Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: NotSure on July 01, 2026, 09:13:02 pm
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Built Dave Hunter's 2 Stroke amp a long time ago. I've always used the setting that uses the entire output winding but with only an 8 ohm speaker. With the Hammond 125SE output tranny this only puts 2500 ohms on the primary. BUT if I use a 16 ohm speaker that puts 5000 Ohms on the primary side. So can I push the limit on this with say a 6L6, 4200 Ohm max load resistance, or really pushing it for a KT66 with only 2200 Ohms max load resistance. BTW the KT66 with a Celection Creamback 16 ohm speaker sounds killer. Don't want to burn up a $50.00 tube. Any comments appreciated.
Dave
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... Don't want to burn up a $50.00 tube. ...
That probably won't happen.
... 16 ohm speaker that puts 5000 Ohms on the primary side. So can I push the limit on this with say a 6L6, 4200 Ohm max load resistance, or really pushing it for a KT66 with only 2200 Ohms max load resistance. ...
Try the amp with "16Ω speaker load" (5kΩ to Primary).
Try the amp with "8Ω speaker load" (2.5kΩ to Primary).
Whichever one is "louder" (assuming similar speaker sensitivity) is "the better load" for this amp, its voltage, etc.
The output transformer primary impedance just winds up setting a ratio of Voltage-change for the Current-change that happens when a signal is applied to the output tubes. The "right load" delivers maximum output power, but that's a matter of supply voltage & overall design.
Most of the time, the "wrong load" is just going to deliver less-power before distortion.
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Just connect the 8Ω speaker to the yellow wire. That's what Dave would do.
(https://www.hammfg.com/files/products/125se/125sesch.gif?v=1697662064)