Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Miyagi_83 on February 08, 2022, 01:40:09 pm
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Hello there, builders, tweakers and gurus.
I've built a MOSFET tremolo following the schematic by 2deaf as posted here
https://el34world.com/Forum/index.php?topic=23811.msg256428#msg256428
It's nearly identical the differences being that
1. I used 1M resistors to grid leaks because I didn't have any 820k's,
2. the supply voltage is 250 V, not 400 V,
3. and it's in a cathode-biased amp.
It worked fine until I installed an intensity pot instead of a fixed resistor.
As I turned the intensity pot up towards the end of the dial, it started arcing inside the potentiometer and, eventually, fried the FETs.
Has anything like that happened to anyone? What could I do to prevent that from happening again? :w2:
Thanks in advance for any help.
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It worked fine until I installed an intensity pot instead of a fixed resistor.
As I turned the intensity pot up towards the end of the dial, it started arcing inside the potentiometer and, eventually, fried the FETs.
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Maybe you wired the pot as a variable resistor, rather than a potential divider?
Need a schematic rather than a text description to do much more than guess.
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Thanks for your reply, pdf64. I wired everything just like on the schematic posted by 2deaf, i.e. cap to the pot's wiper. I'm posting the schematic here for reference.
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the only way I see your fail is;
1 if the pot was bad and the wiper brought down the bottom leg of the pot to ground
2 the cap was bad allowing some of the -45vdc onto the wiper.
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Thank you for the suggestions, shooter. It must have been the pot then because the whole circuit worked perfectly well before I replaced a fixed source resistor with it :dontknow:
I'll get a new one and report back.
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1 if the pot was bad and the wiper brought down the bottom leg of the pot to ground
I disassembled the potentiometer and found that the connection between one of its legs and the resistive trace was burnt open. The way I understand it, that leg must have been loose leading to arcing. Does that make sense?
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that's a chicken an egg sorta ?
when current exceeds a resistors capability to contain it, well.... smoke :icon_biggrin:
I fixed complicated broke stuff for a living, spending time analyzing why it broke took away all the fun of......it's friggin working, i'm outta here :icon_biggrin:
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that's a chicken an egg sorta ?
when current exceeds a resistors capability to contain it, well.... smoke :icon_biggrin:
I fixed complicated broke stuff for a living, spending time analyzing why it broke took away all the fun of......it's friggin working, i'm outta here :icon_biggrin:
I see your point (I think :smiley:). I'm analyzing what happened to prevent it from happening again :smiley:
If that's at all possible :icon_biggrin:
So I'm just gonna replace the pot and retry launching the circuit. Stay tuned :smiley:
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Fixed.
I replaced the pot with a new one and it's working all right. I'll post some sound clips later if anyone's interested.
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Sound sample. Enjoy :smiley:
https://soundcloud.com/user-996907905/lnd150_tremolo_by_2deaf-aac
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:thumbsup: Nice Trem Miyagi (and nice recording)
Regards
Mirek
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:thumbsup: Nice Trem Miyagi (and nice recording)
Regards
Mirek
All the credit should go to 2deaf because he designed it and I just applied his design in my build :angel
Thank you nonetheless :thumbsup: