SVT-CL or SVT-VR output power NOT anywhere close to 300W

[g1]

I am wondering if it may be some other issue than the power output that they are complaining about, like a problem in the eq or transient response, and maybe it's coincidence that power output is low.
The drop in power does not seem all that huge from a SPL standpoint. Although if the customer had just received it back from repair, they may have been sensitive to any drop in power.

Just thought I would toss that in as a suggestion. It may be worth double checking with the customer whether the volume drop was very noticeable, or just sleight. Of course, depending on the customer's trouble shooting skills, cabinet or cable issues could still be in play.
For me, if someone says 'low output' I am looking for a very obvious drop in volume.

[nevetslab]

This is a newly purchased Ampeg SVT-CL Heritage amp from a client AuntieM here in the Los Angeles area. Steady client, smaller than CenterStaging with regards to sourcing backline gear. They have 14 SVT-CL's besides this new Heritage version, and at least 3 SVT-VR's in their rental inventory. When I first put it up on the bench, I noticed it's LF Boost switch wasn't sounding like that of other SVT-CL, plus it was somewhat intermittent. Dealt with the switch contacts with seeping Caig into it, & exercising, now no more intermittency but the LF notch didn't sound normal. axial ceramics, and didn't bother measuring them, though all the resistors in that network were nominal. I'm guessing someone plugged their bass into the amp driving an 810 cab, and thinking it sounded on the weak side, grabbed another SVT-CL from inventory to compare against, and found the other sounded like it had more poop, and sent it over for solving the low output state. they did just drop off another SVT-CL that was now going into protect, so when I get that fault corrected, I'll have another of their SVT-CL's for side-by-side comparison. That one that just came back also has 6550's instead of the KT-88's that I normally change to. Nothing like having sufficient inventory to assist in identifying odd ones, let alone one Brand New!

Onward...

[Helmholtz]

Under Burst Pink, I see AC Wattage consumption peaking around 300-340W where onset of clipping begins.

As power efficiency of a tube class AB amp typically isn't much better than 50%, a mains power consumption of 340W would mean an output around 170W.
Efficiency of big amps amps might be somewhat better.
The theoretical max. efficiency of a pure class B amp is 78%.
But that value doesn't include PT and OT losses, screen dissipation, heater power and preamp consumption which can easily lower total efficiency by 20%.
What am I missing?

I don't think there's a substitute for directly measuring continuous sine power.

[nevetslab]

As power efficiency of a tube class AB amp typically isn't much better than 50%, a mains power consumption of 340W would mean an output around 170W.
Efficiency of big amps amps might be somewhat better.
The theoretical max. efficiency of a pure class B amp is 78%.
But that value doesn't include PT and OT losses, screen dissipation and heater power, which can easily lower total efficiency by 20%.
What am I missing?

I don't think there's a substitute for directly measuring continuous sine power.

I agree. Direct measurement of continuous sinewave Power into dummy load and measurement of the AC Mains wattage/current consumption during that, held at steady 120VAC mains is more meaningful. The readings noted during burst pink is not a steady reading, nor is the envelope of the burst waveform, as the nature of pink noise is such. Mostly an additional point of merit. I'll see as much as a 2-3dB envelope around a nominal SPL reading (RMS SLOW) on my B & K 2130 SPL meter (or 2607) when running at max output in the room, 20 ft from the speaker at the end of the room from my desk where my meter is.

[nevetslab]

I ran Voltage RMS, Power Output, Plate Voltage, Screen Voltage, AC Amps, AC Watts & AC Volts vs Frequency at both 4 ohms resistive and 2 ohms resistive on this SVT-CL-Heritage amp (new amp), placed the data into Excel spread sheet, then printed out a pdf of that (no cells shown, just columns of numbers vs Freq. Highest output was found at 10kHz, and lowest output at 40Hz, though the variation is very small (236W @ 40Hz vs 282W @ 10kHz @ 4 ohms & 231W @ 40hz vs 267W @ 10kHz @ 2 ohms.

Ampeg SVT-CL-Heritage Power vs Freq Data_82224.pdf

Very much in line with what J M Fahey post on #12 earlier.

[Helmholtz]

So full power consumption is around 610W (assuming you measured real power and not V*I apparent power) and total amp efficiency is a mere 40%.
The power tubes might have somewhat high saturation voltage, lowering efficiency and output.
The "saturation" voltage is the Vak drop at max. signal plate current and is the only tube parameter that affects amp output.

[nevetslab]

So full power consumption is around 610W (assuming you measured real power and not V*I apparent power).

The bench gear I used on the data recorded used the Magtrol 4614B 100A 3-Phase True RMS Power Analyzer, sourced from a Superior Electric Powerstat Model 146 0-140V/30A, and sourced thru a Topaz 5KVA Isolation Transformer, fed from 240VAC source, strapped for 120VAC Out. Regulation of course still subject to AC Mains in the building shared by three large Rehearsal Studios. I have a Weston Model 341 AC Line Monitor (large Analog Meter, traditional Mahogany box) at the head of the AC Mains distribution feed to all the quad boxes feeding the benches along one wall. My coffee pot is ahead of that, and when it cycles, I can see minor drop in Mains voltage from the rooms' breaker panel. But essentially very stable power.

My checkout bench uses a smaller General Radio W20 0-140V/20A Variac, feeding a Valhalla 2101 Power Analyzer (20A max).



On the first photo, just to the right of my wood Machinist Tool Chest you can just barely see the Weston 341 AC Mains Line Monitor. In the foreground next to my load bank is the large Magtrol 4614B Power Analyzer and on top of it the Superior Electric Powerstat Model 146 0-140V/30A, and on the bench above it is the Topaz 5KVA Isolation Transformer.

In the second photo, to the left of the service cradle holding the Marshall JCM900 chassis, the Valhalla 2101 Power Analyzer can be seen (used for the Check-Out bench to the left of this early phase of the test bench, and against the wall to its' left is the General Radio W20 0-140V/20A Variac

MAGTROL 4614B Instruction & Reference.pdf

Valhalla_2100_2101_Manual.pdf

Both Power Analyzer Operators/Service Manuals attached, both instruments have True RMS Converters within their circuits. Top Quality gear. Industrial grade gear meant to last decades properly treated. Power Cables for the Magtrol is 10ga/3-wire, as is the in/out cable to/from the GR W20 Variac, which feeds back of the Valhalla Power Analyzer. Output of that passes thru a separate Circuit Breaker Box that I normally use on backline gear, unless I need the higher current. It has a 2A and 5A magnetic circuit breaker inside. That will be updated to have a switch to change it to a 20A Magnetic Ckt Brkr when I pick up the 20A SPDT toggle switch. Output is via strain-reliefed Hubble Nema 5-15F AC Mains female connector.

[Helmholtz]

The bench gear I used on the data recorded used ...

Why list all your fancy stuff?
Doesn't mean a thing to someone not familiar with it.

Simple question is if you recorded real (mains) power or apparent power.
Real power is apparent power times power factor.

I use a cheap 20€ "Energy monitor" (I think that's called a Kill-A-Watt in the US).
This gives me apparent power (VA), real power (W), power factor, mains current, mains voltage and more.

[nevetslab]

I provided all that as I got the impression I was being challenged as to how I made the measurements. Isn't Apparent power (VA) derived by multiplying the Mains Current times the Mains Voltage? And one then has to wonder how AC Current and AC Voltage is being measured. I've always been using Power Analyzers with True RMS converters in them, so I provided the doc's to verify that for anyone interested. I can measure Power Factor on the Magtrol as well (setting for it), though never bothered with it for my documentation

So taking one of the measurements from the 4 ohm test chart at 1kHz...6.32A x 120V = 758.4 VA, and Power Factor would be RMS Power / VA or 623W/758.4 VA = 0.82?.

[Helmholtz]

So taking one of the measurements from the 4 ohm test chart at 1kHz...6.32A x 120V = 758.4 VA, and Power Factor would be RMS Power / VA or 623W/758.4 VA = 0.82?.

Yes, apparent power is Vrms*Irms and real power is Vrms*Irms*PF, so real power divided by apparent power gives the power factor (PF), which typically is lower than 1.
Apparent power is real power+reactive power.
For efficiency calculation we need real power, because reactive power isn't consumed power.
I think that's what you recorded. So everything is fine and efficiency Pout/Pin is 40%.

(The term RMS power doesn't make sense.)

[g1]

(The term RMS power doesn't make sense.)

I think this has come up a few times before in discussions. Though not technically correct, it is used to describe power calculated from RMS voltage and current measurements. Also helps differentiate from terms such as 'peak' power or other references.
So I guess when people say RMS power or 'watts RMS' it is actually apparent power that is the true terminology.

[Helmholtz]

I think this has come up a few times before in discussions. Though not technically correct, it is used to describe power calculated from RMS voltage and current measurements. Also helps differentiate from terms such as 'peak' power or other references.
So I guess when people say RMS power or 'watts RMS' it is actually apparent power that is the true terminology.

I think what people mean with "RMS power" is (time averaged) real power or continuous sine power into a resistive load.
Only in the special case of current and voltage being in phase and having the same waveshape (as with a resistor) this would mean Irms*Vrms, because in this case reactive power is zero and apparent power equals real power.

[g1]

I think what people mean with "RMS power" is (time averaged) real power or continuous sine power into a resistive load.

Yes, I agree. I mentioned RMS current, but I think most of us measure RMS volts and calculate using load resistance. (E squared over R).
I think it became widespread when the industry tried to implement a standard for comparing real power in watts for amplifiers from various manufacturers. It may have even been trade publications that started the use of the term 'RMS power' or 'watts RMS'.

For a time it was widespread, then the manufacturers decided that it may not be a good indicator of real live use, especially for things like speakers, and terms like 'program power' started being used. So it's still very difficult to find a standard for comparing specs between manufacturers.

[Tom Phillips]

Lower tier manufacturers really went crazy with their published power specs in the late 1960's & 70s. Unaware consumers were really puzzled when a good "40W" rated amp would outperform their xxxxW amp that was rated in some made up term such as "Peak to Peak Music Power" and other such terms.

[Helmholtz]

I think "RMS power" is just audio jargon used for power amp output.
It is a technically and mathematically wrong term as multiplying two rms quantities like current and voltage does not give another rms quantity (power) in the true sense of root-mean-square.