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what determines the output watt?

Posted: Wed Apr 21, 2021 2:03 pm
by Skrawboy
Hey everybody

I just finnished a 2x el84 build because i wanted a lower-ish output amp for bedroom use (witch in normal people language sound like stadium output).
After all is done, i got a amp that is definitely lower output volume than my 2x el34 amp witch has a pretty similar output section (only som resistor and capacitor values changes).
My ampbuilding knowledge comes from this forum and what ever i can find on the internet. And so far this is waht i got, 4x el34 = 100watts, 2 el34 = 50 watts, 4 el84= 30 watt etc. etc.

So how do marshall and other builders use the the same amount of el34/6l6/el84/whatever, and lower the output to 20-30watts?

Re: what determines the output watt?

Posted: Wed Apr 21, 2021 7:05 pm
by danman
Those numbers you listed are mostly for advertising purposes and give people a general idea of an amp's possible wattage. In the real world, 2 x El34's can actually deliver close to 100 watts if pushed into extreme distortion and a 4 x El34 amp can make almost 200 watts when you are driving the amp hard. Manufacturers will normally list an amp's wattage at a certain distortion level such as 50 watts @ 5% "total harmonic distortion" (THD) but a tube amp can make much more power when pushed well into clipping. This is why speaker manufacturers recommend doubling your speaker wattage when purchasing speakers for a tube amp.

The power tube type, the amp's power supply, plate voltage, tube bias and the output transformers primary impedance are all factors in how much output wattage an amplifier can provide. Marshall is able to squeeze some of their newer 2 x EL34 amps down to 20 watts by lowering the amps B+ voltage and using a smaller OT. Even at 20 watts though, they can still be quite loud but they do run out of clean headroom before their larger, 50 watts do.

Re: what determines the output watt?

Posted: Thu Apr 22, 2021 3:59 am
by Skrawboy
Thanx Danman

Another mystery solved.

B+ powerscaliing, here i come :thumbsup:

Re: what determines the output watt?

Posted: Thu Apr 22, 2021 7:00 pm
by danman
Power scaling will work to lower volume and headroom if that's your goal. Some folks will scale only the power amp section so that the preamp still sees it's correct voltage. This method works well and helps to retain the amp's original tone because the preamp voltages are not lowered. Other's will scale the entire amp with good results but some folks have claimed that it does not sound good with all amps. I don't have any experience with it so I can't comment on which method is better. I will mention that cathode biased amps are much easier to scale because you don't have to adjust for the change in bias voltage like you would on a fixed bias amp. I believe that London Power does make a scaling kit for fixed bias amps though.