Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: thetragichero on October 29, 2021, 04:13:53 pm
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i've got a 2x6v6 build that briefly motorboats while warming up (i'm impatient so i strike a chord until i hear it). it goes away almost immediately. has a bridge rectifier and a fairly large choke (9H) before the screens and rest of the power supply... is this just caused by the caps not being fully charged? not a huge problem but i'm thinking a small film cap across either the plate or screen electro cap might eliminate this?
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Are you sure one of the existing HT caps isn’t a bit dodgy?
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i suppose anything is possible but it only happens for a second or two and then everything's fine
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okay finally got a chance to go under the hood and do some experimenting. does not seem to be a filter cap issue, but what DID get the brief motorboating to disappear was putting a large resistor (first tried 1M, then 100k) from the plate node to the screen node, basically bypassing the choke. so i think that at startup a combination of barely right-sized power transformer (it's a torroidal transformer i believe intended for a tube record player), bridge rectifier, and big choke (9h) don't allow the screen filter cap to charge quickly enough when slamming an E chord as soon as it's flipped on. so i'm assuming the bypass resistor allows just enough through until the choke is fully energized, at which point it's effectively out of the circuit (100 ohm or so resistance of the choke is much more enticing a path than a 100k resistor)
is does this theory hold water?
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> when slamming an E chord as soon as it's flipped on.
Chill out! Sip a beer. Wink at the hotties.
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Try some different PI tubes.