I currently have an OT with 4 and 8 ohm secondaries. Is there a way to wire it with a 16 ohm option as well?
Thanks!
Jay K.
Increase Output Impedance
Moderators: pompeiisneaks, Colossal
Re: Increase Output Impedance
Sure. Just double the primary label and the two secondary labels will double also. You lose the 4Ω label. But there is no way to have 4, 8, and 16 ohm labels.
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Stevem
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Re: Increase Output Impedance
The closest you can get will be for a 12 ohm load.
What’s the primary impedance of this transformer stated as being?
What’s the primary impedance of this transformer stated as being?
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Increase Output Impedance
Transformers do not have impedances, they have ratios.
A transformer labeled as (for instance) 4.4k to 8 ohms does not force those impedances to happen. The "4.4K to 8 ohms" just tells you that if you happen to put 8 ohms on the secondary, the apparent load on the primary will be 4.4K ohms. For this same transformer, if you put a 6 ohm resistor on the secondary, the primary will have an impedance of (6/8)*4.4K = 3.3K.
If your transformer has listed output impedances of 4 and 8 ohms, no, you can't wire it for 16 ohms without that also changing the loading on the plates driving the primary. Comments from sluckey and stevem are kinda right, but have asterisks and footnotes to look at. If and only if the 4 and 8 ohm secondaries are independent windings, you can wire them in series to make an unchanged plate to plate load on your output tubes for 12 ohms. However, I've never seen OTs other than the Williamson OT and a couple of other random hifi OTs wound this way. In general, guitar OTs are wound with the 4 ohm being a tap on a bigger 8 ohm secondary, so you can't wire them in series like you might if they were separate. Sluckey is right that if you call the 8 ohm output "16 ohms" then it works, but the plate to plate impedance on your output tubes doubles too.
This is important because output tubes work best when they are loaded with a specific plate to plate impedance. In fact, this is kind of the whole point of having an output transformer. A pair of 6L6s (as a f'rinstance) produces the most power output when loaded with about 4K to 4.4k plate to plate. Load them at 6.6K and they produce less power output, but at a lower distortion. The hifi people use 6.6K for their 6L6s for lower distortion, the guitar amp guys use 4K to 4.4K for more power. So if you double the load impedance on the tubes, you get less measurable power out. Sluckey is right - there isn't any way to wire the typical output transformer up to get three output impedances. You're stuck with only two, 4 and 8 if you want the most power out.
A transformer labeled as (for instance) 4.4k to 8 ohms does not force those impedances to happen. The "4.4K to 8 ohms" just tells you that if you happen to put 8 ohms on the secondary, the apparent load on the primary will be 4.4K ohms. For this same transformer, if you put a 6 ohm resistor on the secondary, the primary will have an impedance of (6/8)*4.4K = 3.3K.
If your transformer has listed output impedances of 4 and 8 ohms, no, you can't wire it for 16 ohms without that also changing the loading on the plates driving the primary. Comments from sluckey and stevem are kinda right, but have asterisks and footnotes to look at. If and only if the 4 and 8 ohm secondaries are independent windings, you can wire them in series to make an unchanged plate to plate load on your output tubes for 12 ohms. However, I've never seen OTs other than the Williamson OT and a couple of other random hifi OTs wound this way. In general, guitar OTs are wound with the 4 ohm being a tap on a bigger 8 ohm secondary, so you can't wire them in series like you might if they were separate. Sluckey is right that if you call the 8 ohm output "16 ohms" then it works, but the plate to plate impedance on your output tubes doubles too.
This is important because output tubes work best when they are loaded with a specific plate to plate impedance. In fact, this is kind of the whole point of having an output transformer. A pair of 6L6s (as a f'rinstance) produces the most power output when loaded with about 4K to 4.4k plate to plate. Load them at 6.6K and they produce less power output, but at a lower distortion. The hifi people use 6.6K for their 6L6s for lower distortion, the guitar amp guys use 4K to 4.4K for more power. So if you double the load impedance on the tubes, you get less measurable power out. Sluckey is right - there isn't any way to wire the typical output transformer up to get three output impedances. You're stuck with only two, 4 and 8 if you want the most power out.
Re: Increase Output Impedance
One little point. You can't just add the impedances of the secondary windings. If you had a tapped secondary with 16, 8 and 4 Ohm taps, the 4 Ohm tap would be the center tap. Do the math(s). Calculate the Voltage needed for say 25W and 4, 8 and 16 Ohms. 25W @ 4 Ohms is 10Vrms. 25W @ 8Ohms is 14.142Vrms. 25W @ 16 Ohms is 20Vrms. If you could wire a 4 Ohm secondary in series with an 8 Ohm secondary, the load impedance that would reflect the same primary impedance would be 23.3 Ohms. (24,142V^2)/25W=23.3 Ohms.
Some old Hammond Output transformers have an unusual load wiring. There is a 4 Ohm secondary and an 8 Ohm secondary that is tapped at 4 Ohms. For a 4 Ohm load, you connect the 4 Ohm winding across the 4 Ohm tap on the other secondary and connect your 4 Ohm load to the 4 Ohm tap. For 8 Ohms, you just move the load to the 8 Ohm tap. For 16 Ohms, you must wire the 8 Ohm secondary in series with the 4 Ohm secondary and connect the load to the (if the 8 Ohm secondary is on top) 4 Ohm tap. If you connected to the 8 Ohm tap, the load impedance required to reflect the same impedance to the primary would be 23,3 Ohms.
Weber used to sell a transformer that would allow you to change impedance from 2 to 16 Ohms. Can't find it on their website just now.
Some old Hammond Output transformers have an unusual load wiring. There is a 4 Ohm secondary and an 8 Ohm secondary that is tapped at 4 Ohms. For a 4 Ohm load, you connect the 4 Ohm winding across the 4 Ohm tap on the other secondary and connect your 4 Ohm load to the 4 Ohm tap. For 8 Ohms, you just move the load to the 8 Ohm tap. For 16 Ohms, you must wire the 8 Ohm secondary in series with the 4 Ohm secondary and connect the load to the (if the 8 Ohm secondary is on top) 4 Ohm tap. If you connected to the 8 Ohm tap, the load impedance required to reflect the same impedance to the primary would be 23,3 Ohms.
Weber used to sell a transformer that would allow you to change impedance from 2 to 16 Ohms. Can't find it on their website just now.