How do you measure OPT impedance?

[pfrischmann]

Hi Guys,
I have a new booteek amp and need to measure the tap the builder used to wire to the speaker jack. There's some question as to what the impedence is. I know enought not to hurt myself and bias my own amps and have done some simple repairs (caps, etc)

Anybody know how to do this?

Thanks,
Paul

 

[John Phillips]

It's not that easy - by far the simplest option is to ask the builder.

If you can't do that for whatever reason, you need to measure the power output of the amp at a fixed point (either the onset of clipping, which you may need a scope for, or absolute maximum power) into different loads. Whichever produces the most power is the correct match.

The crudest (but accurate enough) method is to connect an AC voltmeter across the speaker, dime the amp and blast away. Power = voltage-squared divided by impedance, so you can calculate the power. Do this with whatever different loads you think the amp might be designed for. If you have two different possibilities, it's fairly safe. If you have all three (4, 8 and 16) it's probably safest to start with 8, then 4 - and stop as soon as possible if the power into 4 looks lower than that into 8, in case the amp is 16 ohm. It's probably marginally safer to run a 16-ohm amp into 4 ohms (though very hard on the tubes) than the other way round, which is a hazard to the OT.

 

[pfrischmann]

Thanks for the info but I don't get your last comment.

I was always told it was safer to put a lower impedence amp into a higher imp cab. I was always under the impression that say a 8 ohm amp into a 4 ohm cab would cause a backup which would kill the opt. The other way would strain it which is not nearly as bad.

Where did I go wrong?

Thanks,
Paul

 

[aleclee]

Originally posted by pfrischmann
I was always told it was safer to put a lower impedence amp into a higher imp cab. I was always under the impression that say a 8 ohm amp into a 4 ohm cab would cause a backup which would kill the opt. The other way would strain it which is not nearly as bad.

The impedance preferences for solid state are opposite from those of tubes. Are you sure you don't have the two confused.

 

[John Phillips]

Yes, exactly what Alec says. The difference is cause by the output transformer.

Solid-state amps (almost always no OT) have a MINIMUM impedance which you must never run the amp under, or you will almost certainly kill the output transistors (very quickly), due to the excess current drawn. They are also almost always safe with any load above the minimum, and simply operate at lower power. This is fairly obvious... tube amps (with an OT) are more complicated.

They have a MATCHING impedance, and mismatching in either direction is safe within reasonable limits. Very low loads (ie under half) do try to draw excess current, but due to the fact that the efficiency of the transformer falls the further away from matching you get, the overall power developed is reduced, which limits the potential harm to some extent. Also, tubes are tougher than transistors when overloaded and don't tend to fry so quickly.

The problem with over-high loads (ie over twice matching) is the back voltages developed in the transformer, due to the inductive nature of both the transformer and the speaker. As the load impedance rises, so the back voltage does too, and can quickly exceed the insulation voltage rating of the transformer wiring. If this arcs through, it will usually cause a permanent short and the transformer is now scrap. Even if it doesn't fry the transformer, arcs can occur at the power tube sockets or in the tubes themselves. It's usually a bad idea to run a tube amp at over double its matching impedance.

Running with no load at all - which I'm sure you're aware is a very bad idea! - is simply the extreme case, equivalent to 'infinite' impedance. It's actually safer to run a TUBE amp into a dead short than this - which is why many amps have a 'shorting switch' in the speaker jack, in case nothing is plugged in.

 

[pfrischmann]

I get it!!

Thanks. That's the best explanation of impedence mis-matching I've ever heard..

Thanks again.