Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on May 31, 2017, 06:03:36 pm
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Negative feedback variable control
I'm taking a 250K pot and installing a 100k resistor from leg 1 to leg 3.
Then taking the #2 leg on to the circuit .
When I measure, it only goes up to 74k. Should I add more to get to 100?
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Remember with resistors in parallel the highest value it can get will always be lower than the lowest value, so the 74k is lower than the 100k you put on it. If you need over a specific value, you need a resistor over that, but you can also calculate resistors in parallel via this formula:
1/Rt = 1/R1 + 1/R2 ... + 1/Rn
in your case:
1/Rt = 1/250,000 + 1/100,000 = 71.42k
You could use an online calculator like this to get what you want:
https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/ (https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/)
~Phil
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Very cool and I worked it out. Thanks!
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Negative feedback variable control
I'm taking a 250K pot and installing a 100k resistor from leg 1 to leg 3.
Then taking the #2 leg on to the circuit .
When I measure, it only goes up to 74k. Should I add more to get to 100?
In hi gain amps this is a very good tone tweak that is often over looked . The Splawn, Quickrod used a 220k NFB resister so I will use a 250k pot. Ive found from putting a few into the amp. That once I find the sweet spot it stays and doesn't get changed so I dial it in when Im getting ready to stick it in the cab. Then find the closest resister and put that in. I don't put adjustable NFB pots in anymore. That being said if Im building an amp for someone I will use there guitar and speakers to dial it Into the sweet spot. Speakers pickups and transformers all play a roll in where the sweet spot is. So put this in your arsenal of things to use.
Bill
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I ended up using a 220k with the 250k pot. Seems to be ok and measures 112k. I have a practice tonight and I'll know better then how it sounds.