question about amps running 6L6GC tubes

[pompira]

hi, i'm new here.

i asked this question @ another forum and someone suggested i should post it here.

if i had one 120W amp and one 100W amp, both running on a quartet of 6L6GC tubes, and both with a plate voltage of 450V, and i wanted to bias each of them at 65% plate dissipation, i'd use this formula for the first one:

30W / 450V x 65%

and this formula for the second one:

25W / 450V x 65%

???

or would it be more correct to use the first formula for both amps?

[diagrammatiks]

you bias to the tube, not whatever the amp is rated.

How do you know that one is actually 120watts and the other is 100?

[pompira]

you bias to the tube, not whatever the amp is rated.

thank you

How do you know that one is actually 120watts and the other is 100?

i don't. it's a hypothetical question. an example, if you prefer.
my current amp actually runs a pair of EL34, not a quartet of 6L6GC.
i was just curious.
so, thanks again for the info, i appreciate it

[Firestorm]

There lots of different "watts" in amps: there's power consumption from the wall, which is measured in watts; there's output power which is measured in watts (sometimes watts RMS, but that's another can of worms); there also plate watts dissipation, which is what you care about -- how much heat can the tube plate dissipate before it hurts itself. As diagrammatiks said, you bias the tube, not the amp. The formula you would use in all cases (w/ 6L6GCs) is: 30W x .65= 19.5; 450V * XmA = 19.5. Solve for X: .043333...mA.

[pompira]

There lots of different "watts" in amps: there's power consumption from the wall, which is measured in watts; there's output power which is measured in watts (sometimes watts RMS, but that's another can of worms); there also plate watts dissipation, which is what you care about -- how much heat can the tube plate dissipate before it hurts itself. As diagrammatiks said, you bias the tube, not the amp. The formula you would use in all cases (w/ 6L6GCs) is: 30W x .65= 19.5; 450V * XmA = 19.5. Solve for X: .043333...mA.

i was referring to the "xxx watts rms @ x ohms" found at the speaker outputs.

and thanks for confirming the formula.

[Firestorm]

i was referring to the "xxx watts rms @ x ohms" found at the speaker outputs.

On most production amps, that number is very scientifically made up by the marketing department.

[pompira]

i was referring to the "xxx watts rms @ x ohms" found at the speaker outputs.

On most production amps, that number is very scientifically made up by the marketing department.

the 30 watt rating for the 6L6GC tubes or, for that matter, the 25 watt rating for the EL34 tubes, are accurate rather than made up, right?

[Super_Reverb]

One way to look at this issue is, according to your example:

Amp(120) Ibias(per 6L6) =0.65*30/450 = 43.3mA

Amp(100) Ibias (per 6L6)=0.65*25/450 = 36.1mA

If the two amps have similar linearity, gain, power supply capacity, output tubes, and performance in general, the RMS (AC) output current to speakers will be same, right? Same input signal * same gain = same AC signal, riding on top of two different output tube bias levels, right? Except the output tube will have a different operating point (load line) due to different bias current. But, bias current isn't part of the signal, so we have to get more AC signal out out of the OT secondary to achieve higher power output.

If you accept the above argument, then differences between a 100W and 120W 4x6L6 amp cannot be explained by output tube bias current. In my mind, to get more power, you need to have more PS current capability, more voltage gain, and adequate linearity (OT saturation is part of this). To achieve higher gain through the amp, you need to set the gain stage bias currents and plate resistor values up to have maximum signal swing through the preamp, PI, and output stage.

As the earlier posters indicated, what is the output voltage requirement for maximum power? With the same p-p voltage, a square wave has more power than a sine wave, but a square wave is heavily distorted (assuming you are sending sine waves from the guitar).

cheers,

rob

[Firestorm]

the 25 watt rating for the EL34 tubes, are accurate rather than made up, right?

Those are pretty accurate, normal variability of analog things notwithstanding.

[pompira]

One way to look at this issue is [...]

thanks for your explanation

Those are pretty accurate [...]

thanks

=================

by the way, and regarding this:

i was referring to the "xxx watts rms @ x ohms" found at the speaker outputs.

On most production amps, that number is very scientifically made up by the marketing department.

if a user trusts the speaker output label (for example, 100W rms @ 8ohm)
and connects it to a 100W rms @ 8ohm cabinet
and then the speakers blow because the amp actually outputs 150W
would there be a way for the marketing department to be held responsible
for the damage?
(apologies for the off topic question)

[Firestorm]

if a user trusts the speaker output label (for example, 100W rms @ 8ohm)
and connects it to a 100W rms @ 8ohm cabinet
and then the speakers blow because the amp actually outputs 150W
would there be a way for the marketing department to be held responsible
for the damage?

No marketing department ever understated the output power of anything. As a hypothetical, you can sue anyone for any cause. Whether you prevail is a different matter entirely.

[pompira]

No marketing department ever understated the output power of anything.

excellent! i mean, it's bad that they lie, but a little less power never hurt a speaker.

No marketing department ever understated the output power of anything. As a hypothetical, you can sue anyone for any cause. Whether you prevail is a different matter entirely.

i hear you.

thanks again

[Jana]

A healthy 100 watt Marshall will put out about 100 watts of clean power. Crank the amp and really push it and there can be close to 180 watts of power. It isn't clean, but it is there.

[pompira]

A healthy 100 watt Marshall will put out about 100 watts of clean power. Crank the amp and really push it and there can be close to 180 watts of power. It isn't clean, but it is there.

that sounds like the opposite of what firestorm posted.

what gives?

[diagrammatiks]

tube amps put out more power when overdriven.

all wattage ratings are always measured at clean power.