Without the schematic is difficult to advice. You can have SE amps designed for 1.5K, 3K, 10K or any other plate loads. But, if your amplifier was designed to use a 3K:8 output transformer, then yes, you do need to connect your 4 ohm speakers to the 4 ohm OPT tap.
I'm afraid you may need new speakers, unless you are happy with the way it sounds with 4 ohm speakers. I'd expect distortion to be quite a bit higher.
Edit: Some Lundahl transformers have a lot of connection options for different impedance etc. Again, it would be easier if you show the schematic, transformer model etc.
The only other alternative to changing either both speakers, or both output transformers . . .
Have a pair of Auto transformers custom wound.
The laminations should be interleaved E / I, the ratio of turns is 100 / 71.
The full 100% windings are connected to the 8 Ohm Lundahl secondaries, and the 71% windings are connected to your 4 Ohm speakers.
The performance of such an Auto transformer can be as good as, or even better than your Lundahl output transformers.
This is an easy task for someone like Jack Elliano of Electra Print Audio, or other custom winders out there.
Just my opinions.
Have a pair of Auto transformers custom wound.
The laminations should be interleaved E / I, the ratio of turns is 100 / 71.
The full 100% windings are connected to the 8 Ohm Lundahl secondaries, and the 71% windings are connected to your 4 Ohm speakers.
The performance of such an Auto transformer can be as good as, or even better than your Lundahl output transformers.
This is an easy task for someone like Jack Elliano of Electra Print Audio, or other custom winders out there.
Just my opinions.
Which speakers do you have? Maybe they are actually higher in impedance than their nominal rating.
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Most loudspeakers have lower impedance at some frequencies versus their nominal ratings.
Take an Ohmmeter or DMM and measure the DCR of the speaker terminals.
The impedance at very low frequency, well below the ported tuning, or well below the closed box woofer resonance is about the same as the DCR.
The impedance at the impedance dip of a ported tuned speaker port tuning is about the same as the DCR.
The impedance in the range somewhere from about 150 to 400Hz is about the same as the DCR.
All Generalizations Have Exceptions
Take an Ohmmeter or DMM and measure the DCR of the speaker terminals.
The impedance at very low frequency, well below the ported tuning, or well below the closed box woofer resonance is about the same as the DCR.
The impedance at the impedance dip of a ported tuned speaker port tuning is about the same as the DCR.
The impedance in the range somewhere from about 150 to 400Hz is about the same as the DCR.
All Generalizations Have Exceptions
Just speculating. A 4 ohm speaker needs about 1.4 times more current and 0.7 times the voltage vs. a 8 ohm speaker, for the same output power. The other thing to consider is the output impedance of the amplifier. If there is no global negative feedback, then the amplifier designed for 8 ohm provides higher output impedance. But a global negative feedback makes the difference non issue.
It is not a transmission line, where impedance matching is important for maximum power transfer and minimum reflexions. The output impedance of the amplifier is not constant, neither is that of the loudspeaker. Impedance matching in a tube amplifier is not so important, IMHO.
I would use the amplifier as is, and don't overdrive it. If you badly overdrive it, the anodes might turn red, but then it is very loud!
It is not a transmission line, where impedance matching is important for maximum power transfer and minimum reflexions. The output impedance of the amplifier is not constant, neither is that of the loudspeaker. Impedance matching in a tube amplifier is not so important, IMHO.
I would use the amplifier as is, and don't overdrive it. If you badly overdrive it, the anodes might turn red, but then it is very loud!
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santitruco,
Ask your self . . .
How good or how bad does your combination of 8 Ohm tap, and 4 Ohm speaker sound?
And can it play loud enough to satisfy you, before it sounds distorted?
Are you spending more time worrying about the sound, versus the time you spend enjoying your playback system?
Unless your single ended 300B amplifier has too much plate voltage, and/or too much plate current,
then I doubt that you would ever have a red-plate on the 300B, no matter how loud you play it.
But if you drive the amplifier real hard, it will sound more like a guitar amplifier on-overload,
versus the sound of a Hi Fi playback amplifier.
I have a pair of single ended 3k Electra-Print Audio output transformers. They have Jack's early 4 sections secondaries; they can be hard wired for 1 Ohm, 4 Ohm, 9 Ohm, or 16 Ohm output tap.
If I could only find the one that is "hiding" from me, we could talk about me sending the pair to you.
I really have to clean out and clear out the downstairs room. I am "downsizing".
I can say that the shipping would be the biggest expense.
Would you be interested?
Let's make a deal.
Happy Listening!
Ask your self . . .
How good or how bad does your combination of 8 Ohm tap, and 4 Ohm speaker sound?
And can it play loud enough to satisfy you, before it sounds distorted?
Are you spending more time worrying about the sound, versus the time you spend enjoying your playback system?
Unless your single ended 300B amplifier has too much plate voltage, and/or too much plate current,
then I doubt that you would ever have a red-plate on the 300B, no matter how loud you play it.
But if you drive the amplifier real hard, it will sound more like a guitar amplifier on-overload,
versus the sound of a Hi Fi playback amplifier.
I have a pair of single ended 3k Electra-Print Audio output transformers. They have Jack's early 4 sections secondaries; they can be hard wired for 1 Ohm, 4 Ohm, 9 Ohm, or 16 Ohm output tap.
If I could only find the one that is "hiding" from me, we could talk about me sending the pair to you.
I really have to clean out and clear out the downstairs room. I am "downsizing".
I can say that the shipping would be the biggest expense.
Would you be interested?
Let's make a deal.
Happy Listening!
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An SE amp isn’t going to red plate if somewhat mismatched. But it will put out less undistorted power than it would if it were matched. Push pull AB are less sensitive to this as the current is free to go up until hitting some sort of limit. If it sounds fine to you and plays loud enough just listen and enjoy. You can always buy the “right” transformer later and you’ll have a way to compare what the “right” one buys you.
I talked to a recognized winder here in Arg and he proposed this to me.
The LUNDAHL are good trafos and I think it would be best to keep them in their post. The optimal solution that I propose is to make a pair of transformers that adapt from 8 to 4 ohms.
It may seem strange to you, but the truth is that they have been used normally in these cases, people have used them to couple old Hi end line transistors with only 8 ohms output, to 4 ohms speakers.
As these transformers have a very low transformation ratio, the high response easily reaches 80/100,000c/s, meaning that the harmonic structure remains intact.
At the low end, it is determined by the type and size of the core, and since this is about transferring full power with a minimum of insertion loss, the core must have room for thick wires since it is a current of 1 .42 amps in the case of 8 watts into 4 ohms. With a good core, I can secure a low end of 15 cycles with ease, and since there is no direct current flowing through the windings, it adds absolutely no distortion. In short, it is absolutely transparent, it perfectly complies with the impedance matching, and allows you to keep the original equipment.
If you are interested in the proposal, I will calculate a couple of these for you..
Is this a good option ? Is better the autotransformer ?
Santiago
The LUNDAHL are good trafos and I think it would be best to keep them in their post. The optimal solution that I propose is to make a pair of transformers that adapt from 8 to 4 ohms.
It may seem strange to you, but the truth is that they have been used normally in these cases, people have used them to couple old Hi end line transistors with only 8 ohms output, to 4 ohms speakers.
As these transformers have a very low transformation ratio, the high response easily reaches 80/100,000c/s, meaning that the harmonic structure remains intact.
At the low end, it is determined by the type and size of the core, and since this is about transferring full power with a minimum of insertion loss, the core must have room for thick wires since it is a current of 1 .42 amps in the case of 8 watts into 4 ohms. With a good core, I can secure a low end of 15 cycles with ease, and since there is no direct current flowing through the windings, it adds absolutely no distortion. In short, it is absolutely transparent, it perfectly complies with the impedance matching, and allows you to keep the original equipment.
If you are interested in the proposal, I will calculate a couple of these for you..
Is this a good option ? Is better the autotransformer ?
Santiago