How Much Power can Cranked 100 Watt Superlead Produce?

[Pete Farrington]

The amplitude of output voltages generated by the amp doesn't increase linearly with mains voltages into the amp, does it?

Not linearly, but following the square law.

In reality it’ll probably be somewhere between the 2, as to the right of the knee, the anode curves of the output pentodes we use show that the current pretty much flatlines.

 

[TheKman76]

The amplitude of output voltages generated by the amp doesn't increase linearly with mains voltages into the amp, does it?

Very likely not, but the point remains, higher HT creates higher output tube currents, which in turn create greater EM fields in the OT, equals more output power.

 

[XTRXTR]

The bias voltage would drop as AC wall goes up so I think that balances the output. True whether dedicated bias tap or from the HT tap.

 

[Scumback Speakers]

My amp tech in So Cal measured old Marshalls running about 600 plate volts on a 100w chassis. The clean tone was 140w with 3% distortion, and over 220w cranked at 10% THD. If you want to know why Marshall recommended two 4x12's, wonder no more.

 

[playloud]

The bias voltage would drop as AC wall goes up so I think that balances the output. True whether dedicated bias tap or from the HT tap.

Yes exactly.

My amp tech in So Cal measured old Marshalls running about 600 plate volts on a 100w chassis. The clean tone was 140w with 3% distortion, and over 220w cranked at 10% THD. If you want to know why Marshall recommended two 4x12's, wonder no more.

Are you sure those figures are RMS? I suspect a sqrt(2) factor may have slipped into those calculations somehow.

600V plate voltage also sounds weird. The only 100W Marshall PT which I'm aware of coming close to that is the Drake 1204-43, which was ~560V at correct voltage, but could reach 600V with US mains voltage mismatch I suppose (GECs would be struggling though!) That amp only put out ~80W RMS clean though.

 

[mickeydg5]

It seems about right with tolerances taken into consideration but 220W will not be much if hardly any louder than 170W.

 

[TheKman76]

The bias voltage would drop as AC wall goes up so I think that balances the output. True whether dedicated bias tap or from the HT tap.

It's not just about bias though, is it? Higher HT will mean higher headroom across the entire pre-amp and potentially significantly higher swing from the PI. Even if the static bias current doesn't shift at all there's still the potential for more signal and higher headroom.

 

[Pete Farrington]

The bias voltage would drop as AC wall goes up so I think that balances the output. True whether dedicated bias tap or from the HT tap.

I can’t envisage how that would happen, could you explain your thinking?

But also, within a reasonable range, bias doesn’t affect power output per se, rather linearity. So the degree of non linearity (% distortion) at the onset of clipping will tend to change with bias. Also that non linearity may tend to reduce the ‘area under the curve’, so the rms voltage will reduce. Hence if power output is defined at x% distortion then yes, it may change with bias. But that’s not really pertinent to our application, peak anode current and hence speaker excursion shouldn’t be affected.

 

[TheKman76]

I can’t envisage how that would happen, could you explain your thinking?

Meaning in a fixed bias amp the negative bias voltage would become more negative as the AC input voltage increases. Same basic rectifier, filter and divider voltage will change with AC input.

 

[XTRXTR]

Well, I think of bias as a type of volume knob, the way a vibrato/tremolo oscillation does, I mean I know technically you don't typically use that type vibrato on a 50 watt or higher on the output tubes but the bias does change the output.

In this case of higher wall AC and HT it has to mean lower bias point assuming no one re-biases the output. Lower Bias lowers or reduces the DC electron flux to the plate.

So I know we are talking AC signal running on the top of the DC plate voltage and current VA = power and the AC power is developed by the swing above and below the DC line just as you point out.

But if the bias point is lower then it takes less to get to cutoff clip and grid clip and the delta is lower - doesn't that mean lower power? Then with that the grid current takes away from the plate current. It may be small but it must be there. Then you have bias excursion which turns to blocking distortion still lowering the AC output.

I'm not an expert but that is how I would explain how lower bias means lower power output over all. If I don't understand the physics of this I accept a description that explains it better to me. I am more about learning here anyway. Seriously, I am not getting my back up here I would rather learn the finer points of it. Especially obvious points I am completely missing.

 

[TheKman76]

I'm hearing you, and there's merit to this thinking. However, the bias point is about which part of the input signal will produce cut-off. Would make an interesting test.

 

[FleshOnGear]

600V plate voltage also sounds weird. The only 100W Marshall PT which I'm aware of coming close to that is the Drake 1204-43, which was ~560V at correct voltage, but could reach 600V with US mains voltage mismatch I suppose (GECs would be struggling though!) That amp only put out ~80W RMS clean though.

I know I’m talking about a different amp, but my Traynor YBA-1A had 560V HT, and it put out 75 clean watts RMS with only two EL34’s.

 

[Pete Farrington]

In this case of higher wall AC and HT it has to mean lower bias point assuming no one re-biases the output. Lower Bias lowers or reduces the DC electron flux to the plate.

Thanks, I see where you’re coming from now.
I think of voltage or other energy sources etc in terms of their magnitude.

And bias in terms of the idle dissipation being hotter or cooler.

That helps to avoid any ambiguity.

In regard of the valve operating point, anode current is largely determined by the relative magnitudes of the voltages on the control grid (bias) and screen grid, both with respect to their cathode.
Anode voltage has a small effect on anode current, as it’s the screen grid that’s accelerating the electrons which make it past the control grid. Hence the virtually horizontal anode current to the right of the knee.

So, both control grid and screen grid voltages will track the magnitude of the mains voltage. The result is that anode current (at idle) doesn’t change much, it will tend to increase a little, so the idle dissipation increases, but not by the square of the increase in mains voltage.

if the bias point is lower then it takes less to get to cutoff clip and grid clip and the delta is lower - doesn't that mean lower power?

No, peak anode current occurs when the peak voltage of the upper half of the signal at the control grid equals the bias voltage. That (momentarily) results in Vg1-k=0, which defines max power with typical AB1 load lines.
So provided the preceding stage hasn’t maxed out / isn’t clipping (which won’t be the case because all HT node voltages will increase, so more signal voltage will be available) the peak signal condition of Vg1-k=0V is still achieved.
But what’s changed is that due to the load impedance being the same, provided we’re not too far right of the knee, more anode voltage means more anode current means power output increases according to the square of the increase in mains voltage.

Phew, hope that made sense

 

[Pete Farrington]

my Traynor YBA-1A had 560V HT, and it put out 75 clean watts RMS with only two EL34’s.

Those poor valves may be under some serious stress!
Usually a dual rail HT arrangements is needed if much more than 50W per pair is required (and dissipation limits are to be respected).

https://el34world.com/charts/Schematics/files/Traynor/Traynor_yba_1a_bass_master_mk_ii.pdf

https://frank.pocnet.net/sheets/010/e/EL34.pdf

 

[FleshOnGear]

Those poor valves may be under some serious stress!
Usually a dual rail HT arrangements is needed if much more than 50W per pair is required (and dissipation limits are to be respected).

https://el34world.com/charts/Schematics/files/Traynor/Traynor_yba_1a_bass_master_mk_ii.pdf

https://frank.pocnet.net/sheets/010/e/EL34.pdf

I ended up installing HT reduction circuitry on the center tap of the PT - a zener amplified by a MOSFET. It’s been ages since I did that, but IIRC the HT became 500V and the power output was reduced to 50w RMS. I was afraid of the stress on the tubes causing a major failure.

 

[XTRXTR]

But what’s changed is that due to the load impedance being the same, provided we’re not too far right of the knee, more anode voltage means more anode current means power output increases according to the square of the increase in mains voltage.

That makes great sense
Fixed Load with increased B+ equates to higher AC pre-amplification which increases the output plate current and higher voltage (I know that is simplifying it) P = IV

It helps to understand it for a specific frequency. The load is reactive per frequency so it has some finer details there. But if you quantize each frequency per moment in time t=tau the same can be said at each time period. Am I still getting it here?

Thus more AC mains certainly increases the probability of blowing tubes, burning coils on speakers, burning the OT, the PT if not fused properly, even blowing weak filter caps, especially when running an amp at extreme output volumes and or long periods of time.

side note:
FYI Hendrix looks like he is using Ampeg 4 full stack size cabs with 2x15" or even 2x18" speakers maybe larger, not even using Marshall amps here. Sounds very ear splitting beam peaks:

Ouch and that mic straight into it. Headaches and tinnitus all around.

 

[Pete Farrington]

That makes great sense

That’s a relief, often though my posts might seem clear to me as I write them, when I review them later they can be gobbledygook

It helps to understand it for a specific frequency. The load is reactive per frequency so it has some finer details there

I’m sure, though impedance might be above nominal, the up to 90° phase shift between current and voltage at some parts of the spectrum can only make things worse really.

Thus more AC mains certainly increases the probability of

Yes, though most amps / rigs should be designed to cope with mains voltage at its upper limit, vintage amps designed for 110V (as per early export Marshalls) might struggle.

even 2x18"

Yikes those cabs are massive!

 

[XTRXTR]

@Pete Farrington Well you have an engineer type mind. If you are anything like me I worry about every word in the syntax of the sentence being misunderstood. I struggle with being too direct and coming off as pompous. In reality I feel I only know enough to be dangerous but I continue to learn. Someday I hope to see the whole amp in a mental three dimensional real time electrical amplifier. Voltage, current, impedance, power, electromagnetic etc. like Neo in the Matrix. Frequency vs amplitude vs load all pumping like an engine. Someday.

Anyway I feel I learned from what you said so thank you.

 

[Pete Farrington]

Someday I hope to see the whole amp in a mental three dimensional real time electrical amplifier. Voltage, current, impedance, power, electromagnetic etc. like Neo in the Matrix. Frequency vs amplitude vs load all pumping like an engine.

That’s brilliant!
Kinda like being able to see the wood and the trees (as opposed to ‘can’t see the wood for the trees’).
I suspect though that all but a few geniuses have to manage with ‘peering through the glass darkly’ or having tunnel vision on some particular thing.

 

[TheKman76]

Geez, I'm really in good company around here. This must be the most like minds I've ever encountered in one place!
It's a pity we're all on (mostly) opposite corners of the planet. Tell me you're not all whiskey drinkers too?