amp power vs.speaker wattage?

[Bugman3183]

Hello all.  I'm in the process of deciding on my next build.  I'm interested in buildings a 5e9a tremolux using a pair of nos ts 5881's I have, and the 5e3 ot Mr Hoffman offers that I believe is rated for 25 watts.  The dilemma I'm having is that I have a pair of vox silver bells that I would like to use in this combo.  One of them has the h1777 cone and the other has the 003 cone.  I've been reading a lot about them and there seems to be some confusion as to how many watts they are rated for 15/20 watts or perhaps both, due to the different cones.  I'm just wondering if it'll be a bad idea to use these speakers with this amp or am I just being paranoid.  Thanks.

[HotBluePlates]

If you want to be certain the speakers will never blow, you want total speaker power to be at least 2x the amp's clean output power. You'll also want to put the speakers in parallel.

... I'm interested in buildings a 5e9a tremolux using a pair of nos ts 5881's I have ...

The Tremolux used a pair of 6V6's, and would likely be safe with the pair of speakers you mentioned.

If I used a pair of 5881's or 6L6's, I might assume 35-40w of clean output power, so I would assume I'd be at risk of blowing those speakers.


Using the Hoffman 5E3 OT wouldn't make much sense with 5881's, as it would limit the output power somewhat (but we can't know what that powe would be without more figuring). Also, using 5881's or 6L6's would be a waste of tubes unless you used a bigger PT than the 5E3/5E9, as you won't get more power output without additional current draw from the high voltage supply.

Not to change the subject, but I can show how you can get "clean output, higher on the volume knob" by using 6L6's in a 6V6 amp without any substantial increase of power output. If you're gonna spend the money on 5881's/6L6's, you need to also choose a bigger PT, different OT with a lower primary impedance, and obviously speakers that can handle the extra output power.

[Bugman3183]

Thanks for the reply hbp.  I have a suitable pt for the 5881's and they've just been sitting around collecting dust so I figured it would be a way of getting a slight boost in headroom while using the 25 watt ot without dramatically increasing power as I'm not looking to get 35/40 watts from the amp.  Although I totally overlooked the impedance mismatch.  I have a suitable ot for these as well rated for 40 watts.  Perhaps I'll use that and just slap some different speakers in there.  40 watts just seem like it would put me on the other side of the equation possibly having too much headroom.  I guess there's only one way to find out.  in addition to extra headroom I was also just looking for a different flavor as I have already built a 5e3 with 6v6's (which I love btw).  I guess a 40 watter it is then.  Thanks.

[Quatro]

Not to change the subject, but I can show how you can get "clean output, higher on the volume knob" by using 6L6's in a 6V6 amp without any substantial increase of power output. If you're gonna spend the money on 5881's/6L6's, you need to also choose a bigger PT, different OT with a lower primary impedance, and obviously speakers that can handle the extra output power.

i'd be interested in hearing more about this if you have the time.

[SoundmasterG]

People used to use Celestion Blues and Silvers in Twin Reverbs without problems. I used a pair in an amp I built with 54 watts...no problems after a year of hard use. Vox AC100's ran though four of these speakers, and  Vox AC50's ran through two of them.....they are stout speakers!

If you are talking about Weber silver bells...that might be an issue because Weber rates their speakers for the power that they really are.

Greg

P.S. I too would like to know what you're talking about HBP. Always up for new tricks to use!

[Bugman3183]

I forgot to mention in my last post that I too would be interested in hearing more about that trick.  I'm sure it'll come in handy.  The speakers I have are vintage silver bells with t1088 code on frames not the Weber version.  Also, I looked in the parts catalog at the 25 watt deluxe ot I have and it only mentioned the secondary impedance and not the primary.  My thinking being that if the primary is 6k6 opposed to 8k it may be possible to use with the 5881's.  Any info on how to find out the primary impedance would be greatly appreciated.  Thanks.

[HotBluePlates]

... Vox AC100's ran though four of these speakers, and  Vox AC50's ran through two of them.....they are stout speakers! ...

For the price of originals, I'd be cautious.

Let's say these are truly 20-25w speakers. As long as you stay clean, you're fine. Once you get a 50w RMS (clean) amp cranked up to distort enough to produce a square-wave, the output power will be 100w RMS.

Age, heat, humidity, previous rough use could all weaken voice coil formers and risk a blown speaker, even when apparently being used within its ratings.

I used to have an early-70's Marshall 4x12 with the "100w" badge. Inside were 4x G12H30 speakers, for 120w of total rating. No way would I have used it alone with a 100w head, though I used it a lot with the 50w head I had at the time. When it was new, a blown speaker wouldn't be that big a deal to have replaced, but prices aren't what they used to be.

A problem you will have is you don't know if the original rating is conservative or not. From the man who designed the JBL D130F for guitar use:
"If you look at the original spec sheet for the speakers in that series you’ll see that the D-130 is rated at 25 watts. ... If you put it in an undersized and over damped box like a Benson then the speaker will take more power, but it will need it to get the volume.

On the other hand, I saw a lot of D130′s come through with fried voice coils that were running off a 12 Watt Williamson amp during the 50′s and
60′s. Integrated music from HiFi systems caused one kind of problem – using the D130 as a musical instrument speaker created other problems.

That’s why I suggested the D130F (which was a redesigned D130), made expressly for musical instrument amps ... Power specifications for the F series were nominally 35 to about 60 Watts. How did I arrive at these figures? Pretty simple, I played guitar and bass through them and kept increasing the power till they blew. Then I downrated them from the power that fried them. Pretty hi-tech, huh? It seemed to work pretty well (of course we didn’t have synth players back then)."

Harvey said there wasn't much change in the speaker itself for the uprating in the power handling number, as the speakers had surround changes to withstand outdoor use better. Notice also the bit that they could be blown by an amp having advertised power less than the original conservative speaker rating.

So, now that you're armed with all the considerations, it's up to you to choose what amount of risk you're willing to tolerate.

Oh yeah, I'll be back later with the amp power explanation, as that will take some write-up.

[HotBluePlates]

Sorry for the delay.

i'd be interested in hearing more about this if you have the time.

... I too would like to know what you're talking about HBP. Always up for new tricks to use!

I forgot to mention in my last post that I too would be interested in hearing more about that trick.  I'm sure it'll come in handy.  ...

Maybe I phrased my statement poorly the first time around. There is no trick.

People tend to think 6L6's output more power because commercial 6L6 amps are more powerful than commercial 6V6 amps. So some swap 6L6's into 6V6 amps thinking they'll get more power (they won't). Yet we all hear people describe how their amps gained more headroom, and evidenced by a higher volume knob setting at the onset of distortion.

What I should have said the first time was, "I can provide a technical explanation for why others get "clean output, higher on the volume knob" by using 6L6's in a 6V6 amp, yet they are fooling themselves because there was no substantial increase of power output."

So first off, I'm assuming we're building a 5E3 or 5E9 type circuit here. The original 5E3 schematic and layout don't provide voltages, so let's work from the 5C3 layout. This shows that the cathode bias voltage is 18v (across a 250Ω resistor), with a plate voltage of ~360v and a screen voltage of 308v. Because the tube sees the difference between plate-cathode and screen-cathode voltage, the tube feels as though it has 342v at its plate and 290v at its screen.

Look at the triode curves below for the 6V6 and 6L6. Each has a green line which represents a 500Ω cathode resistor. We're gonna assume you're building a tweed Deluxe or Tremolux, and would use a shared 250Ω resistor for the output tubes, which equates to a 500Ω resistor per tube.

Time for a trick: the 6V6 data sheet doesn't have plate curves for a screen voltage higher than 250v, while the 6L6GC data sheet jumps from a screen voltage of 250v to 400v for its curves (leaving our 290v G2 condition missing on each). Because the triode curves on the data sheet assume the plate is tied to the screen, and because in pentode mode the screen voltage (along with G1 bias voltage) is what control plate current, we can determine bias at our actual screen voltage by using the triode curves.

On the 6V6 triode curves, a vertical line is placed at 290v (our schematic screen voltage minus cathode voltage). This crosses the green 500Ω line at a voltage between -15v and -20v, but closer to -20v (I might guess -19v). The plate current at this point is something less than 40mA (I might guess it to be 38-39mA).

The 5C3 layout shows 18v across 250Ω, or 18v/250Ω = 72mA/2 = 36mA per tube. So a volt and a few milliamps difference, but we see how the triode curves predicted what bias voltage we'd see when using a specific cathode resistor, and when we know what our screen voltage will be.

Now look at the 6L6 triode curves. The same 500Ω resistor is shown in green (cause we're taking the same amp and dropping 6L6's in the 6V6 sockets). ow the green line crosses the red line between -20 and -25v (we might estimate that as -23.5v). If you're sharp, you'll notice I moved the red line to just under 285v, as 308v - 23.5v = 284.5v. The crossing looks like it happens at ~47mA to me, and 47mA * 500v = 23.5v. So the math checks with the triode curves and lines drawn.

Key takeaway at this point: 6L6's in the 6V6 sockets resulted in a slightly larger bias voltage, given the same cathode resistor and power supply voltages.

[Bugman3183]

hbp, you are the man!  When I grow up, I wanna be just like you!

[HotBluePlates]

Now what about power output? How do we determine that?

In Radiotron Designer's Handbook (RDH4), it is shown that the power output of a push-pull pentode output stage can be calculated by by knowing the peak current maximum of one side of the output stage (which happens at the knee of the 0v grid curve), and the supply voltage and plate voltage where the loadline intersects the knee. See the graphic below.

The diagonal line AB is the loadline of the output stage, and (because this is a class AB stage) represents the OT primary plateto-plate impedance divided by 4.

Point B is at "Ebb" which is RDH4's shorthand for "supply voltage". Point A is at the knee of the "Ec1=0" curve, which means the 0v gridline from a set of plate curves, and which is the point reached on the loadline when the peak input signal driving the tube just equals the bias voltage (because 18v peak signal plus -18v bias equals 0v at that instant).

"Ibm" is RDH4's shorthand for "plate current, maximum", which happens at point A on the loadline. "Emin" is shorthand for "minimum plate voltage" which also happens at point A.

The equation for power when you know all these quantities is:
Po = 1/2 * Ibm * (Ebb - Emin)

Now if you stop and think about voltage, current and resistance, you may realize that Ohm's Law says they're all linked, all the time. In fact, the diagonal line AB represents the resistance of the circuit (in this case, 1/4 OT primary impedance), and its slope is the magnitude of the impedance. More-vertical equals less impedance (which should make sense, because for the same voltage change you get more current change, which is what Ohm's Law also tells you).

We don't have curves for the 6V6 and 6L6 at 290v on the screen, so what do we do?

Let's assume we again use our 5E3-type amp and the same OT exists regardless of whether we plugged in 6V6's or 6L6's. Let's also assume the powe supply voltage doesn't drop with the 22mA extra total plate current the 6L6's are pulling.

Given these assumptions, point B on our diagram doesn't change. Also, because the same OT (and same OT primary impedance) is being used, the slope of AB doesn't change. The only variable that can change output power, then, is if the 6L6's can pull their plate a little closer to 0v so Emin is a smaller quantity.

If you look at the bottom graph of page 4 of the 6V6 data sheet, and the top graph of page 4 of the 6L6 data sheet, you'll see a knee for G2=250v happens at ~35v for the 6V6 and ~50v for the 6L6. So it would seem the 6V6 is more likely to output more power with G2=290v.

Now that presumed the OT was the right load for the 6V6 to make maximum output power...

[HotBluePlates]

... I looked in the parts catalog at the 25 watt deluxe ot I have and it only mentioned the secondary impedance and not the primary. ...

Hoffman's Transformer Catalog page does say, "Part number 041318". At the top of that page is a link: "Transformer hookup diagrams, spec's, technical info and weights for the transformers on this page". Under the Fender style Transformer link, it says this part number has an 8500Ω primary.

So let's cross-check our info, working backwards:
8500Ω/4 = 2125Ω
Ebb = 290v (pretend plate equals screen-cathode voltage)
Emin = 40v  (let's take this average number for both tube types)

(290v-40v) / 2125Ω = ~118mA peak
1/2 * 118mA * 250v = 14.75w

Can the 6V6's make 118mA peak? Look back at those triode curves, and note where the red vertical line runs into the 0v gridline... It's just under 150mA, so 118mA is well within the capability of the 6V6 (it oughta be, since this is a 6V6 amp).

Can the 6L6's make 118mA peak? Look at the triode curves again, and the intersection with the 0v line is well off the chart and over 140mA. So the 6L6's can pass 118mA, and probably could pass much more than the 6V6's, but the load impedance is too great to allow the 6L6's to pass more current in this situation. By the time the 6L6 is passing 118mA, its plate voltage is being pulled down to 40v, and the drop in the gridlines below the knee shows that plate current rapidly drops.

All this said another way: The existing 8.5kΩ OT allows the 6V6 to make full clean output power in this amp; but the same OT limits the 6L6 plate current because a smaller load impedance is needed to allow the 6L6 to pass more current before it reaches a low plate voltage.

You should also see then that to get more output power from a given amp, it's not enough just to plug in bigger tubes... You also need an OT with a lower load impedance (to allow more current to pass), and that implies you need a bigger PT that can supply the extra current.

[HotBluePlates]

"Now wait... I thought you said you were gonna show how the 6L6's seem to have more headroom than 6V6's in the same amp!"

If you've stayed with me this long, you now know the power output is essentially unchanged when plugging 6L6's into a 6V6 amp. So why do people report "more headroom" when they do it?

Did you notice how I showed that 6L6's would have a slightly larger bias voltage than the 6V6's? 23.5v versus 18v??

If our max power happens when the grid is driven momentarily to 0v (as shown in the examples of the last couple posts), and the 6L6 and 6V6 make essentially the same power output because the OT primary impedance hasn't changed, then the bigger bias voltage of the 6L6 means it takes a bigger input signal to drive it to 0v!!

So you actually get the same power output, but you have to turn your volume knob higher to get to that output!

If one went by an electrical measurement of power at the speaker (or dummy load), or a SPL meter in the room, you might notice little change. But the fact you could turn the volume knob from 7 to 9 before distortion began can trick you into thinking you got more out of the amp.


Now I assumed earlier that supply voltages didn't change with the swap to 6L6's. They might change... in fact, they might be pulled lower. That would tend to reduce power output from the amp, even with the bigger tubes.

Or you might have tubes that idle a little cooler or hotter than the "ideal tube" shown in the curves. That could change how much difference (or not) you get when swapping 6L6's for 6V6's.

All that said, I know I've swapped 6K6's into a 5E3 amp before. It's a lower-power tube than a 6V6, so the average joe might think you'll get less power output. In fact, the amp does break up a little lower on the volume knob, but I'm not convinced that happens at a lower acoustic power... For reasons I've shown, I tend to think it happens because the 6K6's settled at a smaller bias voltage than the stock 6V6's.

When I grow up, I wanna be just like you!

When I grow up, I wanna be like PRR.  

[Bugman3183]

Extremely informative post!  I really appreciate you taking the time to break things down in such a detailed manner that guys with less experience like myself can easily absorb the knowledge being imparted upon us.  In other words,  you ARE the man!