Bass Thump: EL84 v. 6L6

[The New Steve H]

There is no reason why a 4xEL84 amp should have less bass thump than a 2x6L6 amp, is there? I mean, just based on the output tubes.

I was playing my Bassman clone yesterday, and I really had to turn the bass down. I'm used to my 5F6A/Rocket hybridy thing, which has four EL84's.

I don't consider this a problem. Just a difference.

[Reeltarded]

6L6 has extension that tiny baby itty bitty 84s do not. They stand pretty hard against 34s too.

[The New Steve H]

Okay, so it's not my imagination. This time.

[Malcolm Irving]

Tubes can operate at any low frequency you like, all the way down to DC. But there are lots of other things that affect the amount of bass: speakers, cabinets, output transformer, fixed or self-biased, various resistor capacitor combinations in the amp circuit, power supply sag, global negative feedback, etc.

[pdf64]

Absolutely, but even when all other variables have been accounted for / eliminated, 6L6 seem to retain a solid bottom end even when overdriven, whereas lower rated tubes have a tendency to mush up more and produce a more middy tonal balance.
Maybe it's related to some obscure characteristic, eg g1-k impedance when positively biased?

[Malcolm Irving]

Absolutely, but even when all other variables have been accounted for / eliminated, 6L6 seem to retain a solid bottom end even when overdriven, whereas lower rated tubes have a tendency to mush up more and produce a more middy tonal balance.
Maybe it's related to some obscure characteristic, eg g1-k impedance when positively biased?

Yes, I agree, it must be something like that. The plate curves for EL84 seem to have a more rounded 'knee' than 6L6 - perhaps that could account for 'earlier' distortion?

[pdf64]

Ha, thanks, I never clocked that before!
My guess is that with any tube, the saggy screen grid supplies of most guitar amps may act to round the knee off more than the published curves, in which the g2 voltage is held solid?
But the screen grid supplies traditionally used with pentode amps often tend to be saggier anyway (eg higher resistance chokes), and their higher ratio of screen grid-to-plate current may exacerbate the resulting sag.

Back on topic, I think EVH is noted for liking a big bass tone, with the 5150 popularising the resonance control; the amps he's used tend to be 6L6 types.

[The New Steve H]

I posted a while back about my friend who wants an amp. He is trying to decide between a Bassman clone and the 4xEL84 amp I designed (Bassman front end running through EL84's).

I am thinking the rumbly nature of the 5F6A might get on his nerves eventually.

[pdf64]

I hear that a fellow called Jim something had some success in running a 5F6A with EL34? After some initial trials with KT66.

[Malcolm Irving]

...
I am thinking the rumbly nature of the 5F6A might get on his nerves eventually.

Just my opinion, and a bit off topic, but I think that back in the good old days when we thought that 100W bass amp must be a good match for a 100W guitar amp, some thump from the guitar amps was probably a great thing. Now we have 300W, 500W or even 1000W bass amps, it is better (IMHO) for the guitar to 'get out of the way' and leave any 'thumping' to the bass player.

[NickC]

I hear that a fellow called Jim something had some success in running a 5F6A with EL34? After some initial trials with KT66.

Indeed, his JTM45 version with KT66s is a thing of glory.

[The New Steve H]

I made a guy a JTM45 clone with KT66's. It was very nice.

[teemuk]

Tubes can operate at any low frequency you like, all the way down to DC.

This. Tubes have no inherent "frequency response". Any of those tubes, as is, can amplify from DC to radio frequencies.

It's other parts of the circuit that define frequency response.

The plate curves for EL84 seem to have a more rounded 'knee' than 6L6 - perhaps that could account for 'earlier' distortion?

Certainly.

EL84 is a true pentode. 6L6 is a beam power tetrode a.k.a. "kinkless tetrode". While people usually categorize both as "pentodes" the two are inherently different: Aligned grids of the beam tetrode:

- reduce screen current draw
- reduce 3rd order harmonic distortion, which true pentodes create significant amounts
- allow more range of linear amplification at given operating conditions

The latter is due to "harder" knee characteristics. Basically beam tetrodes can provide more headroom than true pentodes. The side effect is that clipping becomes harder and the onset of it becomes more abrupt.

[img:486:400]http://www.r-type.org/static/grid14b.jpg[/img]
Note how the A-B loadline of a beam tetrode can extend farther to left before it crosses the zero grid volts curve.

So yes, pentode would inherently clip "softer" than a beam tetrode, but the side effect of increased harmonic distortion at non-linear range of amplification is that intermodulation distortion also increases, which means you eventually loose "note definition". Amplifying low frequencies probably exaggerates the issue.

The lower screen current draw will also introduce effects if screen supply voltage modulation is allowed by the design. Usually it is, at least in moderate amounts.

Because a beam tetrode draws less screen current it develops lower voltage potential at the screen terminal than a pentode when both use equivalent screen supply circuit. Because screen voltage modulates less the gain compression effect due to screen voltage variation is also lower. When you drive output tubes to grid conduction, skyrocketing screen current draw, I think the beam tetrode will also draw less screen current than a true pentode in equivalent condition.

These will all affect overdrive characteristics and harmonic distortion created by overdrive. A beam tetrode should amplify "cleaner" than a pentode but clip harder, creating more high order harmonic distortion, which makes the sound "brighter". A pentode will clip softer but the soft clipping is "muddier" than hard clipping, especially due to intermodulation distortion it creates. Screen current draw effects in a pentode are also more "violent".

All these are audible effects and make the amplifier's tone different but none has anything to do with frequency response. As said, as is, those tube are capable to amplify from DC to radio frequencies. If frequency responses are different it's because the entire circuits are different, not because one tube could "extend lower" in amplification than another.

Now we have 300W, 500W or even 1000W bass amp

Higher power = more headroom, more linear range of amplification

If the amplification is non-linear it means harmonic distortion. Side effect of harmonic distortion when amplifying anything but a single frequency is intermodulation distortion, which in greater magnitudes "muddies" the signal. The problem is exaggerated if low frequencies need to be amplified.

A bass amp should amplify at moderately wide bandwidth (40 Hz - 15 kHz). It needs to amplify low frequencies because it is a bass amp, and it needs to amplify high frequencies because these make the instrument sound more "natural" and are also important for "slap" and "pop" -type sounds, which consist of higher order harmonics. A bass amp should also do this moderately cleanly because low frequency clipping generally doesn't sound too different from "farting" and because harmonic distortion increases IMD, which makes the effect even worse. So, high headroom is beneficial, and the high output power is about the only way to achieve that.

Guitar amps are entirely different animals and typical applications are based in ovedriving them more extensively. But a high gain channel of a guitar amp may have a bandwidth that ranges only from about 1 kHz to 5 KhZ. Low frequency harmonic distortion, and IMD, is substantially reduced because of that. The limited high frequency range of typical guitar speakers introduces a steep low-pass at about 5 Khz, which nicely removes all "fizzy" high order harmonics generated by overdrive and clipping. Because they are operated so differently than bass amps headroom requirement is much less. If you can cope with ovedriven tones even a 10W guitar amp cranked is probably loud enough to hang with a drummer and despite high amounts of distortion sounds "traditional".

Try that with bass guitar.

[Structo]

IMHO, I don't like the EL84 in higher power amps.

I have an amp that has 4x EL84's and although it sounds nice, I prefer the 6L6 or EL34.

Bigger, bassier and more punch.

I do like some 18 watt derivatives, they can sound great.

But, I guess I favor the Octals.

[RJ Guitars]

There is an inner circle that refers to the TrainWreck Rocket (4 x EL84 output section) as a "Fat Bottom Girl" for the reason that these amps tend to put out a ton of low end.

I recently started using one of my 4 x EL84 amps based on the Dr. Z "Z-Wreck" as a bass amp (I call it the "Tejas Twister"). It really puts out the goods for tone including lots of bottom end. Not a monster bass amp but a very good sounding small venue bass amp with plenty of headroom and more bass response than you might expect: http://diyguitaramps.prophpbb.com/topic18.html

I was surprised by this result but in a very pleasant way...YMMV