currently looking into this as a viable option for the same type of situation.

As mentioned by another member, this device does a few studio tasks. If I understand correctly, one of those tasks is to take the output of a low wattage amp (speaker output) and push bigger cabinets using the power station as an additional power amp between the two to make it louder/more powerful and sculpt the tone further. The power station power amp is either 50 or 100 watts depending on the model.

I am considering the Fryette PS to use in the following way to achieve something akin to either a B15 tone, or a sound that's really interesting without buying more amps/gear


bass into bass preamp -> guitar amp "power amp in" -> guitar amp speaker output into power station -> bass cabinet

In theory, I think this will allow me to utilize any tube guitar amp & any bass cab to become an all tube bass amp.

 

[agedhorse]

In theory, I think this will allow me to utilize any tube guitar amp & any bass cab to become an all tube bass amp.

Except that most tube guitar amps do not make very good bass amps since they are voiced (some quite extremely) for guitar.

 

[Ox Han]

Except that most tube guitar amps do not make very good bass amps since they are voiced (some quite extremely) for guitar.

That's why I would bypass the preamp stage on *most of them by using the "power amp in" jack

This is especially true if you have a guitar amp with 6L6 power tube(s). If you do, then you might be sitting on a potential bass amp under the right circumstances. And, some guitar tone stacks can work fine with a bass especially for a more vintage sound. Just connect to a bass cab and give it a spin. Worst thing is that is doesn't sound good

 

[agedhorse]

Possibly, IF the amp has a power amp input (many do not).

Many guitar power amps, regardless of the power tubes used, still make poor bass amps because the output transformer is optimized for guitar and is often limited in low frequency capabilities (and at higher levels can saturate at low frequencies which can damage power tubes over time, besides it sounds pretty bad).

 

[Ox Han]

I have not heard about low frequencies damaging power tubes, which seems to be odd considering many bass amps and guitar amps use 6L6 tubes

I guess this is another reason why the fryette power station is attractive

 

[N. Kentucky Bass]

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?

Easy solution: get a bigger amp.

 

[agedhorse]

I have not heard about low frequencies damaging power tubes, which seems to be odd considering many bass amps and guitar amps use 6L6 tubes

I guess this is another reason why the fryette power station is attractive

You just aren't understanding the reasons why.

In a guitar amp, the output transformer is not generally designed to handle below about 60-80Hz at rated power. What happens is that as the frequency decreases and the power level increases, the core runs out of ability to increase magnetization and reaches its flux limit. When this occurs, the primary (driven) impedance begins to fall and with some alloys that are used, it falls like a rock. As the primary impedance falls, it places additional plate load on the output tubes which cause them to attempt to deliver more power than they were designed for. Over time this can shorten the life of a power tube by quite a bit.

It's not odd at all once you understand how a tube power amp really works (part of my day job).

 

[Ox Han]

You just aren't understanding the reasons why.

In a guitar amp, the output transformer is not generally designed to handle below about 60-80Hz at rated power. What happens is that as the frequency decreases and the power level increases, the core runs out of ability to increase magnetization and reaches its flux limit. When this occurs, the primary (driven) impedance begins to fall and with some alloys that are used, it falls like a rock. As the primary impedance falls, it places additional plate load on the output tubes which cause them to attempt to deliver more power than they were designed for. Over time this can shorten the life of a power tube by quite a bit.

It's not odd at all once you understand how a tube power amp really works (part of my day job).

I wasn't trying to disagree or argue. You are not wrong about me. I have zero knowledge of tube amps, circuits, or electrical engineering.

I defer to you and others with the knowledge.

 

[Ox Han]

You just aren't understanding the reasons why.

In a guitar amp, the output transformer is not generally designed to handle below about 60-80Hz at rated power. What happens is that as the frequency decreases and the power level increases, the core runs out of ability to increase magnetization and reaches its flux limit. When this occurs, the primary (driven) impedance begins to fall and with some alloys that are used, it falls like a rock. As the primary impedance falls, it places additional plate load on the output tubes which cause them to attempt to deliver more power than they were designed for. Over time this can shorten the life of a power tube by quite a bit.

It's not odd at all once you understand how a tube power amp really works (part of my day job).

Question for you - safe to assume the same is NOT true for guitars going through bass amps?

Or, does a guitar going through a tube bass amp lower power tube life also?

Asking because I could be using the V4B for guitar sometimes

Thanks!

 

[agedhorse]

Question for you - safe to assume the same is NOT true for guitars going through bass amps?

Or, does a guitar going through a tube bass amp lower power tube life also?

Asking because I could be using the V4B for guitar sometimes

Thanks!

The bass amp output transformers are generally much more robust at LF and therefore can't saturate at guitar frequencies no matter how hard you push it.

 

[Wasnex]

Question for you - safe to assume the same is NOT true for guitars going through bass amps?

Careful if your cab has a tweeter. I suppose the warning could be valid for a mid-driver as well.