Tube circuit question

tmazz · April 19, 2021, 04:56:41 PM

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 · April 20, 2021, 05:18:14 AM

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 · April 20, 2021, 10:27:57 AM

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 · 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?

tmazz · May 11, 2021, 05:04:49 PM

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?

12AX7s

steve · May 13, 2021, 09:38:25 AM

Quote from: tmazz on April 20, 2021, 10:27:57 AM
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?

Hi T,

When I designed my personal phono stage, the filament circuit was completely isolated from everything except for one connection at one point. There are no common lines of filament/power/signal grounds.

That one connection occurs when floating the filament circuit above ground with the resistor divider network to provide a positive filament to cathode potential. (Stay within the filament to cathode voltage limits.)
Even then, location of the resistor grounding is crucial.

But as previously mentioned, I also do not know the circuitry/layout of "your" particular situation.

cheers and hope this helps.

steve

P.I. · May 13, 2021, 04:56:36 PM

Quote from: steve on May 13, 2021, 09:38:25 AM
Quote from: tmazz on April 20, 2021, 10:27:57 AM
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?

Hi T,

When I designed my personal phono stage, the filament circuit was completely isolated from everything except for one connection at one point. There are no common lines of filament to power/signal grounds.

That one connection occurs when floating the filament circuit above ground with the resistor divider network to provide a positive filament to cathode potential. (Stay within the filament to cathode voltage limits.)
Even then, location of the resistor grounding is crucial.

But as previously mentioned, I also do not know the circuitry/layout of "your" particular situation.

cheers and hope this helps.

steve

In many ways circuit design is lie real estate: Location, location, location. I know: been there, done that.

tmazz · May 13, 2021, 10:15:09 PM

Thanks for you input guys. I kinda fatigued that this would be more involved than just a quick simple answer.

I do not know much about the particular amp that had this problem. I don't even personally know the guy who owns it. He was just a guy I was chatting with online.

My interest in the topic was strictly from a point of intellectual curiosity.

Thanks again for sharing your ideas.