Self correcting hum on 18W Plexi

Johnny-B

I’m having a problem with an 18W Plexi kit I built in 2015. For a short while after power up, it hums and then corrects. The power supply drops from its nominal 340Vdc to about 280Vdc and the sawtooth ac component rises from typically 15Vp-p to 60V p-p, thus creating a power supply hum.
Is it likely to be the 6CA4EH rectifier tube or a smoothing capacitor?

Yeti

I'm really just starting out on valve amps and don't have any trouble shooting experience so take with a suitable pinch of salt. My gut feeling is that the rectifier tube is having trouble charging up the filter caps.
This would explain the noise, the low voltage and the fact that it returns to normal after a period of time.

Maybe to taking the other tubes out to limit the current draw and then try again.

petercornell

Hi Johnny
60v p-p sounds excessive. Hard to believe the caps are already going being only a few years old. Could be either of the components.
Are you handy with electronics? Remember SAFETY FIRST in a valve amp
Try taking out the rectifier and putting in a couple of diodes. Remember the voltage will be higher now, so check if your filter caps can handle the higher voltage.
What is the value of the smoothing cap? One way to try is to tack in a new cap across the old one and see if that helps.
If I can help, let me know, I have parts on hand.
Peter

petercornell

Sorry, obviously you are au fait with electronics, you built the amp in the first place!

Johnny-B

Mystery finally solved! The length of time for which the dc power supply voltage reduced and the amp had a mains buzz continued to increase. The amp played fine all day Saturday while I practiced for Sunday service but when I switch on at church at 7am on Sunday morning, it buzzed and died. On investigation, the 1500 ohm 10W resistor first in the dc power supply chain was open circuit, so I replaced it. Resistors, of course, don't usually burn out for no reason so I suspected something else was wrong. When I checked voltages, I was dropping over 300V through the resistor 1500 ohm which meant that something was drawing heavy current. With only the rectifier tube in circuit, all power supply voltages looked fine. After re-installing only the preamp tubes, all voltages still looked fine. When I installed the power tubes, they drew the heavy current, dropping the dc power supply voltage.
I took the two EL84s up to JD kustom in Woodmead and they kindly tested them without charging me. One tube was faulty. Shout out for JD. I ordered two new JJ EL84s from Tone Tubes in PE last Thursday and they were delivered by courier to my home this afternoon. Shout out for John Astbury at Tonetubes. I plugged in the new tubes and the 18W Plexi is again playing beautifully.
So it was a run-away power tube that was causing the dc power supply to drop.