Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Peter212 on September 15, 2022, 08:40:20 am
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Recently I worked on a homemade amp with a single 6V6. Skipping the unnecessary details, I noticed the output transformer was marked as 500 - 4 - 8. Quick DC test showed the primary at 40 ohms... :w2: Further research yielded the Triad line transformer S-65X with the primary impedance of 500 ohms, secondary 4 and 8. Now, how in the world this thing is working just fine with a transformer that's clearly not designed for tube amps.
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Impedance is not the same as DC resistance. Impedance is ‘resistance to AC’, and measuring DC resistance on a transformer winding won’t give you the reflected impedance. You need to pass AC through one of the transformer windings while measuring AC on each other winding - in order to figure out how much impedance the OT will reflect.
so the OT is probably fine (if it’s a singled ended OT designed for a single 6V6). You just need to test it the right way.
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I'm clear on DC resistance versus impedance. The specs clearly states the input impedance is 500 ohms. The DC resistance is only 40 ohms. Reading the specs of 6V6, plate load, this transformer should yield really crappy results. Instead, it sounds great
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Have you measured maximum clean power output? I'd expect maybe 1 Watt instead of 4 Watts. This may not be obvious without a direct A/B test against a conventionally loaded 6V6 on the same speaker.
Or put in a standard Champ/ValveJr OT and played?
DC Ohms is imperfect guide to Audio impedance but 5X to 10X is often a fair bet. The sheet says 37 Ohms DC so we would be thinking 185 to 370 ohms. But it is a very small core and factory rated for 300Hz, so a 13X multiplier may be correct.
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Call it 1 Watt at 13% THD
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It's definitely more than 1w.
As for distortion, unless cranked up all the way, my imperfect ears could not detect any. I am curious who and how came up with such a design. I've seen all sorts of weird tube amp design solutions , this one being the weirdest.
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Some folks have found that line transformers can do the job of output transformer. There are videos on it explaining it better than I could. They're cheaper than a typical OT too and tinkerers sometimes like saving money. I can't tell if you are asking a rhetorical question or if you are looking for an answer. How many 1 watt amps have you heard? I've been surprised at how loud my 1/3 watt amp is. All I could tell ya is there's always going to be a circuit that surprises me out there, enjoy the surprise!