Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on May 26, 2025, 08:07:15 pm

Title: 800 circuit power section hum help!
Post by: joesatch on May 26, 2025, 08:07:15 pm: ok i have tried many things. currently i have both preamp tubes removed and the input to the P.I. from the master disconnected. reservior cap is grounded to chassis along with the PT CT and power tube grounds. Heater is artificially grounded to chassis with two 100R resistors (i have tried removing this as a test no difference). B+1 grounded to chassis along with bias ground, OT ground, presence ground. With the master connected to the P.I. i get good signal and preamp section works fine but the hum is there.
Title: Re: 800 circuit power section hum help!
Post by: AlNewman on May 26, 2025, 08:22:37 pm: Sounds like you have your thumb on the input cable.
Title: Re: 800 circuit power section hum help!
Post by: HotBluePlates on May 27, 2025, 06:25:49 pm: Quote from: joesatch on May 26, 2025, 08:07:15 pm
... currently i have both preamp tubes removed and the input to the P.I. from the master disconnected. ...

What happens if the Phase Inverter tube is removed?
Title: Re: 800 circuit power section hum help!
Post by: joesatch on May 27, 2025, 08:42:05 pm: noise goes away if the PI tube is removed. this is 60 cycle hum correct? so it's in the audio path? PI input is disconnected.
Title: Re: 800 circuit power section hum help!
Post by: mresistor on May 28, 2025, 12:15:36 pm: Sounds like 60hz to me. Check all grounds for good connection. could be injected into signal via a problem with small bottle tube. So have you substituted all the tubes?
Title: Re: 800 circuit power section hum help!
Post by: joesatch on May 28, 2025, 02:19:19 pm: i pulled the board. i believe it's conductive. built too many amps and this is a simple circuit. must be the board. i'll put a turret board in
Title: Re: 800 circuit power section hum help!
Post by: joesatch on May 29, 2025, 08:55:50 am: :huh:
Title: Re: 800 circuit power section hum help!
Post by: danhei on May 29, 2025, 08:16:56 pm: Quote from: joesatch on May 28, 2025, 02:19:19 pm
i pulled the board. i believe it's conductive. built too many amps and this is a simple circuit. must be the board. i'll put a turret board in

Why do you believe the board is conductive? That doesn’t seem particularly likely for what appears to be a fiberglass/epoxy PCB.
Title: Re: 800 circuit power section hum help!
Post by: joesatch on May 30, 2025, 06:31:36 am: entire bottom of the board has a ground plane around all holes
Title: Re: 800 circuit power section hum help!
Post by: pdf64 on May 30, 2025, 07:35:24 am: Quote from: joesatch on May 30, 2025, 06:31:36 am
entire bottom of the board has a ground plane around all holes
Are you using it as a shield, by only making a single connection between the chassis and ground plane, or are you using it as a conductor for 0V common, with 2 or more connections to it?
Whatever, the likelihood of a good question board substrate becoming conductive seems low, much lower than other possible causes of hum.
Title: Re: 800 circuit power section hum help!
Post by: joesatch on May 30, 2025, 09:06:53 am: spent too many hours troubleshooting the hum. i ripped the board out will build a turret board over the weekend and be done with it