Rocket Power Dissipation

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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jkey04
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Rocket Power Dissipation

Post by jkey04 »

What is the true power output of the Rocket circuit? By my calculations, each EL84 is dissipating ~13W for a total output of ~52W!

Formula used: (plate voltage-cathode voltage)*cathode current

Cathode current=cathode voltage/cathode resistor

Does this look correct? If so, how healthy is this in the long run for the 35W OT?
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brewdude
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Re: Rocket Power Dissipation

Post by brewdude »

Plate dissipation and output power are not the same thing.
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martin manning
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Re: Rocket Power Dissipation

Post by martin manning »

Plate dissipation is not output power! Note that with no signal the plates are dissipating 50W and the output power is zero.
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jazbo8
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Re: Rocket Power Dissipation

Post by jazbo8 »

It might be a typo... the worksheet shows a total plate dissipation of 52W, perhaps that's what he meant, in any case, 13W dissipation per tube is well over the max Pda of 12W. Again, the rule of thumb for Class AB amp bias is 0.7*12W = 8.4W, so the output stage is biased way too hot now.
jkey04
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Re: Rocket Power Dissipation

Post by jkey04 »

Sounds like I need to brush up on a little tube theory dealing with output sections and power...

Thanks! Pardon my ignorance. :)
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martin manning
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Re: Rocket Power Dissipation

Post by martin manning »

jazbo8 wrote:It might be a typo... the worksheet shows a total plate dissipation of 52W, perhaps that's what he meant, in any case, 13W dissipation per tube is well over the max Pda of 12W. Again, the rule of thumb for Class AB amp bias is 0.7*12W = 8.4W, so the output stage is biased way too hot now.
The 70% rule is more for fixed bias. Cathode bias amps are typically run higher (close to 100%) since the cathode voltage will rise when signal is applied.
jkey04
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Re: Rocket Power Dissipation

Post by jkey04 »

After comparing my voltages to the Rocket voltage chart in the Trainwreck files section, it appears that my Anode voltage is about 10Vdc higher. I have build two Rocket clones and plate voltages for both were in the 295-300Vdc range. Is this too high?

Should i go the route of increasing the cathode resistor of the EL84s to around 60ohms?

The first Rocket clone was built over 2 years ago and ive never had any issues out of it. Its still got the original set of JJ EL84s in it and they sound great.
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martin manning
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Re: Rocket Power Dissipation

Post by martin manning »

Sounds like it's all good then!
jkey04
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Re: Rocket Power Dissipation

Post by jkey04 »

So is it safe to assume that the Rocket is class A, and that the output power is approximately 90% of the dissipation?

Thus,

0.9*52W = 46.8W

If not, how would you calculate the output power seen by the speaker load?
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martin manning
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Re: Rocket Power Dissipation

Post by martin manning »

It will not run in Class-A all the way up to full power. You can calculate expected power output from the load line assuming the grid voltage swings all the way to zero volts Vg-k sinusoidally, and you will get a theoretical power output of a bit over 31W. See: http://www.valvewizard.co.uk/pp.html It won't achieve that in practice, though, and the best answer comes from a measurement using a signal generator, an oscilloscope, and a dummy load.
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xtian
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Re: Rocket Power Dissipation

Post by xtian »

Using a scope, I measured 15 watts clean power output with a 1kHz sine wave into a 4ohm dummy load for my Rocket build.
I build and repair tube amps. http://amps.monkeymatic.com
jkey04
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Re: Rocket Power Dissipation

Post by jkey04 »

Thanks for the info. It sounds like i need to invest in a signal generator and a dummy load....

Seems like the rocket does operate in the 30-35W range.

Xtian -- you are saying that the rocket produces about 15W of clean power before it begins to distort the input signal at all? Im sure this varies for frequencies other than 1kHz, correct?
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jazbo8
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Re: Rocket Power Dissipation

Post by jazbo8 »

martin manning wrote: The 70% rule is more for fixed bias. Cathode bias amps are typically run higher (close to 100%) since the cathode voltage will rise when signal is applied.
I think we just went through a similar discussion on another forum, IIRC, whether cathode biased output stage could be biased hotter (as some have suggested to 90-100% of max Pda) would depend on if and how much the cathode is bypassed, if it is well bypassed like the Rocket, then perhaps 70% max Pda is still good rule to follow, then again if the field report says no issue with the hot bias then why not...
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xtian
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Re: Rocket Power Dissipation

Post by xtian »

I remeasured my Rocket.

Plates = 295v
50R cathode resistor (measures 49.2 ohms) drops 8.4v
shows ~42mA per tube
I also measured voltage drop and resistance across both sides of OT secondary, shows about 37-39mA per tube.
So I've got very near almost exactly precisely 100% dissipation at idle, using the Saratov 6P14P tubes that RJ provided.

I measured about 16 watts into 8 ohms clean before distortion. Distortion does not change much when I sweep the frequency. Lowest distortion seen with cathode bypass off, Bass and Cut controls at zero.

You can see my build log here: http://ampgarage.com/forum/viewtopic.php?t=17584
I build and repair tube amps. http://amps.monkeymatic.com
Cliff Schecht
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Re: Rocket Power Dissipation

Post by Cliff Schecht »

jazbo8 wrote:
martin manning wrote: The 70% rule is more for fixed bias. Cathode bias amps are typically run higher (close to 100%) since the cathode voltage will rise when signal is applied.
I think we just went through a similar discussion on another forum, IIRC, whether cathode biased output stage could be biased hotter (as some have suggested to 90-100% of max Pda) would depend on if and how much the cathode is bypassed, if it is well bypassed like the Rocket, then perhaps 70% max Pda is still good rule to follow, then again if the field report says no issue with the hot bias then why not...
The cathode bypass capacitor will determine how long the output stage cathodes take to change from their no-output bias point to the full output bias point. The cathode voltage raises which in effect limits the potential seen across the tube and has the effect of causing the output stage to self-limit (because of the resistor, not the cap). The more current you try to push through, the more limiting will occur. Even if you have a massively large bypass cap, the time it takes for the tube to start self-limiting is still usually under one second and the cathode will work as hard as it can to charge that cap as fast as possible so this voltage will change quickly.

The point at which the cathode cap makes the output stage sound like a fixed bias amp is usually much smaller than the cap value you'd need to require a 70% bias point.
Cliff Schecht - Circuit P.I.
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