Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bnwitt on September 09, 2016, 10:27:14 am
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I noticed that two different resistor values are shown in the Hoffman AB763 PDF file for the resistor which grounds the negative feedback. One is 47 ohms and the other 100 ohms. I doubt this makes much difference but it's curious as I see that this value also varies between the different models of AB763 amps (Bandmaster, Deluxe Reverb, Super Reverb etc..) Was it just incomplete schematic updates that made this happen?
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Check out the AB763 Showman schematic (http://el34world.com/charts/Schematics/files/fender/Fender_Showman-AB763-schematic.pdf) for context.
When Fender's amps had a single speaker (8Ω load), they used a 47Ω resistor. When the amp used multiple speakers (most often 2x speakers for a 4Ω load), they used a 100Ω resistor.
This partly compensates the feedback for a drop in source voltage at the speaker when a lower load is used. For example, a 40w 2x 6L6 output stage with an 8Ω load will have √(40w * 8Ω) = ~17.9v RMS present at the speaker as a source for the feedback loop. The same amp with a 4Ω load will have √(40w * 4Ω) = ~12.6v RMS, or 1/√2 as-much source voltage.
The lower 47Ω resistor doubles the applied feedback voltage to offset the 1/√2 as-much source voltage with the lower load. You might argue a better match of feedback for the two cases would be if Fender used something closer to 70.7Ω (like 68Ω). However, it does give an exactly-equivalent amount of feedback with the 2Ω load of the Super Reverb... Perhaps Fender was looking to only stock 2 values of resistors for that spot, and didn't figure exact matching justified the cost of using a third value.
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Thanks HPB. Never too old am I to learn something new. :worthy1: