SE amps and speaker load

[TIMBO]

Hi guys, a question???

What effect does a SE amp suffer if you put a low impedance speaker on a high impedance output. Thanks

[Paul1453]

Hi guys, a question???

What effect does a SE amp suffer if you put a low impedance speaker on a high impedance output. Thanks

I could be mistaken, but my understanding is that this lighter than designed load will allow the output tubes to push more current through the OT than they should.  Heating up the tubes and OT, resulting in even lower resistance, which creates more current, equals more heat / lower resistance etc.

The result is that you can burn up your tubes or OT.  It might sound cool until it does, but not worth the risk IMO.

I'm sure someone will set me straight if I've got this concept wrong.   

[TIMBO]

Yeah, I thought it would be the same as the PP amps having a light load on the output.

[John]

IIRC, you can get away with 2x either way. You can put an 8ohm speaker on 16 ohm tap, or 4ohm on 8ohm tap. Or 8ohm on 4ohm tap.


Matching is still best. :)

[PRR]

Self-biased amps, which includes most SE amps, can be loaded with ANY impedance.

The power and current input is closely controlled by the self-bias. At idle the tube must dissipate all of this, so the amp must be designed to stand this. Large signals tend to displace this, and you can have single large current peaks. But over many cycles the current has to balance and the self-bias will not allow much rise of current.

If the load is not near the V/I of the tube, max clean power output will be down, ultimately in proportion to the mis-match.

Low load is higher distortion.

There is a second limit for high-volt amps with transformers (most tube amps). A too-high load, and the amplifier over-driven, the transformer voltage "spikes" to very high values, possibly 10X the B+. As we run 300V-400V supply and single-paper insulation is good for 500V long-term, even multiple layer insulation can be punched-through. This is why Champs die when used without a load (or a shorting jack).

[TIMBO]

Thanks guys, This relates to the Rex mascot.
The little info on them has a 15ohm speaker.
This one arrived with a replacement 8ohm.
I thought I was on a winner with a neat ROLA that I took from a valve stereo and I was thinking it was a 15ohm but I found that it was a 2ohm
When I fired up the amp it was fairly clean to about "3" on the dial but anything above that, had a bit of marshall crunch happening.
I talked to a mate that has one and his had been replaced as well and he has a couple of spares (15ohm) I can try.

Could anyone calculate the power output.Thanks

[tubenit]

From Kevin O'Connor and London Power Amps  FAQ:

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

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.

With respect, Tubenit

[TIMBO]

Thanks T.

[Paul1453]

Thanks T.

Yes, Thanks Tubenit!

I knew somebody would straighten me out if I got this wrong.  That's why I took a shot at it.   

So, I guess my conceptual understanding of this only works for transistors. 
Thanks for teaching me the exceptional properties of tubes.
These tubes are unique devices aren't they?
I guess that's why we love their sound so much.   

[HotBluePlates]

... lighter than designed load will allow the output tubes to push more current through the OT than they should.  Heating up the tubes and OT, resulting in even lower resistance, which creates more current, equals more heat / lower resistance etc. ...

This is an accurate description of thermal run-away of transistors which don't have thermal compensation or external protection circuits.

In contract, a tube's internal resistance is a function of the voltages applied to its elements. A tube can only pass more-current if the applied element voltages and the external load/power supply allow it. If load resistance/impedance is too high, the tube will be unable to pass more-current even if the applied element voltages would otherwise allow it. Likewise, a tube could have a 0Ω load (as in voltage applied by perfect d.c. power supplies with no external load resistance) and yet the tube won't pass more current if the element voltages allow it (most data sheet conditions, except for output tube typical operation, are derived by applying the indicated voltages with d.c. supplies and no load/cathode resistors).

[TIMBO]

Thanks guys, A 15ohm speaker is a way better match for the OT, so all is good