How Doubling Guitar Amp Power Output Affects Speaker Performance?

[K Teacher]

Hi Helmholtz,

The original question was "How Doubling Guitar Amp Power Output Affects Speaker Performance?"...
I was trying to understand the benefit(s) (reason) of doubling the output power (2x 6V6 parallel vs 1x 6V6) and supposed there was some benefit other than loudness.
According to theory, an increased of 3dB in loudness is barely perceived by the average listener...

However this has nothing to do with increasing speaker performance... the Jensen F10-U needs around 80mA to work. Like you pointed out, the typical plate current in SE class A is only around 50mA... the easiest/ more economical way to increase the total current to 80mA is to add another 6V6 wired in parallel to the first one...

[Helmholtz]

Curiosity Satisfied…

First, I want to thank you all for your insights…

I guess I ended-up answering my own question, after thinking for a couple of days on what Enzo posted about power & speaker efficiency.

I think I approached the 2x 6V6's - parallel, single-ended design inquiry from the wrong point of view…
it is not about the amp… is all about the speaker.

The speaker field coil requires some minimum power to produce enough magnetic flux, for the speaker to be able to work. The Gibson amps BR-9 & early GA-9 models, use the Jensen F10-U field-coil speaker, which is what was available at the time these amps were manufactured.

Based on various articles from the web, I estimate that the F10-U speaker field-coil requires about 5.5W to work, or approx. 68V @ 81mA (= 850 ohms not 1000 ohms indicated in the schematic).

An amp with (1-6SJ7 and 1-6V6) OR (1-6SN7 and 1-6V6) will idle at approx. 46~48 mA, not enough for the speaker to work properly. If we used a 6L6 power tube instead, then the B+ would be way too high for the available 450V electrolytic filter caps… 2x 6V6's fit the bill perfectly… lower operational voltage, 80 mA idle current.

I would appreciate if anybody has any specs or info on the F10-U to confirm (or not) my estimates. Thanks.

Sorry, I absolutely don't understand what all this has to do with your original question.

- I thought you were going to use a PM speaker, so why worry about the field coil speaker and its energizing current?
- It is true that the efficiency of a field coil speaker increases with the field current. The generation of the magnetic field doesn't require power though, rather there are power losses caused by the DCR of the coil wire. All this power gets dissipated as heat. I assume there is an optimum field current for the speaker.
- There is no reason why a 6L6 would increase the B+ or need higher B+. Typical plate current in SE class A is around 50mA.
- Nothing wrong with using 2x6V6 in parallel if the PT can supply enough current.

[K Teacher]

Curiosity Satisfied…

First, I want to thank you all for your insights…

I guess I ended-up answering my own question, after thinking for a couple of days on what Enzo posted about power & speaker efficiency.

I think I approached the 2x 6V6's - parallel, single-ended design inquiry from the wrong point of view…
it is not about the amp… is all about the speaker.

The speaker field coil requires some minimum power to produce enough magnetic flux, for the speaker to be able to work. The Gibson amps BR-9 & early GA-9 models, use the Jensen F10-U field-coil speaker, which is what was available at the time these amps were manufactured.

Based on various articles from the web, I estimate that the F10-U speaker field-coil requires about 5.5W to work, or approx. 68V @ 81mA (= 850 ohms not 1000 ohms indicated in the schematic).

An amp with (1-6SJ7 and 1-6V6) OR (1-6SN7 and 1-6V6) will idle at approx. 46~48 mA, not enough for the speaker to work properly. If we used a 6L6 power tube instead, then the B+ would be way too high for the available 450V electrolytic filter caps… 2x 6V6's fit the bill perfectly… lower operational voltage, 80 mA idle current.

I would appreciate if anybody has any specs or info on the F10-U to confirm (or not) my estimates. Thanks.

[pdf64]

Y...I've been in bands almost constantly since the blackface Fender days. I have never, ever, EVER run across anyone who wished his amp had less power.

A JMP Super Lead was too much for me. I certainly wished that the other guitarist didn't also have one, or at least didn't insist on 'natural' valve overdrive, ie ear bleeding volume, for his solos

[K Teacher]

Thank you all for your answers

@Enzo,
Good point... didn't think of that... thanks

@nhbassguitar,
I do not have the amps... I am deciding on a new build, GA-5 vs GA-8 with either Eminence LEGEND-1028K or GA10-SC64.

What would be the advantage to build the GA-8 if the perceived loudness is about the same?

[nhbassguitar]

First, I'd like to hear from the OP why this topology appeals in the first place. What particular amp did the OP hear and like, or did I miss that? What driver was it using? How hard was it being driven? What mic and processing were being applied? These are all important details. But is any of that even knowable?

Second, much of an amp's charm can come from its circuit design alone. Overdrive is another matter entirely. So I think it's important to know what goal we're after. Are we after the charm of a topology, or are we after what happens when overdrive occurs? And, a topology's inherent nonlinearities are sometimes mistaken for overdrive-induced distortion. So it's important to be able to discern one from the other. You have to know what you're hearing, and to be able to identify the exact cause(s).

Third, whether in a studio or live, what sounded so awesome in one's bedroom (running a design at or beyond its clean limit) may not work at all in a mix. It'll often turn to sludge and essentially disappear, i.e. wash out, so it sounds like someone’s running a table saw in the background. Or the Ramones. A related point is that if you give a producer a (relatively) clean signal, that gives the producer something to work with. It gives the producer some flexibility to re-amp, add effects, and so on. But you give the producer something that’s already overdriven, and that will be a constraint during mixdown. You’ve essentially chained the producer’s wrists.

[Enzo]

An important consideration is the speaker. A speaker that is 3db less efficient would require twice the power to be as loud as the more efficient speaker. Someone might want more power to effectively drive an inefficient speaker they like.

[K Teacher]

Maybe that is the reason why amps with parallel wired 6V6's fell out of fashion… even Dave Hunter's TWO-STROKE revised version dropped the 2x6V6 parallel for a single 6L6…

[K Teacher]

Thank you all for the answers… but I am still a little confused…

These are from Google:

- Can you hear 1dB difference?
“You should be able to hear 1 dB increase or decrease of a steady single tone. With music, such a difference may or may not be that noticeable, it really depends on the kind of music and the volume level one is starting from. - Jun 19, 2014”

- Can you hear 3dB difference?
“A change of 3 dB is accepted as the smallest difference in level that is easily heard by most listeners listening to speech or music. It is a slight increase or decrease in volume. - Aug 4, 2019”

This was the explanation that was given to me regarding an audio amplifier frequency response – is when the output drops 3dB in relation to a reference point at 1KHz (or for some old texts at 400Hz), because beyond 3dB the listener can perceive a volume decrease in output.

So, it must be some advantage in doubling output power other than loudness…
Headroom increase perhaps?

[The Dude]

......if the pre-amps are the same it would just be a tube socket and a few resistors to go from one to the other......

...... and a beefier output transformer with lower input impedance. And, possibly a larger PT to handle the extra current.

[g1]

Lots of guys want to crank their amps for power tube distortion without being too loud. In a small room or for re-amping.
Those are the guys who wish their amps had less power. I've sure run into them. Not everyone is playing in clubs or situations that require big power.
How many guys even with a Twin that wish they had a Deluxe, to hit that 'sweet spot' at a lower loudness.

[Bloomfield]

I'm not sure if you are planning on building or buying the amp(s.) If you are building it would be easy to try both circuits in the same chassis; if the pre-amps are the same it would just be a tube socket and a few resistors to go from one to the other. If you are buying of course it would be easier to modify the 9 W down to 4.5 W if you decided you wanted less power.

[Enzo]

No one ever wishes they had less power, on the other hand MANY people confuse output power with loudness and even gain.

It is just ONE factor among many. Really the thing to do is play through both and decide which you prefer. A couple decibels is detectable, just detectable. It isn't like turning the knob from 4 to 8.

[nhbassguitar]

You say you want to exclude loudness from the calculus. Will you regret that at some point in the future? There are two types of "loud." Clean (dynamic) loud, and overdriven (highly compressed) loud. It would be a shame not to have the former available when you need it.

Cone control is typically not a concern in guitar work. The driver suspensions are too stiff. And at these power levels you're talking about, the cone suspension will hardly be moving. The driver is basically going to be sitting there wondering if it's ever going to be asked to do any real work. Also, only a tiny portion of each guitar string's output consists of the fundamental tone. The vast majority is harmonics. This will especially be the case in an amp that's using a single-ended output stage.

Finally, I've been in bands almost constantly since the blackface Fender days. I have never, ever, EVER run across anyone who wished his amp had less power.

[Helmholtz]

Now, if I understood correctly, doubling the output power, only produces +3dB increase, which is barely perceived by the human hearing.

A sound level difference of 3dB does make a difference (even 1dB is perceivable) especially when playing with other musicians. Doubling output power also increases dynamic headroom.

Regarding speaker resonance damping, amp output power doesn't matter. Rather it's the amp's output impedance that damps the resonance. Most probably both amps have similarly high output impedance in the 100 Ohm range, so little damping. Better speaker damping would require global NFB - at the cost of lower gain.