Tube amps/sealed cabs

[emblymouse]

I have been noticing folks recommending using a sealed cab design for use with a tube amp. I understand the difference of the 2, but not why one would be better suited for use w/ tubes. Or is this just another case of personal taste?

[Peg_legs]

No expert, but I've used both tube and solid state amps with both ported and sealed cabs. I prefer sealed cabs with All tube amps to tighten up the bottom. Sealed cabs roll off the lowest frequencies gradually but start at a higher frequency (typically). Ported cabs roll off more steaply and often at a lower frequency depending on the frequency the port is tuned too. I also "believe" sealed sounds "tighter" and more controlled

I've read plenty on the differences between the 2 types of enclosures, but those are my laymans definitions.

I don't see as much difference when using solid state amps.

I personally think that a properly designed sealed cab will give you all of the frequencies you need for playing bass live.

And now for some ADD thoughts..In my car though, I want dead flat down to 20hz, then slight EQ adjustments for personal taste and I usually use ported enclosures in the car.

[emblymouse]

Ok, so this may be why the low end is kind of flabby with what I've got paired up. I was hoping that a different cab could tighten that up and it wasn't the head (Fender Super Twin).
But the question remains, why this phenomena with tubes and not soild state?

[Mr. Foxen]

The difference is more on the speaker properties, the sort of speakers that suit sealed cabs suit valve amps, because they have a more even impedance curve. If you put those sort of drivers in ported cabs (like the classic Ampeg job with the double baffle) they work too, but don't gain as much advantage from the porting..

[billfitzmaurice]

Originally Posted by emblymouse

I have been noticing folks recommending using a sealed cab design for use with a tube amp. I understand the difference of the 2, but not why one would be better suited for use w/ tubes.

SS amps don't like low impedance loads, tubes don't like high impedance loads. Sealed cabs have a lower average impedance than ported, as shown here, comparing the same driver in sealed (green) and ported (red) cabs:

[eastcoasteddie]

Maybe I got lucky, but my SVT 2-Pro hookedup to a pair of Goliath II cabs (rear ported) sounds amazing. Or maybe because the 2-Pro is sort of a "modern" tube amp.

[Bassmec]

Originally Posted by emblymouse

Ok, so this may be why the low end is kind of flabby with what I've got paired up. I was hoping that a different cab could tighten that up and it wasn't the head (Fender Super Twin).
But the question remains, why this phenomena with tubes and not soild state?

Its a myth left over from the days of when people thought the earth was flat.
The port phase relationship problems makes all non ported (infinite baffle) enclosures tighter in the low end whatever the amplifier.
The problem is infinite baffle need far more internal volume to work properly at low frequencys.

[AwkwardLoudness]

In my experience, tube amps have great CLEANS with ported cabs (and sealed cabs too for that matter). However, I have found that ported cabs generally do not handle overdriven tube amps with lows and low mids dialed in....compared to sealed cabs. Output transformers on tube amps exert dynamics more wildly than solid state output transistors, so I believe that to be a factor. Speakers in ported cabs are pushed to their maximum excursion more easily with that extra air moving around. Compressors can help tame dynamics.

Something I like to do is run a couple tube amps in stereo: one clean and low and one middy and overdriven.....ah sweetness.

[BbbyBld]

Some classic tube power amps don't have a lot of low frequency damping and CAN have very floppy low end. Most classic sealed designs (like an 810) have no real low end response. That combination kind of balances out, which is why some say a sealed cab is a must. You can actually pop all the voice coils out of their magnets with one good slap if you use some tube amps with some ported cabs, even if the cab has a higher power rating.

That's one advantage of Peavey tube bass heads going back to the Classic 400. The resonance control lets you adjust the power amp response to suit the cabinet.

[Mr. Foxen]

Originally Posted by BbbyBld

You can actually pop all the voice coils out of their magnets with one good slap if you use some tube amps with some ported cabs, even if the cab has a higher power rating.

How's that happen?

[billfitzmaurice]

Originally Posted by Mr. Foxen

How's that happen?

Only the lack of high-pass filtering would explain it. But that's not at all unusual with tube amps, especially vintage models. Since they were typically used with sealed cabs and drivers with low xmax and a high xlim to xmax ratio there was little need for high-pass filtering, so they didn't have it.

[Southpaw5]

Originally Posted by BbbyBld

That's one advantage of Peavey tube bass heads going back to the Classic 400. The resonance control lets you adjust the power amp response to suit the cabinet.

Can you explain this further? I have a vb-2 but don't really understand the purpose of the resonance control.

edit* I ask because it feels like i have a lack of low end with this amp unless it drive the lows seriously high and cut mids and highs very low.

from peavey:

Resonance Control -
Adjusts the low frequency damping factor of the power amplifier, which gives the amp a "tigher" sound
when fully counter-clockwise and an increasingly "looser" sound when adjusted clockwise


in the context of this discussion, what does this mean?

Thanks - Dave

[Peg_legs]

Damping factor - Wikipedia, the free encyclopedia

Explanation

In loudspeaker systems, the value of the damping factor between a particular loudspeaker and a particular amplifier describes the ability of the amplifier to control undesirable movement of the speaker cone near the resonant frequency of the speaker system. It is usually used in the context of low-frequency driver behavior, and especially so in the case of electrodynamic drivers, which use a magnetic motor to generate the forces which move the diaphragm.

Speaker diaphragms have mass, and their surrounds have stiffness. Together, these form a resonant system, and the mechanical cone resonance may be excited by electrical signals (e.g., pulses) at audio frequencies. But a driver with a voice coil is also a current generator, since it has a coil attached to the cone and suspension, and that coil is immersed in a magnetic field. For every motion the coil makes, it will generate a current that will be seen by any electrically attached equipment, such as an amplifier. In fact, the amp's output circuitry will be the main electrical load on the "voice coil current generator". If that load has low resistance, the current will be larger and the voice coil will be more strongly forced to decelerate. A high damping factor (which requires low output impedance at the amplifier output) very rapidly damps unwanted cone movements induced by the mechanical resonance of the speaker, acting as the equivalent of a "brake" on the voice coil motion (just as a short circuit across the terminals of a rotary electrical generator will make it very hard to turn). It is generally (though not universally) thought that tighter control of voice coil motion is desirable, as it is believed to contribute to better-quality sound.

A high damping factor indicates that an amplifier will have greater control over the movement of the speaker cone, particularly in the bass region near the resonant frequency of the driver's mechanical resonance. However, the damping factor at any particular frequency will vary, since driver voice coils are complex impedances whose values vary with frequency. In addition, the electrical characteristics of every voice coil will change with temperature; high power levels will increase coil temperature, and thus resistance. And finally, passive crossovers (made of relatively large inductors, capacitors, and resistors) are between the amplifier and speaker drivers and also affect the damping factor, again in a way that varies with frequency.

For audio power amplifiers, this source impedance Zsource (also: output impedance) is generally smaller than 0.1 ohm (Ω), and from the point of view of the driver voice coil, is a near short-circuit.

The loudspeaker's load impedance (input impedance) of Zload is usually around 4 to 8Ω, although other impedance speakers are available, sometimes as low as 1Ω.

[billfitzmaurice]

Originally Posted by Peg_legs

A high damping factor indicates that an amplifier will have greater control over the movement of the speaker cone, particularly in the bass region near the resonant frequency of the driver's mechanical resonance.

The point of diminishing returns with respect to damping factor is arrived at very early on.
http://www.diyspeakers.net/Articles/...G%20FACTOR.pdf

If your eyes glass over halfway through what it boils down to is that a damping factor of 20 is plenty.

[Mr. Foxen]

Originally Posted by billfitzmaurice

Only the lack of high-pass filtering would explain it. But that's not at all unusual with tube amps, especially vintage models. Since they were typically used with sealed cabs and drivers with low xmax and a high xlim to xmax ratio there was little need for high-pass filtering, so they didn't have it.

Doesn't the output transformer act as a high pass, or is it more a max power transfer thing that makes big ones better for bass?

[billfitzmaurice]

Originally Posted by Mr. Foxen

Doesn't the output transformer act as a high pass

I suppose it could be so configured, but there's so many inter-stage pass-through caps in the signal chain that if each were configured with a 30Hz corner you could get at least 4th order and at zero cost, as the caps are there anyway. I doubt engineers in the 50s and 60s ever considered it as the need just wasn't there.

[eyeballkid]

so I was kinda wondering about this myself cos I have a fender 300Pro and the matching 610 and 215, both of which are ported and it sounds absolutely PRIMO.. but with the Fender Bassman 1200, which is the solid state/hybrid rack version of the same amp it sounds kinda muddled and boomy on the same settings. It just struck me as ODD that the ported cab works WAY better with the tube head than the solid state. Put that 1200 on the sealed NV610 and MY GAWD!!! awesomeness abounds.

same thing happened with my V4B and the 610. That combo was balls out great, but put the V4B on an NV610, and while I LOVE that cab, it didnt really shine to me then.

[77PBass]

Originally Posted by billfitzmaurice

SS amps don't like low impedance loads, tubes don't like high impedance loads.

Why is it each amp type "don't like" these loads?

[eyeballkid]

77: just so Bill and all dont have to repeat themselves, go check out this short thread: this is why.

it starts out with a different matter somewhat, but gets to the nitty gritty real quick starting around post #6.

[fdeck]

Originally Posted by BbbyBld

You can actually pop all the voice coils out of their magnets with one good slap if you use some tube amps with some ported cabs, even if the cab has a higher power rating.

Is that due to an inductive kick-back from the energy stored in the output transformer?