Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rdrgtr on September 22, 2011, 11:07:15 pm
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I have this old piece of iron that I have been tripping over the last few years. It is an output tranny out of a leslie that is used to drive a 16 ohm load with 2 6550 tubes. I am thinking about using it in my next build. The amp will be a plexi type build so a single 16 ohm output is a-ok with me. All of my cabs are 16 ohm. I am trying to learn a little theory with this tranny. I put a 1v ac signal into the secondary and measured 17 vac out. Now, with that said, my 1v ac was measured with the signal generator hooked up to the transformer. There will be voltage drop from the signal generator right? I did my calculations and came up with a projected 22.5 ohm with a pair of el34's. Would I be better served to use 6550's instead? A projected 13 ohm load with the 6550's. There has to be some tolerance with these loads. People change tubes all the time without considering changing out the iron. Am I worrying to much?
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17^2 * 16 is Z primary. 4624ohms with a 16ohm speaker.
--DL
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So, The el34's would like to see 6,500 ohms in AB1 or AB2. Technically, the 6550's (3500 ohms) would be a better match for this chunk of iron. Correct?
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You could easily run 2 x EL34s in PP with 4k load resistance
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So, The el34's would like to see 6,500 ohms in AB1 or AB2. Technically, the 6550's (3500 ohms) would be a better match for this chunk of iron. Correct?
It took me some time to realize that most guitar amps don't have a perfect impedance match.
Sometimes even greater than the 20% tolerance you see so often in a schematic.
Some times a perfect match wont sound as rich as a mis-match.
Efficiency does go down a tad, but it's all about tone.
Ray
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Ok. With the 17^2*8= 2312 ohms...Would I be pushing it with an 8 ohm load with 2 el34's?
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marshall 2xEL34 amp uses 3K4 load, bassman & super reverb use 4k load. you have 4k6 - it will work work with EL34, 6L6, 6550, KT66, KT88, etc..
copy the very nice leslie output stage - glue marshall or fender pre & PI.
--DL
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I took it a step further. I hooked up a toa impedence meter to the primary and a 16 ohm speaker to the secondary. I read 6k. I then hooked up an 8 ohm speaker and read a little less that 4k. I am not sure how accurate this meter is. I guess it is only measuring the total resistance to the 1k tone that the meter is providing. I think I trust the 17:1 ratio method more than the analog meter...
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I put a 1v ac signal into the secondary and measured 17 vac out.
what frequency? 60Hz? 100Hz? 1KHz?
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It was 1k also. I used a HP tone oscillator. What Hz should be used?
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I just checked the ratio with a 60hz tone. I measured 1 volt in with the transformer hooked up. As I shifted the freq. higher the voltage climbed way up. With 1v in the ratio was basically the same.
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Don't forget about power-handling capacity. Many of those Leslie amps used a low supply voltage, which means the OT primary impedance is lower for a given output power. If you're planning to use a higher supply voltage, you could exceed the power rating of the OT. On the other hand, OTs for guitar amps can be smaller because they don't have to reproduce low frequencies.
To test the turns ratio, 1kHz is good (or any mid-frequency). Avoid very low or very high frequencies. Measure the input voltage across the transformer leads and the output voltage. As already stated, the impedance ratio is the square of the turns ratio.