• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Do i have a problem with my DC filaments?

DAK808

Member
2012-05-27 7:34 pm
Hi, I made a 6vdc filament supply for my 2 x 6sn7 line stage preamp. It seems to work ok as now there is no hum from the AC on the filaments. The only problem is that there is a voltage offset on the 2 legs. One leg reads + 2.8v while the other leg reads - 3.3vdc. Here is a link to the schematic that I used, it is at the bottom of the page;
The Valve Wizard
Except I did not have any heaters on AC before for FWBR. Also, I am still in the "tweaking" stage and I have not adjusted the final voltage as I am running the preamp off a Variac with only 90 VAC on the primary. I probably will need to put in a series resistor to adjust the voltages to come in at a total of 6.3vdc. Any help is most appreciated. Thank you , 808
 
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If you leave your heater supply without a voltage reference (= a path to circuit ground) it will always try to find it on it's own. The only route thru which to find it is the tube cathode.

The tube cathode - heater filament system acts as a diode. If you give the heater a positive charge relative to the cathode, the diode is cut off.

A leaking current from heater to cathode may - in addition to injecting noise into the audio circuit - weaken the heater insulation over time.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi MrCurwen,
If you leave your heater supply without a voltage reference (= a path to circuit ground) it will always try to find it on it's own. The only route thru which to find it is the tube cathode.
In this case, there isn't a circuit that has been interrupted for current to flow. The heaters are part of their own completed circuit path. There is a ton of test equipment that operates on the premise of floating power supplies. For anything to seek a ground path, it must be charged by a source that does have a connection to that ground. I use that to my advantage for heater supplies as you've noted. Reverse biased thermionic diodes. As long as no one connects the center tap of the heater winding to chassis common, no problem. A similar thing happens if a hum balance control is introduced with a path to common. However, a completely floating heater supply can be connected through high impedance leakage currents in the transformer, tube sockets or the tubes themselves. Each tube possibly having it's own leakage source. The most likely path would be between two tubes. The common practice of biasing the heater supply positive swamps out any normal leakage currents. That way you know what the potential is on the heaters, most of the time anyway.
A leaking current from heater to cathode may - in addition to injecting noise into the audio circuit - weaken the heater insulation over time.
Absolutely! A few really good studies on this "wear-out mechanism". Some of these studies are fascinating to read.

-Chris
 

DAK808

Member
2012-05-27 7:34 pm
If you leave your heater supply without a voltage reference (= a path to circuit ground) it will always try to find it on it's own. The only route thru which to find it is the tube cathode.

The tube cathode - heater filament system acts as a diode. If you give the heater a positive charge relative to the cathode, the diode is cut off.

A leaking current from heater to cathode may - in addition to injecting noise into the audio circuit - weaken the heater insulation over time.

The heater circuit is center tapped and is therefore grounded. regards 808