mismatching OT and speaker.

[martin manning]

The other big thing being left out of this discussion is - higher load risks the screens from burn out if they're not adequately protected, low load risks the plates from redplating if the load line moves in a way to cross over the max power dissipation curve by too much. Both moves will result in less power output but stress the output valves in a different way.

Yea, good point. No screen resistors in this thing and just a 470 ohm dropper between the screen and plate nodes on the power string.

[EtherealWidow]

On the other hand too much load resistance is bad, as it is effectively the same as having infinite load resistance (which amounts to the same thing as having none - if you can get your head around that contradiction).

If I could just chime in with the little that I know, although it seems like a contradiction it's the wording of that statement that isn't quite right.

When you unplug a speaker it's not a short circuit (no resistance), because the tip and sleeve aren't connected. Instead, it's just open. So instead of having an 4-16 ohm speaker for the load, you have air, which is a LOT higher in resistance, for guitar amp purposes, it's infinite.

[tubeswell]

Yes agree on that (sorry I should've made myself clearer.)

[10thTx]

from Kevin O'Connor London Power

http://www.londonpower.com/tube-amps-faq

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

In most amps, you can set the impedance selector to whatever sounds best. The one caveat is: NOT in English amps. Having replaced more Marshall OTs than anything else, I would advise that the impedance selector always be set to the rated load, or less.

[Reeltarded]

And I will stand by 100% in either direction never having killed any of my stuff. Not saying I haven't killed anything, just that is wasn't because of a 16 ohm load on an 8 ohm output. (Marshalls)

Now, I don't like it going the other way. My mind paces around in circles with a 4 ohm on an 8 out or similar but I have done it too. (Fender no selector having cheap assed surfboard lookin'... fribbin' frabbin'..)

Who makes a 2 ohm cabinet? WTF LEO FENDER!! Sheesh. Get with it.

[sepulchre]

Okay, once again I am a bit confused.

I understand how running a 16R speaker from a 4R output (or some other such higher load mismatch) could risk the OT, punching holes in the insulation. But does running a lower load (eg: 8R spkr from the 4R output) risk red plating? I read in another thread that doing so would lead to more power tube break up at lower volume because the PA is not delivering its full wattage.

Is this so? What are the risks of running lowered speaker impedance. I understand that most say one notch is okay but what happens beyond that? Not that I'm about to try, I'm just contemplating the idea. However, if lowering the speaker Z does indeed promote greater output tube break up with little risk I might give that a try.

[martin manning]

I think you miss typed above, meaning to say running 4R load from 8R tap. Yes, this will increase the current through the tubes and OT, potentially resulting in over dissipation in the power tubes- provided that the power supply can keep up.

If the amp has a robust power supply, and the voltage and load as designed are already skirting the limits of plate dissipation, then you might get red-plating. However, if there are large screen resistors then they will limit the current through the tubes by reducing screen voltage and help to keep the plate dissipation under control. I suppose the screen resistors could be driven over their dissipation limits if they are not stout enough, though.

If the power supply is the limiting factor, then the result could be an overheated power transformer.

You can give it a try, just watch for these problems.

[talbany]

To keep things in perspective and try not to scare the children ..It really all depends on how you run the amplifier!

Running a sine wave input with the output not clipped, there is no danger of either flyback voltages or screen overdissipation. The problems occur when the output stage is heavily overdriven into a square wave.(a duty-cycle modulation of the waveform as you would get in a non-master Marshall cranked way up) But how many would do that with a guitar amp these day's ..Especially 100w..

BTW.If you do run the amp into a hard clip and are are concerned about fly back voltages a few diodes to ground off the plates will help with arcing and the carbon it leaves behind..

If you use an attenuator (restive load) the flyback voltages aren't bad at 2x mismatch, but at 4x mismatch they can get rather high, primarily dependent upon the leakage inductance of the transformer used.Screen current might be a concern but again run into a hard clip..The damage caused on output transformers running certain attenuators improperly are well documented..

You guy's are basically discussing running these amps right at or beyond their operating limits aka...Jimi Hendrix style.

If you run it like a race car you have to design for a race car!
Tony

[sepulchre]

Yes, Martin, I did mistype. Sorry, glad you saw what I was asking.

Thanks, guys. My amp is Marshall 2204 style 50 watt with EL34s. It's been awhile since it was complete so I'll have to check the PT capabilities, voltages, etc. I was just wondering if I could get a bit more power tube break up at lower volumes by doing this. I'll pull the back off and try it, keeping an eye on the tubes. I'm not looking to drive the amp all that hard; my wife would pack a bag and the fish would go belly up.