Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AlNewman on January 12, 2024, 07:46:10 pm
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So I've heard the analogy of output transformers compared with bicycle gears and transmissions, which makes sense. But there are primaries on transformers with the same impedance which have different resistance on the output tubes. For instance, I have 2 similar transformers, both rated at 8.5k primaries, one with a resistance of say 350 ohms across each winding, and another with a resistance of say 150 ohms across each winding. One would be a 1750 PA, rated at 18 watts, the other a 1750 E, rated at 15 watts.
I'm curious to know how that effects the output wattage and clean headroom? In my limited experience, a lower DC impedance allows for higher voltage at the same current. Does that equate to higher clean wattage at the speaker?
I'm looking to build a pp marshall style amp with approximately 300V B+, and I'm wondering which OT would be better suited for more better results.
Thanks in advance.
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The resistance is just dead loss, like friction in the gearing. As long as it is "small" compared to the working load, the effect is small.
Higher resistance can be longer wire for deeper bass, or skinnier wire for less cost. They rarely take it so far that performance per dollar really sucks.
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I believe the 1750PA is the replacement for a Marshall 18w so it is "Mo Bettuh" for an 18 watt Marshall style amp it also has paper former instead of bobbin it's made like they used to make em
It is a beefy OT and sounds great in my 18w amp. It is open coil and the 1750E has end bells. I would say the 1750PA has larger guage wire in it and is probably heavier because of that.
Just my opinion and I've never used a 1750E.
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I have two 1750PA OTs. One is in my Sluckey Dual Lite and the other is in my 1974X clone. Both sound excellent if that helps.
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Bass response, that might come in handy. The 1750 PA is certainly a heftier piece of iron, and it is also slightly less expensive.
I have actually tried both in the same amp, mainly because I needed the room. I believe I thought I would see a much higher voltage, but ultimately I think the effect was negligible. I did get slightly more output at the speaker.
I'm always using these old radio transformers, so every bit of voltage I can squeeze out is a good thing. But in this case, since real estate isn't really an issue, and I have a fairly beefy PT, I might as well use the bigger transformer.
Plus, I have another project that the 1750E should work well in, where it uses the same footprint.
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... both rated at 8.5k primaries, one with a resistance of say 350 ohms across each winding, and another with a resistance of say 150 ohms across each winding. One would be a 1750 PA, rated at 18 watts, the other a 1750 E, rated at 15 watts. ...
The resistance is just dead loss, like friction in the gearing. As long as it is "small" compared to the working load, the effect is small.
Higher resistance can be longer wire for deeper bass, or skinnier wire for less cost. ...
Hammond 1750E (https://www.hammfg.com/files/parts/pdf/1750E.pdf) says 150Ω for each half-primary (300Ω total), and a Primary Inductance of 24H.
Hammond 1750PA (https://www.hammfg.com/files/parts/pdf/1750PA.pdf) says 229Ω + 335Ω (634Ω total), and a Primary Inductance of 70H.
1750PA has 2.1x the DCR, and 2.92x the primary inductance. Some of that increased inductance is due to the extra core-mass of the 1750PA, weighing in at 4lbs to the 1750E's 1.1lbs.
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Hmm, didn't even notice the inductance ratings on the primaries in the datasheet.