Using a High Power OPT (400W) at half power

[Steve Conner]

Hi guys,

Yes, I meant L-R as in inductor-resistor. Linkwitz-Riley is just a filter shape like Bessel or Butterworth anyway, they come in all orders.

Anyway let's run some numbers on that. The relevant measurement is 12.5H plate to plate. At 100Hz that has a reactance of 7850 ohms, at 50Hz, 3925 ohms, and at 25Hz, 1963 ohms.

DVMs are notorious for failing to measure iron cored inductors properly, but these figures are in the right sort of ballpark so we'll press ahead. Let's say the amp is called to deliver 400W at 33Hz, the low B on a 5 string bass. 400W into an 1100 ohm load is around 660V and 600mA, both RMS. Now with 660V at 33Hz across it, the magnetizing inductance will pass 250mA RMS.

So it will be starting to steal a little power from the speakers. But bearing in mind that eight 6550s can supply something in the region of 1.2A of current, there's plenty to go round.

Now remove half of the tubes and calculate what will happen when you hit that low B again.

[TDS]

Steve, As I indicated, I had some misgivings about the inductances measured with the DVM, your comment reinforced them, so I decided to do them again, using the old fashioned method of feeding a voltage direct from the 240V mains via a potential divider (22K/3W and 10K/1w) and measuring the voltage drop to the primary across a selected 1k resistor inseries with it.

The results were:

Secondary Open Circuit
Voltage across primary 70.7V rms
Voltage drop across 1k resistor 1.063V rms = 1.063mA
Calculated Z = 65917 ohms
Calculated Inductance = 209H

Secondary Loaded 4 ohm load on 4 ohm tap
Voltage across primary 10.1V rms
Voltage drop across 1k resistor 8.5V rms = 8.5mA
Calculated Z (i.e reflected load) = 1188 ohms
Calculated Inductance = 3.78H

Where does that leave me now?

[Steve Conner]

It leaves you somewhere between 12.5 and 209H But probably nearer the 209 end. The DVM may use such a small test current that it gets fooled by the shunt equivalent resistance of the iron losses. (I think that's what people mean when they say that DVMs can't "magnetize the core properly" of an iron cored inductor.)

Nevertheless, the inductance is non-linear and will fall with increasing voltage and decreasing frequency, until it saturates and gives up completely. This hopefully happens somewhere in the region of 600V RMS plate-plate at 20-30Hz, but before this point windows will crack and beer glasses fall off the bar :-)

Try the measurement again using the full mains voltage (or even more if you dare) and see if you get a lower reading. If it is still up in the 100s of henries then you may well be in luck with the tube pulling.

Why are you calculating the inductance with the transformer loaded anyway? You are just measuring an inductor shunted by a resistor, so converting the result to henries makes no sense.

[TDS]

This is getting good No messing about, using an HT winding to buck the mains voltage up, I managed to get 625 volts across the OPT primary, with a 1k series resistor to measure current.

The results are now:

Secondary Open Circuit
Voltage across primary 625V rms
Voltage drop across 1k resistor 4.86V rms = 4.86mA
Calculated Z = 128608 ohms - strangely, this appears to have increased!
Calculated Inductance = 409H

What you said about the loaded secondary makes sense. It is useful to confirm the reflected load. I guess I was assuming that the load on the secondary reduced the inductance of the primary.

[Bruce / Mission Amps]

Bruce,

It’s the shunting effect of the primary inductance in parallel with the reflected primary impedance which causes the bass to roll off.

I think the OT designer will have chosen the primary inductance to give the desired low frequency response with a 4 ohm load on the 4 ohm tap. If you ‘mismatch’ the OT by connecting an 8 ohm load to the 4 ohm tap I’d expect the bass to start rolling off an octave higher than with 4 ohms on the 4 ohm tap because the reflected impedance has doubled but the inductance (No. of turns) is unchanged.

Dave H.

Steve and Dave...
Hmmm... this is subtle but I sometimes hear an amp with an OT run with a mismatched load, (in order to achieve a proper primary load), as being very slightly thinner sounding then when properly matched.
When I have my custom OTs wound I always spec them with at least two secondary windings so I can mix and match cabinet loads.
When the OT is wound correctly, with multi tapped secondaries, does this bass roll-off not exist then when shifting the loads to the proper secondary?
And also, why does the amp sound better to me when the secondary load is run at it's max impedance?... ie., a 16 ohm load correctly coupled to an OT with 4-8 and 16 ohm secondaries.
I have convinced myself many times this is true ....but am I really suffering from a preconceived, acoustic expectation rather then fact.

[Bruce / Mission Amps]

Bruce,

It’s the shunting effect of the primary inductance in parallel with the reflected primary impedance which causes the bass to roll off.

I think the OT designer will have chosen the primary inductance to give the desired low frequency response with a 4 ohm load on the 4 ohm tap. If you ‘mismatch’ the OT by connecting an 8 ohm load to the 4 ohm tap I’d expect the bass to start rolling off an octave higher than with 4 ohms on the 4 ohm tap because the reflected impedance has doubled but the inductance (No. of turns) is unchanged.

Dave H.

Steve and Dave...
Hmmm... this is subtle but I sometimes hear an amp with an OT run with a mismatched load, (in order to achieve a proper primary load), as being very slightly thinner sounding then when properly matched.
When I have my custom OTs wound I always spec them with at least two secondary windings so I can mix and match cabinet loads.
When the OT is wound correctly, with multi tapped secondaries, does this bass roll-off not exist then when shifting the loads to the proper secondary?
And also, why does the amp sound better to me when the secondary load is run at it's max impedance?... ie., a 16 ohm load correctly coupled to an OT with 4-8 and 16 ohm secondaries.
I have convinced myself many times this is true ....but am I really suffering from a preconceived, acoustic expectation rather then fact?
Another reason why I leave the actual magnetics up my winders like Heyboer, Lenco, MM or who ever I'm using!

[Dave H]

Hmmm... this is subtle but I sometimes hear an amp with an OT run with a mismatched load, (in order to achieve a proper primary load), as being very slightly thinner sounding then when properly matched.

You’d expect it to be subtle with a guitar unless the OT is a tiny one with poor bass response. I’d expect it to be more obvious for bass guitar.
When you noticed it sounding thinner was that with an OT that required an upwards mismatch (e.g 8 ohm speaker on 4 ohm tap) to get the correct primary impedance? If you had to do a downwards mismatch (e.g. 4 ohm speaker on 8 ohm tap) to get the correct impedance it should have better bass response.

When the OT is wound correctly, with multi tapped secondaries, does this bass roll-off not exist then when shifting the loads to the proper secondary?

There will always be a bass roll off but the frequency of the roll off won’t change with tap as long as the load is correctly matched to the tap.

And also, why does the amp sound better to me when the secondary load is run at it's max impedance?... ie., a 16 ohm load correctly coupled to an OT with 4-8 and 16 ohm secondaries.

I don’t really know. Perhaps it’s something to do with the 16 ohm tap using all the winding but the 4 and 8 ohm taps only using part of the winding, that’s assuming it’s a guitar OT with a single tapped secondary. A hi-fi OT usually uses all the winding all the time (multiple secondaries connected in series/parallel).

Dave H