davis419b Posted November 28, 2015 Share Posted November 28, 2015 My tube amps have a 4 ohm and a 8 ohm tap but they are not marked. How can I tell which is which ? Quote Link to comment Share on other sites More sharing options...
mike stehr Posted November 28, 2015 Share Posted November 28, 2015 You can pull the bottom plate and look at the secondary taps to see what color the wires are, or you can measure the resistance between each tap and ground. 1 Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 I tried measuring each tap and ground and they measured the same 5 ohm which doesn't make sense. I guess it is possible I am using the wrong setting on my ohm meter. Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 I have used the Dodd Audio 120wpc monoblocks in my system for a long time. They use EL34's. I just recently purchased a newer set of Dodd Audio 150wpc monoblocks which use the KT77's. Using the marked 8 ohm taps they sounded much brighter which I don't like. I put the EL34's in thinking that would help but it did not. When I got the 120's Dodd told me to use the outside taps and I have always been satisfied. I would ask Gary these questions but he is no longer with us. (RIP) On the 150's the 8 ohm is the inside tap which makes me wonder if I have been using the 4 ohm tap on the 120's all this time and didn't know it. I had myself convinced that the extra power would not change the sound but it has. Quote Link to comment Share on other sites More sharing options...
jorjen Posted November 28, 2015 Share Posted November 28, 2015 My tube amps have a 4 ohm and a 8 ohm tap but they are not marked. How can I tell which is which ? Is the negative tap/terminal identified? If so, most amps I have seen over many years start out with the 4 ohm tap closest to the negative terminal, then 8 ohm, 16, etc. Oh crud, never mind. I just noticed in your photo that it appears the negative terminal is in the middle. So much for that idea, sheesh. Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 My tube amps have a 4 ohm and a 8 ohm tap but they are not marked. How can I tell which is which ? Is the negative tap/terminal identified? If so, most amps I have seen over many years start out with the 4 ohm tap closest to the negative terminal, then 8 ohm, 16, etc. Oh crud, never mind. I just noticed in your photo that it appears the negative terminal is in the middle. So much for that idea, sheesh. My pCats are that way but I have never tried anything but the 8 ohm tap. Quote Link to comment Share on other sites More sharing options...
Jim Naseum Posted November 28, 2015 Share Posted November 28, 2015 I tried measuring each tap and ground and they measured the same 5 ohm which doesn't make sense. I guess it is possible I am using the wrong setting on my ohm meter. I think you need more precision than 1 digit. Your meter should be able to read 5.108 or 5.673 and so on. I believe the 4ohm tap has less turns of wire than the 8 ohm, so it will have a slightly lower DCR. 1 Quote Link to comment Share on other sites More sharing options...
jorjen Posted November 28, 2015 Share Posted November 28, 2015 My tube amps have a 4 ohm and a 8 ohm tap but they are not marked. How can I tell which is which ? Is the negative tap/terminal identified? If so, most amps I have seen over many years start out with the 4 ohm tap closest to the negative terminal, then 8 ohm, 16, etc. Oh crud, never mind. I just noticed in your photo that it appears the negative terminal is in the middle. So much for that idea, sheesh. My pCats are that way but I have never tried anything but the 8 ohm tap. The sound always strikes me as being thinner with my Jube Clones on 4 ohm taps. I found this to be the opposite with my stock Khorns if memory serves. My pCats were this way and so are my VRD's. 8 ohm taps sound much more pleasing with my current load. Better bottom end, fullness/richness and more balance from top to bottom on the 8 ohm taps. It will not hurt the amps to experiment. Select the tap that sounds more "right" to you. I see no need to go through measuring, curiosity aside. I think I will go throw the Clones on the 4 ohm taps on the VRD's for a listen just because it has been awhile. You never know. Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 My tube amps have a 4 ohm and a 8 ohm tap but they are not marked. How can I tell which is which ? Is the negative tap/terminal identified? If so, most amps I have seen over many years start out with the 4 ohm tap closest to the negative terminal, then 8 ohm, 16, etc. Oh crud, never mind. I just noticed in your photo that it appears the negative terminal is in the middle. So much for that idea, sheesh. My pCats are that way but I have never tried anything but the 8 ohm tap. The sound always strikes me as being thinner with my Jube Clones on 4 ohm taps. I found this to be the opposite with my stock Khorns if memory serves. My pCats were this way and so are my VRD's. 8 ohm taps sound much more pleasing with my current load. Better bottom end, fullness/richness and more balance from top to bottom on the 8 ohm taps. It will not hurt the amps to experiment. Select the tap that sounds more "right" to you. I see no need to go through measuring, curiosity aside. I think I will go throw the Clones on the 4 ohm taps on the VRD's for a listen just because it has been awhile. You never know. Harshness in the highs is what I don't like. The newer amps on the 8 ohm tap is very harsh to my ears. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 28, 2015 Share Posted November 28, 2015 I tried measuring each tap and ground and they measured the same 5 ohm which doesn't make sense. I guess it is possible I am using the wrong setting on my ohm meter. 5 ohms doesn't sound right, the DC resistance of that winding should be much smaller. I would make a safe assumption in that the global feedback is being taken from the highest tap i.e. 8 ohms. Open her up and see which tap has the feedback connected to it. Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 I tried measuring each tap and ground and they measured the same 5 ohm which doesn't make sense. I guess it is possible I am using the wrong setting on my ohm meter. 5 ohms doesn't sound right, the DC resistance of that winding should be much smaller. I would make a safe assumption in that the global feedback is being taken from the highest tap i.e. 8 ohms. Open her up and see which tap has the feedback connected to it. The meter reads .5 ohms not 5, which doesn't make sense . Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 28, 2015 Share Posted November 28, 2015 .5 ohms makes much more sense than 5 ohms. 1 Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 .5 ohms makes much more sense than 5 ohms. Why would both the 4 and 8 ohm taps read the same ? Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 28, 2015 Share Posted November 28, 2015 (edited) Your meter might not have the resolution to read the difference. What meter is it? Use the lowest resistance setting for better resolution. For example your meter say .5 for both taps on one setting but on another it might read 508 for one and 585 for the other. EDIT: Basically you should set your meter to read milliohms if you can. Edited November 28, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 28, 2015 Author Share Posted November 28, 2015 Your meter might not have the resolution to read the difference. What meter is it? Use the lowest resistance setting for better resolution. For example your meter say .5 for both taps on one setting but on another it might read 508 for one and 585 for the other. I tried it with the same ohm setting on a K-33 woofer and it read 3.7 so I figured it was set richt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 28, 2015 Share Posted November 28, 2015 (edited) I tried it with the same ohm setting on a K-33 woofer and it read 3.7 so I figured it was set richt. You are measuring two different devices, one is a transducer (k-33 woofer) and the other is a transformer (output of your tube amp). When a tube amp has speaker taps of "4 ohm" and "8 ohm" that is what load they want to see not the actual resistance of that transformers winding. Edited November 28, 2015 by xxJPMxx 1 Quote Link to comment Share on other sites More sharing options...
Jim Naseum Posted November 29, 2015 Share Posted November 29, 2015 When you measure OHMS across a transformer winding all you are measuring is the actual DC resistance of the WIRE in the winding. The meaning of "4 ohms" on an amplifier though is an IMPEDANCE, not a RESISTANCE. Impedance must consider a vector of both voltage and current in an AC signal. A "transformer" then, "transforms" a high impedance of the power tube to the low impedance of a speaker. It does this by magnetically coupling a big winding on the power tube to a smaller winding on the output tap. Since the signal is AC is tightly couples from one winding into the other. This "transformation" makes the voltage lower and the current higher at the output tap. 1 Quote Link to comment Share on other sites More sharing options...
davis419b Posted November 30, 2015 Author Share Posted November 30, 2015 Thanks to everyone for your info and advice. After switching to the 4ohm taps and trying every preamp in the house I definitely like 4ohm vrs 8ohm ! Quote Link to comment Share on other sites More sharing options...
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