Systemic Development > Amplification and Preamplification

Tube circuit question

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tmazz:
I was reading a thread on another board from a guy who had a hum problem with a tube phono stage that he had just picked up on the used market.

After looking into all kinds of things he finally found that the DC from the power supply was wired backwards to the tube heater circuit. He reversed the polarity to the proper way and the hum disappeared.

My question to you tube design gurus out there is why should this have made a difference. The filament is not in the signal path and all it does is provide the heat needed to operated the tube, so why should it be polarity sensitive?

From a high level look I don't see why , but I am sure there is something I am missing here.

Anyone have any ideas?

BobM:
I don't know enough about tube design to properly answer this, but I have found that the ground plane on a phono stage is uber sensitive and implementing it wrong results in hum. A bad solder join, a loose connection, etc .... hmmmmmm.

tmazz:
That's a good point Bob. I wonder if the problem was not so much the polarity as it was the connection and in then process of doing the lead swap the hum went away not so much because of the polarity but because the connections were done properly.

A number of years back I had an intermittent hum problem with my Moscode 600. Brought it over to Scott and he found the problem in no time flat, a cold solder joint on the underside of the board. Heated it up to reflow the solder and it has run like a champ ever since.

Still wondering if any of ur tube design experts had any thought ? Steve?  Bill?

Response Audio:
That is very interesting but as mentioned, it may have likely been a bad connection or proximity issue. That's not to say polarity didn't have an effect. Without knowing the circuit layout and filament/power supply designs, it's hard to say for sure.  What tubes are used in the unit?

tmazz:

--- Quote from: Response Audio on May 11, 2021, 03:52:59 PM ---That is very interesting but as mentioned, it may have likely been a bad connection or proximity issue. That's not to say polarity didn't have an effect. Without knowing the circuit layout and filament/power supply designs, it's hard to say for sure.  What tubes are used in the unit?

--- End quote ---

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