Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: capn_starsky on April 29, 2020, 10:13:02 am
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Good morning again! More research into parts from a gutted amp and I've come across another item I'll nee a little assistance with. The output transformer secondary taps are all connected to one of those terminals and they're unlabeled both on the transformer itself and on the schematic for the amp they were robbed from. I do know that they're 8 and 16 ohm secondaries but I'm not 100 percent sure which is which. The colors are black, green, and orange. I'm assuming the black is speaker return and I've input a small voltage from black to each of the other two and measured on the primary side. Is the higher voltage from the 16 or the 8 ohm tap? I ask because I'm overthinking the math side of it. I figured going the other way, fewer turns == fewer volts, so smaller of the two? Thanks again everyone. I'm also trying to wrap my head around the turns ratio and what exactly it means, but I've got Uncle Doug's video queued up for later on that subject.
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with a good meter you should be able to put 6VAC on primary, ballpark a 30:1 step-down for something close to 200mV ac on secondary's. so 8ohms steps down LESS than 4ohms - I think :laugh:
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Is the higher voltage from the 16 or the 8 ohm tap?
The higher voltage on the primary is from the 8 Ohm tap. If BLK is really common, the 8 Ohm tap will make a voltage on the primary that is about 1.4 times larger than the voltage made by the 16 Ohm tap. If one voltage is way higher than 1.4 times the other, then BLK is not common or the transformer is not 8 and 16 Ohms.
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Thanks again for everyone's help! Feeding 1 VAC in to the secondary taps and measuring, my orange gave me 15.35 and the green was 21.56...1.4 times higher. Figuring the impedance ration and using plate to plate impedance for EL34s like this amp had in it, I came out with 16 and 8 as expected on the two with black being common. Can't wait to share progress when I actually get parts in and hopefully shouldn't have too many bugs to work out afterwards.