Otange AD140 real output power?

[GainFreak]

Hi,

I have an AD140 on my bench retubed. All voltages check according to the schematic however it wouldn't produce more than 100 Watts of clean power (not that I was expecting 140 full clean Watts).
Is that normal for this amp? If yes why bother with those high voltages and iron?
If no what am I missing?

[J M Fahey]

That a "100W" rated one might have probably have only some 80/85 W before clipping.

Oh well.

[Chuck H]

Well let's look at the facts...

Max plate volts at 800. Max screen volts at 500. Max plate diss at 25W.

That orange AD140 put's 545V on the plates and 380V on the screens. I seriously doubt the tubes are conducting 35W each of continuous music power. That rating must be relative to something other than RMS. Which they shouldn't do but does allow them to put a big fat number on the amp face and in the spec sheet.

[teemuk]

Few of my observations....

- Whenever there's a number in the model name, as in AD140, the number doesn't neccessarily have anything to do with rated output power.

- Yes, Orange AD140 has quoted output power rating of 140 watts. But we don't know if that's 140 watts at 0.001%THD or 140 watts at 30 % THD. ...Or if that rating even refers to continuous average power instead of peak power or some other arbitary figure they can derive - like peak transient power. So when you have just the power rating without additional details about how the rating was derived the rating is practically worth nothing because it cannot be compared to any meaningful reference.

Is that normal for this amp? If yes why bother with those high voltages and iron?

IMO, the voltages aren't that high and the iron isn't that big for a tube amp with a moderate output power rating. That's just the overall nature of tube amps: They are big and heavy in respect to their output power and they operate at high voltages.

[GainFreak]

That a "100W" rated one might have probably have only some 80/85 W before clipping.

I'm aware of that marketing "trick" but still I was expecting more power from the amp.

IMO, the voltages aren't that high and the iron isn't that big for a tube amp with a moderate output power rating.

The voltages (plate) are higher than found in an average tube amp and the iron is also bigger than in most amps (maybe due to the lack of FB) and I've seen quite a lot.
I checked several datasheets and in fact there is such operating point listed (500V plates, 400V screens) where an amp is expected to produce 70Watts (two tubes) that's why I was expecting some more than 100W that I got (before clipping). Maybe the old production tubes were capable of that or the PI starts clipping earlier.

[Chuck H]

I checked several datasheets and in fact there is such operating point listed (500V plates, 400V screens) where an amp is expected to produce 70Watts (two tubes) that's why I was expecting some more than 100W that I got (before clipping).

That's Class B at some 6% distortion. And it's not the good kind of distortion, I promise. It's crossover distortion. It's possible then that the amp is biased too hot. Class B maintains the dissipation limit by running the tube for an unequal cycle that forces a cutoff time proportionate to burst of over dissipation made possible by the allowance of a small control grid current. So the waveform peaks are there, but there is an unavoidable anomaly with mating the cutoff/crossover point. I believe this is what some old Traynor amps did to achieve similar results that seem to defy the limits of the chosen valves. (corrections on this accepted)

On the bias note, new tubes don't always behave like they did even ten years ago. So bias voltages may look accurate and the amp may still be biased wrong. I don't know that the AD140 is supposed to be a Class B amp either. I looked at the schematic and while both PI's are typical LTP type I just couldn't figure out what they were doing with them. Hopefully someone can decipher the schematic and enlighten us.

Prowess Amplifiers - Orange - Schematics - AD140TC - First main board

[J M Fahey]

Dear gainfreak, beyond datasheets and simulation, you have the real thing on the bench and you are properly measuring it.

100W just before clipping?
Then that's the naked truth.

FWIW I have always found MI tube amps meeting their rated power .... with quite visible clipping or at least top rounding (even if thecnically "not yet clipping" ) and most of the time with waveform "S" bending/kinking which is also a form of distortion.

In a nutshell, for a measurement "by the book" you should have a pure sinewave out, even before clipping ... that's not common in a MI tube amp which is designed for max power, period, which to boot have little or no NFB.

I'm quite certain that a late 50's Class A 20W RMS Hi Fi power amp driven by a pair EL34 will definitely meet rated power and under 1% distortion ... it simply is designed under different rules.

[GainFreak]

That's Class B at some 6% distortion. And it's not the good kind of distortion, I promise. It's crossover distortion. It's possible then that the amp is biased too hot.

At lowest bias setting (hottest) the tubes are pulling around 28mA each (including the screen current) which is ~5mA so yes, it's kind of class B and the sine wave is not very nice (some crossover). Going to colder bias makes the sine wave look worse (more crossover) and output power goes down.

Thanks to all who chimed in.

P.S. I tried to correct the "Otange" thing but couldn't. It would be nice if moderators did it for further reference when searching.

[g1]

Were you running an 8 ohm load? Did you try 16, and any chance your load is opening or increasing resistance with heat?
At 8ohms, for 100W you need 28V, for 140V you need about 33.5V. I would try measuring the AC volts with your voltmeter, turn it up past clipping on the scope, till you get 33.5V on the meter. How bad does the clipping look?
They do have it labelled as 140W RMS on the rear panel, but there is no mention anywhere of how much THD at that power level.

[xtian]

I repaired an Orange AD200B Mk3 in March. That's a quad of 6550s, 608v on plates, a remarkably cold bias with about 15mA diss per tube when set per factory recommendation. Measured 132 watts clean into 4 ohms. Had a helpful email exchange with tech Len at Orange, who said it was up to spec.

[GainFreak]

After some more searching on the subject here's a quote from Eurotubes page:

The AD140 really makes about 90 watts of clean power but 90 watts is a LOT of power and it really does crank it out.

Obviously that's "normal" although datasheets suggest more power.

[TimmyP1955]

The schematic I have says 545 on the plates. With EL34s I'd bias this at 27ma. I'd expect to get about 100W at the onset of clipping.