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 Post subject: Re: Chasing hummmm
PostPosted: 01 Oct 2018, 20:53 
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Joined: 04 Jun 2008, 20:59
Posts: 3854
Location: Arizona, USA
Hi, Looking at the photo ....I would use shielded cable from the inputs to the board. I find that any time you need to go more than about 2-3 inches from the inputs it ought to be shielded from the input jacks to the circuit board connection. Remember to connect the shield to only one end unless there is no alternate signal ground path. My preferred way is to connect the shield to the ground side of the input jack and run a separate ground wire from there to the circuit board ground connection. That way you get the benefits of the shield but avoid the chance of creating a ground loop (and hum).

The power supply generally looks OK to me. It might not be filtered enough for the 300B though. Singled ended output stages have pretty much no immunity to power supply hum. That is one reason I like push-pull ones. The heater circuit is probably ok. It is typical of DHT ones. Regulation may help there though. The hum itself will almost always be either at the AC mans frequency (50 or 60 HZ) or at twice that (100 or 120 HZ). In the first case it will be signal / ground / shielding related and in the second case it will be power supply related. It is important to figure out which you have (could be both). Fixes to the power supply will not cure problems with the signal side nor will fixing signal problems help with respect to power supply hum.

Good listening
Bruce

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 Post subject: Re: Chasing hummmm
PostPosted: 03 Oct 2018, 04:40 
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Joined: 13 Jan 2018, 21:33
Posts: 93
Location: australia
Thanks Bruce, already have the shielded cables cut and trimmed to give a try. Will solder it in soon. I also got the courage to check the heater voltages myself after having a guitar amp tech look at it. He may have overlooked the 300b heaters? I found the 300b voltages a bit low whereas I would have expected them to be a bit high due the the rectification. Thinking I needed to drop voltage from the 6v secondaries of the filament transformer to 5 volts ac I used 1 ohm 5 watt wirewound resistors for a bit of trial and error. The actual resistance is more like 1.2 ohms so voltage at the edge connector terminals is 4.5 volts ac. I am only measuring 3.5 dc across pins 1 and 4 so something doesnt look right. I will get a few lower value resistors to experiment with to find the goldilocks one. All the other heaters seem good.

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 Post subject: Re: Chasing hummmm
PostPosted: 03 Oct 2018, 21:44 
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Joined: 09 Oct 2012, 19:43
Posts: 318
Location: Vancouver Canada
I notice from the pic you have the HT wires all bunched together from source to destination. Have you tried to route the drive HT close as possible to the metal top and front end HT away from the metal. (threw the air when cover is off but close to the bottom plate when covered)? Then route 300B heater wires separate from those. It helps to lessen any injection from HT into heater wiring and vis-versa.


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 Post subject: Re: Chasing hummmm
PostPosted: 04 Oct 2018, 16:20 
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Joined: 13 Jan 2018, 21:33
Posts: 93
Location: australia
Thanks Laurie. I could have planned the layout a bit better. I'll give that a try. There's always the next amp. Though the pcb layout does limit available choices.

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 Post subject: Re: Chasing hummmm
PostPosted: 06 Oct 2018, 06:23 
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Joined: 13 Jan 2018, 21:33
Posts: 93
Location: australia
swapped over twisted wire with shielded wire. i think the hum’s about the same level. i noticed there’s a ripple sound coming from the mains transformer outside the chassis. wonder whether this would be injecting the hum. got the heater voltages close now at 4.6 volts.

would a different combination of filter capacitors help?

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