Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on March 09, 2013, 08:42:07 pm
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I know this is a silly question but if you had a big enough 6.3V transformer with 120V and 240V input taps could you theoretically use it as a 5.8K:4 PP OT?
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Theoretically ( or in the interest of learning... ) it certainly will work.
When I was an impoverished student in the 1960's there were many old valve televisions being scrapped, so I built guitar amplifiers using two TV power transformers - one as a power transformer, the other as an output transformer. The centre-tapped HT secondary became the output transformer primary, while various combinations of the multiple 6.3V and 5V heater windings became the secondary.
These amplifiers were really heavy, and they didn't sound very good - although the lack of sound quality may not have been solely due to the output transformers . . .
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Hey, I can dig it man. Most my stuff is out of something else. Sewing machine carring case, repourposed military metal boxes and a couple of TV transformers, etc. Wish I was doing this when everyone was scrapin stuff. I love making amps outta things that weren't meant to be amps. Just think, you were recycling before it was cool to "go green".
As far as heavy, I get it. I've used reporpused transformers that I actually don't have a good idea of what a proper sized transformer should look like. Some times when I see a 6V6 amp somewhere I laugh and think to myself "look at the tiny little transformers". All my 6V6 amp's transformers are twice that size.
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Sure, but:
Working at 240V 60Hz, the transformer wants to distort.
When driven by a MegaWatt electric company, the distortion is mild and we really don't care.
When driven by a skinny vacuum tube, the distortion can be severe.
Inversely proportional to frequency. At 82Hz you can hit it 1.36 times higher voltage for the same distortion.
Proportional to voltage. For pretty-clean output at power frequency you want the plate swing to be much less than design voltage. Even 1/3rd of design. So "120VAC" is good for about 40VAC at 60Hz. That's 56V peak. Adding tube losses we might aim at 80V-120V of B+. This is quite low by our standards.
Look another way. A "6VAC" winding will distort bad at 6V (60Hz) or 8V (82Hz). For real-clean we drop to 1.5V-2V. That much voltage in 4 ohms is 0.5W-1W of clean power. 9W-16W very-bent power. NOT a large power.
In special cases (harmonica, ocarina, small female singers) you might aim at 250Hz. Then you can apply much higher voltage and stay clean.
Aside from all that: power transformers are fairly sure to pass 50/60Hz to about 400hz (rectifier harmonics). Anything more is incidental, and not really a bonus for power use. Simple all-in-one winding often goes past 5KHz. Two-bobbin windings often fall off before 1KHz.