mismatching OT and speaker.

[Randall]

I have always thought it is a bad thing to match a lower ohm speaker to an OT's rated value, ie connect a 4 ohm load to an 8 ohm winding. But what is the deal in reverse? I think it's OK, but I have a customer just put a 16 speaker in his '58 Harvard with a 8 ohm OT. I don't know that I'd want to tempt fate with that original OT.

What should I tell him?

[martin manning]

I'd say don't do it. Running higher load impedance will increase the peak voltage that appears on the primary winding, which could result in breakdown of the insulation and a ruined transformer.

[Leo_Gnardo]

I have always thought it is a bad thing to match a lower ohm speaker to an OT's rated value, ie connect a 4 ohm load to an 8 ohm winding. But what is the deal in reverse? I think it's OK, but I have a customer just put a 16 speaker in his '58 Harvard with a 8 ohm OT. I don't know that I'd want to tempt fate with that original OT.

What should I tell him?

Prob'ly not going to blow anything up. Get a little less power. Never hurts to have Ken Fisher's suggested flyback quench rectifiers in the plate circuit. Hey he can always run an extension 16 ohm cab and have a heck of a mini stack.

If you observe the amp's output with a scope you'll see an odd sort of distortion with an underloaded output. The top and bottom of the sine wave test signal seem the "reach" a little further than they should, sort of get pointy. Haven't found it to be a problem though.

[gingertube]

The reverse - that is "matching" a (for example) 8 Ohm Spekaer to a 4 Ohm Tap.
The 8 Ohms Speaker will reflect twice the impedance back to the OT primary that a 4 Ohm Speaker (on the same 4 Ohm tap) would. That means that each output tube is loaded by twice the impedance and for the same voltage swing will conduct half the signal current.

So that means you will get a bit less output power and possibly a bit less power amp distortion, BUT it will not present any threat to your amp since (in a Push Pull Amp) the output tubes would therefore dissipate less power.

It can also give you a tighter sound as the speaker is more "controlled" by the lower output impedance from the lower Ohms OT tap.

You need to think about this a bit "sideways" however because:
The output tubes are a "transconductance" device, that is, they conduct signal current when a signal voltage is applied to the grid. The signal voltage that appears at the anode (and hence the OT primary connection) will be the voltage developed by that signal current through that reflected load impedance. So it is not quite as simple as stated above.

In general though, you can say:
Higher Ohms speaker on lower Ohms OT tap - OK
Lower Ohms speaker on higher Ohms OT tap - SUS

Cheers,
Ian

[EtherealWidow]

It was put to me in these simplified terms.

If your amp has a triode for an output tube (or a pentode bootstrapped as a triode) it is more of a voltage source. Therefore, mismatching upward (4 ohm jack into 8 ohm speaker) is safer to keep the current swings under control and avoid something like red plating.

If it's a pentode, it's more of a current source, so mismatching in a downward direction (16 ohm jack into 8 ohm speaker) is safer to keep the voltage swings under control and avoid punching holes in the windings of the OT.

Sounds smart enough to me.

[gingertube]

Also keep in mind that the impedance of a nominally 8 Ohm Speaker will vary between (typically) 6 Ohms and 40 Ohms across the audio frequency spectrum. I would not normally worry about a 1 step difference either way, however I would not want to put a 4 Ohm speaker on a 16 Ohm Tap. x2 is of little consequence, x4 more of a worry.
Cheers,
Ian

[martin manning]

Also keep in mind that the impedance of a nominally 8 Ohm Speaker will vary between (typically) 6 Ohms and 40 Ohms across the audio frequency spectrum.

This makes the effect of doubling the nominal impedance on the peak voltage even worse IMO.

[Firestorm]

Tubes cannot develop their full rated power into the wrong load. Higher than optimal cuts quite a bit, but even lower than optimal may drop 15% or more. None of this matters (well, there are tonal effects) until you approach peak power. Aye, there's the rub. If the OT is marginally constructed, flyback voltages may puncture the insulation. But I'm with Kevin O'Connor on this: if a factor of 2 or even 4 mismatch kills a tranny, it was destined to die anyway, even if run "by the book."

[pdf64]

Have to bare in mind this is a vintage OT, in its 6th decade. Consideration given to insulation which may have degraded through age, overheating from tube shorts, voltage spikes from speaker failure etc.
Given that, my view is that it may be best to be nice to it and use a speaker of the correct impedance.
Pete

[Randall]

And that is really my point in question, we do not know how robust this 50+ year old OT is, what it's history may have been, or if it's winding's insulation is brittle and possibly cracking, etc. I wouldn't worry about it too much on a modern, freshly wound OT. So why risk it? Not worth it in my opinion.

[Firestorm]

Point taken. It's like the situation with a 50 year old speaker: it sounds great but if you play it hard it will die.

[tubeswell]

Usually running 'one knotch' above or below the optimal load resistance won't do harm, but it won't result in an optimal transfer of power. 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).

[printer2]

The big thing that is being left out of the equation is how hard the amp is being driven and the speaker impedance at resonance. If you have a mismatch and you do not push the amp and like playing clean there is less possibility of harm than if you square wave it. Also if the speaker resonance has a great big impedance peak and the back emf gets to higher levels than a milder speaker you are looking at more potential for harm.

It all depend on how important it is to you that the OT remains original. Heck, cheap insurance, parallel up a high wattage resistor across the output of the amp to give its rated load. Not like the wasted power will make much difference to the speaker output.

[Leo_Gnardo]

It all depend on how important it is to you that the OT remains original. Heck, cheap insurance, parallel up a high wattage resistor across the output of the amp to give its rated load. Not like the wasted power will make much difference to the speaker output.

And for more expensive insurance, swap out the OT for one that is rated 16 ohms secondary. Put the original in your safe.

If the player is the dime & slam type, get those flyback rectifiers on, no matter what.

[katopan]

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.