Having an issue with the power supply in a Marshall tube amp after installing a new set of EL34 tubes. . During biasing I measured the plate voltage after letting the amp warm up for 15 - 20 min, volume down with a 4 ohm load (speaker) connected, it measures 470 volts to start than drops to 448 after a half hour or so, seem s the longer I leave it on the lower it goes. Just measured it again this evening, and it dropped and went back up to 467 at some point. I am thinking the power supply caps might be leaky, any input would be greatly appreciated.
Not leaky caps. Are you also monitoring the mains voltage? You have 120vAC mains, your B+ is not quite 480vDC. That is numerically about 4 to 1. And that means for every volt your mains changes, your B+ changes 4v. So if the mains moves 5 volts - something it does all the time - then your B+ will change 20 volts.
Just a possibility.
Leaky caps would leave you with excess ripple and thus hum.
Just a possibility.
Leaky caps would leave you with excess ripple and thus hum.
Enzo, Thanks I did not think of that, it is summer and the the voltage on the primary of the input transformer is likely fluctuating, I will check that. Thanks so much for the reply.
To satisfy yourself that there is no problem lurking, measure the anode current to make sure it is stable.
> measure the anode current to make sure it is stable.
Or approximately proportional to wall-socket sag.
I have 125V at the street, but I am 500 feet back on a skinny wire. While my A/C (the usual Big Load in summer) is tiny, when the dryer and hot-water run I am down to 113V, and can dip to 108 if other loads happen. This would be a much bigger change of B+ than your 470V-448V. Which is only say 125V to 119 wall-voltage.
Or approximately proportional to wall-socket sag.
I have 125V at the street, but I am 500 feet back on a skinny wire. While my A/C (the usual Big Load in summer) is tiny, when the dryer and hot-water run I am down to 113V, and can dip to 108 if other loads happen. This would be a much bigger change of B+ than your 470V-448V. Which is only say 125V to 119 wall-voltage.
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