Quick speaker question

[martin manning]

Po = Zo'*(Vg*gm)^2

After considering for a day or so, I'm not liking this calculation.

Gm varies from one tube sample to the next and with operating condition, but more importantly it is defined at constant plate voltage which does not correspond with the way the device is being operated.

A much better estimate can be made by using the voltage and current swings from the idle point to Vg-k = 0 on the plate curves, and then calculating output power as (change in V)*(change in I)/2. For example, the Phillips EL-84 data sheet gives 17W for a pair in p-p operation at 300V Va into an 8k load. By this method I get 16.6W, as opposed to 13.8W using the data given at zero signal and the Zo'*(Vg*Gm)^2 method.

Re speaker rating vs. amp power, I think it's pretty common to use a speaker that is rated at twice the expected clean power output to allow for overdriven operation. As an example, I have a Marshall Studio 15 with it's original Vintage 30 speaker (23 years old and still rockin', and in fact the V30 was originally designed for this particular amp). It doesn't sound bad through a cab with 4x that rating, though. If I plug it into a pair of G12-65's, it sounds remarkably similar tone-wise to the V30, just "bigger."

[tubeswell]

Hi Martin

I did say that I was only guessing about the maximum unclipped grid input voltage, and thinking about it, it is likely to be more than 7VAC (RMS), so the equation probably does give in the order of 16V with a more realistic set of variables.

This equation was given to me by Old Tele Man over at ampage some time ago, so I reckon the source is fairly reputable. However I am not beyond being able to be persuaded to another point of view as usual.

Pete

[martin manning]

Pete, you're using it exactly as it was described. I actually dredged up that old thread over there. If you apply it using the actual AC conditions (and the cathode voltage does indeed come up) it gives 25.5W, a result that is too high by 50%. The basic problem is with missusing Gm.

Cheers,

MPM

[martin manning]

Pete, here's the missing piece. Reading more carefully I see you missed out the rp/(rp+Zo) factor above... that makes more sense, but I still can't make it work for the operating poiint on the data sheet.

...if you know some of the signal voltages in your amp, try this:

Po = Zo'*(Vg*gm)^2

where:
Po = Power output, Watts(avg)
Zo' = Effective reflected load impedance, Ohms
Vg = PI drive signal to each power tube, V(rms)
gm = Power tube avg transconductance, Ap-per-Vg

Zo' = Zo*loading factor; Zo = Zpp/4
...the "effective" load impedance (Zo') is the combined effects of OT reflected impedance (Zoo/4) and the combinational loading of the tube(s) itself (also called 'load factor'):

Zo = Zoo/4 <---purely an OT and speaker function

% = (rp/(rp+Zo))^2 <---'load factor', a tube & OT function

Zo' = Zo * %

...thus the "full" original equation is:

Po = (gm*Vg)^2 * (rp/(rp+Zo))^2 * Zo

...which simplifies to:

Po = Zo' * (gm*Vg)^2

...by letting Zo' = Zo * (rp/(rp+Zo))^2

...and, remember that the Vg signal is assumed to be an RMS-value, not a peak value!

[tubeswell]

Gotcha, thanks Martin. Duh - how could I have missed out the load factor? - must've been in a hurry. I'll work it out tonight, I've got to get to work now. Cheers

Pete

[azatplayer]

I think the overspecing speaker rating is all bollocks.
I had a Dr Z maz 18 that shipped from Z with a 15 watt alnico blue.
That amp copped a flogging and they run 350-60 V plates so at the upper end of an 2 x EL84 amp for output, and that speaker has never faulted. I sold that amp after about 3 years, and have used that speaker continuously in other builds.
I have a marshall quad that i got a couple years ago with pre rola greenbacks in it. The guy i bought it from was in a well known cover ban and ran a JCM800 100 watter thru it, gigging 2-3 times a week.
He bought it from Jack Jones, local guitar hero, who had gigged it for years under 100 watt Marshalls.
I pulled those speakers and use them in a couple 2x12's, bloody magnificent tone from original cones.
My celestion gold i bought new a few years ago, has been under heaps of 50 watters, and its the best sounding gold ive ever heard.
That Xits ships with either a greenback or an alnico blue, Surfsup. So im pretty sure that amp aint gonna kill either speaker.

[surfsup]

azat, I ordered a scumback m75 based on scumback's reccomend (I think its 30W?). Should be at the office monday. But I leave for france for a couple weeks so won't be getting to it for a bit. The Xits x10 has the celestion g2 25W.