Why does a different OT soound so different? - diyAudio
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Old 12th January 2014, 06:24 PM   #1
diyAudio Member
Join Date: Mar 2013
Location: Orange County, CA
Default Why does a different OT soound so different?

I've had a Weber OT on my breadboard for all my testing (their "Deluxe" replacement, 6600 ohm CT/8 ohm sec), but wanted to use a used one I had kicking around, from a Hammond organ, its amp had 2 6V6s, so I tried it. Man it sounded louder, and cleaner, and the amp squealed above 7 on vol pot. This OT is about 2/3 the size of the Weber, unknown prim impedance (but had 2 6V6s on it), and 3 secondary leads (8/4 ohm?) originally wired to two 10" spkrs in parallel. I'd been running the 8 ohm Weber into 4 ohms (so that gives me 6600/2=3300 apparent primary?). Tried both secondary options on the little Hammond, not much different, but I like it! Does it would better because it a) is smaller; b) was on 6V6s running only 300 plate volts (I'm running 380v); or c) the impedance is considerably different? DC resistance on the Weber primary was 136/129 ohms, the Hammond 243/274). Would that relate to an impedance of roughly 10K or more? Thanks!
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Old 12th January 2014, 10:28 PM   #2
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Why is the Hammond wire resistance much higher than the Weber?

Not saying that the two primary impedances are the same, but if they are what does the above answer tell you?

What else could be different in the construction of the transformers?

Search how to test transformer impedance and test each.
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Old 13th January 2014, 12:55 AM   #3
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OK....higher winding resistance means more wire, more windings? And if it's impedance is the same as another xfmr, that's because the secondary has more windings as well (so the winding ratio is the same)? But the DC plate current didn't change with the Hammond vs the Weber....does the tube compensate (a constant current source?)? And the higher DC resistance Hammond was louder! I have done some testing, using low AC voltages to determine AC voltage ratio, and that impedance ratio is then that ratio squared.... But that was at 60Hz (used a Variac). Tried a SigGen, plotted induced voltage and current, but it made no sense. Need to revisit. But THANKS for asking good questions!
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Old 13th January 2014, 06:26 AM   #4
sesebe is offline sesebe  Romania
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I'm beginer in tube but why one measurement is 136/129 (=1.054) and another one is 243/274 (=0.887)? I mean for first measurement first no. is biger comparing with the second and for the second measurement first no. is smaler comparing with the second no. You measure from diferent pins?
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Old 13th January 2014, 07:11 AM   #5
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
The impedance is only a matter of load and turns ratio, the DC resistance has nothing to do with impedance. One may measure twice the resistance of the other, but you cannot infer impedance from that.

Why would one have different resistance? wire size and wire length are what determines resistance. Any piece of wire has some resistance per foot. And so many feet of wire in the winding. Calculate the resistance.

If both the primary and secondary have twice the windings, the ratio is the same. So more windings of the same wire means higher resistance. But you also might have the same number of windings, but thinner wire, and have higher resistance. And core size matters. The windings go around the core, and a smaller core means shorter windings, and that lowers the resistance. You cannot assume two transformers with the same impedance ratings are both wound with the same size wire.

As to why it sounds different, many factors. The DC resistance might make an audible difference, sure. But impedance is not precise, especially in a guitar amp, so even if both transformers are meant for pairs of 6V6, we cannot assume they are of the same turns ratio. There is a range of acceptable transformer impedance.

many transformer windings are not perfectly balanced, so the resistance of one half differs from the other half. This can be due to winding size. One turn about the core in a center winding is a shorter wire length than a winding about the outside of the coil, so it will have lower resistance. Depending upon how they are wound, some transformers are very close in resistance, while some have substantial differences.
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Old 13th January 2014, 04:01 PM   #6
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To SESEBE: To answer your questions (we all were newbies once!): The "136/129" refers to the primary winding of a center-tapped (CT) xfmr, the values being the DC resistance from the CT to each winding end. The ratio is not really useful, but the two resistances should be fairly close (for push-pull power tube balance). You measure this from the tube socket pin for the tube's plate (tube usually removed) and the xfmr center tap lead (usually connected to the power supply B+). I'm still learning about how little this relates to IMPEDANCE, and how it affects the sound and tonality.
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Old 21st January 2014, 11:01 PM   #7
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at a guess if your Hammond trany is physically smaller (designed for less watts) its probably going into saturation where the weber is staying clean. but even the different materials can make a big difference just like in a microphone or a speaker.

ps check this page out for how to get impedance
Fat Tubes For Fat Sounds
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