# Lowering B+ on a 5F2A

Discussion in 'Shock Brother's DIY Amps' started by HBamps, Jun 3, 2020.

1. ### HBampsTele-Meister

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I just finished a 5F2A build and am getting about 370V for my B+1. I want to get that down to about 350 so I put a 300 ohm, 10 watt dropping resistor before the for filter cap. It worked but messing around with the amp for about 5 minutes that resistor was pretty darn hot.

I measured the voltage on either side of the resistor to get an idea how much it was dropping and got 355 and 342, so a 13 volt drop.

My calculations say that there's 43mA, hence only dissipating 0.559 watts. This seems very wrong. If it's not even 1 watt of dissipation, why is it so hot??

2. ### dan40Friend of Leo's

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Have you subtracted the voltage across the cathode resistor to find the actual plate voltage? You can also simply measure from the plate to cathode pin on the tube socket to get the same result. Please post this measurement as well as the cathode resistor value.

3. ### tubegeekFriend of Leo's

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Dunno. Your math is correct though. See how much AC your meter registers between those spots - maybe there is a fair amount of AC wave riding on that DC drop, and that isn't getting measured.

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4. ### HBampsTele-Meister

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I messed up and asked too quickly. I had it temporarily in there and now that it's placed and soldered properly, it's not too hot. Waiting on zener's to do this a better way but this will do for now.

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