Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jimmybjj on April 11, 2010, 12:45:26 am
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I have a champ aa764 circuit that I've installed a 6l6 in. I got a Hammond 125ese to try and take full advantage of having a larger tube in there. I am still unsure about OT's. I understand the OT is kinda like a bridge between the power tube and the speaker. What i don't understand is the impedance relationships and when to use what as far as taps go. The 125ese has 2,500/10,000 primary, this confuses me, is the transformer only capable of translating the 2500/10000 impedance to the power tube? If that is true, will this transformer not work for a 6l6, average load resistance of 5000ohms. Which secondary tap should I use for a 8 ohm speaker and the 6l6? It seems the more i read about this, the more confused i get. Thanks for any guidance and/or advise.
(http://lh3.ggpht.com/_yFex0loOVcw/S8FhsAn1rEI/AAAAAAAAAXQ/BFVUpKL6Zz8/s800/photo%284%29.jpg) (http://picasaweb.google.com/lh/photo/jzJEOXnHxzcHGVBUg5MCot4is4b4Gscn0zldSkK7W9A?feat=embedwebsite)
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On that OT chart, don't worry about the numbers on the left-hand side (primary).....it's just telling you the "range" of primary imp's available.
Look at the right-hand (secondary) chart #'s.
Each tap (secondary wire) has a list of choices.
1st determine what speaker imp you are using....let's say you have an 8Ω speaker.
Now, you determine which tube you are using (6L6) & what primary imp it "likes" (as you said, ~~5k)
So, look at the chart for an 8Ω load with a 5K (5000, or something close to that) directly underneath it.
The yellow tap has an 8Ω with the # 5000 under it..........so that's the tap you use.
IF you had a 6V6 (wanting a 10k primary) with the same 8Ω speaker, then you would use the Green tap....see the system?
HTH, Geezer
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Another way to look at it: transformers are ratio device. Its a way of converting high voltage to low voltage or high impedance to low impedance.
All they are saying is: the ratio between input and output stays constant, so you set the primary load by putting on a different load of speaker.
if you attached a 8ohm speaker to the white wire, you will have a 2500ohm primary load. 2500/8 = 312.
if you attached a 16ohm speaker to the white wire, you will have a 5000ohm primary load. 5000/16 = 312.
Select a different winding, you get a different ratio.
So... like geezer says: pick your primary impedance based on the power tube (say 10k) and you have an 8ohm speaker in the box: then select black and green taps for the secondary.
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thanks guys, things are a whole lot more clear now.