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Old 6th August 2003, 12:56 AM   #1
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Default 15 W out of single SE KT88 ?

Guys,
I see number of SE KT88 amplifiers on the market advertising 15W outputs power.
Any ideas how do they do it ? How do they get 15 watts out of single KT88/6550 tube ? I think, they use tube in pentode mode.
I have breadboarded a simple KT88 amp using 2.5k OT and I only get about 6...8 watts before onset of clipping. Supply voltage is 400vdc and I used fixed bias.
Any comments ?

Thanks
Sergey
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Old 6th August 2003, 01:21 AM   #2
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SE Amp CAD gives 8.6Wrms / 17.2W peak with a 6550 using your parameters and 80mA bias when driven almost to grid zero. Distorion is 8%
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Old 6th August 2003, 01:51 AM   #3
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Default Info.+

Very tough to get KT88 15 watts in triode mode. Must be tetrode operation. Depending on the brand, distortion varies widely.

As Brett mentioned and per your conditions, in tetrode mode, distortion is high. 2.5k OPT is way too low for low distortion operation.

Svetlana, EH, and Sovtek give the lowest distortions, with JJs, MOJOs twice the distortion. In my opinion, Svetlana and EH give the best sonics.

Hope this helps.
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Old 6th August 2003, 02:01 AM   #4
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Default Re: Info.+

Quote:
Originally posted by Positron
As Brett mentioned and per your conditions, in tetrode mode, distortion is high.
That was triode. So I think manufacturers are using the 'peak' number to sell to customers. I only spec in RMS, and define clipping as 1%.
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Old 6th August 2003, 02:17 AM   #5
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Brett,
What's the difference between RMS and PEAK power ?
And how would you measure PEAK power ?

Thanks
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Old 6th August 2003, 02:38 AM   #6
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Default Thanks.

Thanks Brett. I have seen peak or music, instead of RMS, used before.

It wouldn't surprise me to see 8% with a 2.5k tranny, in triode mode, maybe more or less, depending on the brand tube used. Personally, I have never worked with that low tranny with that tube.

Take care and thanks Brett.
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Old 6th August 2003, 07:31 AM   #7
EC8010 is offline EC8010  United Kingdom
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Default Marketing

Quote:
Originally posted by gurevise
What's the difference between RMS and PEAK power?
Lies.

Quote:
And how would you measure PEAK power?
Imaginatively.

Seriously, almost all SE amplifiers lie about their output power. A rough rule of thumb for triodes is that the audio power is 20% of the anode dissipation. (For push-pull, add the anode dissipations together.)
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Old 6th August 2003, 08:56 AM   #8
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Default That old chestnut...

Hi,

As mentioned in previous threads, there is actually no such thing as "RMS power"; There's "Power". The same stuff that heats up a kettle. There is no need to check whether your kettle is "RMS"

"RMS power" was just a term that equipment manufacturers thought up to differentiate real power from ficticious (peak) power measurements.

I know a little about how they go about calculating "Peak power", but I can't for the life of me see how it could be higher than the real power for class A.

Imaginitive power ratings in class A can only depend on the distortion figure referenced.

Quote:
A rough rule of thumb for triodes is that the audio power is 20% of the anode dissipation. (For push-pull, add the anode dissipations together.)
That is for class A1 operation, and is typical of what can be achieved with RC coupling.
With DC coupling it is possible to move into class A2. Just under 40% is possible there, but it is device dependent, and getting good results is a non-trivial task.

Cheers,
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Old 6th August 2003, 10:43 AM   #9
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Peak power is simply the mathematical power given or drawn at the peak voltage of an AC signal. RMS power is power measured at volts RMS, instead of volts peak. RMS is the useful, or "REAL" voltage and is calculated by multiplying .707 to the peak voltage. Rarely, however, is voltage measured peak, but normally RMS. Power is not by designation to be expressed as "watts RMS," it really means, "watts calculated at volts RMS," since volts are supposed to measure at either RMS or PEAK.

Also, note that DC voltages don't exist in either RMS or PEAK, they are simple volts DC.

The full scope of the matter is somewhat complicated, however, I hope I have cleared the air as to what watts RMS and watts PEAK really mean.
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Old 6th August 2003, 10:05 PM   #10
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Original-flavor 6L6, as Pentode, was rated about 10 watts output in SE.

I have on my bench a 1938 PA amplifier that runs the 6L6 a little past the design-center specs and delivers 9 watts despite transformer losses (which are substantial in a SE transformer with good bass response).

FWIW: original 6L6 working somewhat under-rating as SE-Triode claimed something like 1.4W. I bet it could do more, but not a huge amount more inside the original 360V plate rating. (6L6GC with higher plate voltage rating could do a lot better, if you can wind a primary to high impedance.)

As a 6550 is roughly twice the size of an original 6L6, I would expect well over 10 watts if you got it powered and loaded right. For gitar-amp duty, where tube-life and low-THD are not the main selling factors, 15 watts should be easy even with an inexpensive (lossy) output transformer.

As Triode, I'd think 15W from 6550 is awful high. I bet I could demo it with a great transformer and either high THD (15%, tolerable on gitar) or heavy feedback. But the poor tube would go soft and die in hours. (Some cheap 6550 might arc and explode at turn-on at the voltage and current needed to hit 15W SET, and the screen grid would not last long.)

WTH: 6L6GC is so cheap, you can afford two, and easily beat 15W SE Pentode loafing. While the 6550 is a big hunk of power in one bottle, on today's market the 6L6GC is a better watt/buck buy and you have a much wider selection. And 6L6GC are in-stock locally on Saturday when your Friday night gig eats your tubes.

> using 2.5k OT

That is sure too low for 6L6 Triode; probably a bit low for 6550 working 400V because the plate dissipation limits the idle current. Try 5K load. If you don't mind maybe-killing tubes, take the plate voltage higher, though you may already be at the screen-grid (or Triode plate) voltage rating (too lazy to look it up for you).

Don't fret too much about the actual power number. If 10 watts is not enough, then you probably want 40 to 100 watts to make you say WOW!. From 10 to 15 watts is just "hmmm...". Changes in how it clips when the power is goosed will affect the "sound" more than any little 50% change in power.
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