Running Filaments on 12VDC and getting hum should I try AC and artificial center Tap?

[nicksorenson]

I built a small bass preamp using a 12ax7 with a Fender style tone stack and 2 gain stages.

I used an old power amp with a Toroidal transformer (with High voltage and 12 volt heater voltage from individual isolated taps for the 2 voltage levels) as the supply. It's got 160 Volts DC after I full wave bridge rectified for the tubes and 13VDC or so full wave bridged for the heaters. I grounded the heater supply to the main power supply ground.

I get hum while the power supply to the filaments is connected. When I disconnect the hum goes away.

I've always worked on amps with a center tap transformer for the main and filament power I don't know much about using a DC power supply for filaments. How do I deal with the hum issue and where/how do I ground the filament supply? Should I use AC instead and artificially center tap it by the 1 ohm resistors trick?

thanks!
Nick

[daddario]

How big is the cap for the filament supply?

[nicksorenson]

How big is the cap for the filament supply?

I believe it was a 2200uF at 25V.

[loudthud]

You said you disconnected the heater supply and the hum went away. Was something connected in it's place? Tubes don't amplify unless they have something to light up the filaments. Try connecting a 12V battery. If it still hums, the hum isn't coming from the heater supply. Is there a safety ground wire connected to the mains? How much filtering is there on the B+? 160V isn't much for B+, consider a voltage doubler.

[nicksorenson]

You said you disconnected the heater supply and the hum went away. Was something connected in it's place? Tubes don't amplify unless they have something to light up the filaments. ...

Yeah, I temporarily (as in for a second or two) disconnected the heater supply. The hum went away the second the wire was disconnected. The tube still conducts for around 5 seconds as the heater cools. But yes it was still on. I did this as a diagnostic to see if the hum came from the filament. It seems like it did.

[tedmich]

I built a small bass preamp using a 12ax7 with a Fender style tone stack and 2 gain stages.

I used an old power amp with a Toroidal transformer (with High voltage and 12 volt heater voltage from individual isolated taps for the 2 voltage levels) as the supply. It's got 160 Volts DC after I full wave bridge rectified for the tubes and 13VDC or so full wave bridged for the heaters. I grounded the heater supply to the main power supply ground.

I get hum while the power supply to the filaments is connected. When I disconnect the hum goes away.

I've always worked on amps with a center tap transformer for the main and filament power I don't know much about using a DC power supply for filaments. How do I deal with the hum issue and where/how do I ground the filament supply? Should I use AC instead and artificially center tap it by the 1 ohm resistors trick?

thanks!
Nick

The battery trick is a good one as its pure DC; if it still hums the problem is elsewhere; lead dress, shielding, transformer coupling. Also I have recently been using the Duncan Amps PS design tool; its very helpful when designing a PS from scratch! Now I know my new PS with increased caps will come up to full voltage after ~20s, allowing the heaters to warm a bit without a standby..

[loudthud]

Yeah, I temporarily (as in for a second or two) disconnected the heater supply. The hum went away the second the wire was disconnected. The tube still conducts for around 5 seconds as the heater cools. But yes it was still on. I did this as a diagnostic to see if the hum came from the filament. It seems like it did.

Good technique. You need some kind of ground reference but don't let the current flow through the chassis or signal ground. In other words connect the wires from the transformer to the bridge, then to the filter cap, then to the tube socket. A single wire should connect to ground from the filter cap. You'll need atleast two stages of filtering on the B+. Again, wires from the transformer to the bridge, from the bridge to the 1st filter, then to the second filter. Connect the second filter's ground to the chassis. If the torroid has a mounting bolt thru the center, only let one end touch the chassis. If both ends touch, it's a shorted turn!

[nicksorenson]

It seems like grounding the filament supply is where my problem is. Do I just connect it to my B+ Ground or leave it isolated to it's own supply? i.e. do I just leave the two power supply grounds un connected/independent of one another?

[loudthud]

If you leave the heater supply ungrounded, you'll probably be able to measure a substantial AC voltage from either side of the heater supply to ground with a DVM. This is just a leakage from the transformer but tubes will pick it up big time.

It's not too critical where the heater supply gets it's ground as long as it's not between the B+ bridge and the B+ filter caps. Experiment once you knock down the majority of the hum to see if you can make any improvement.

[daddario]

Hmm... 12AC after recifing should make about 16V under load. That 13V is weird.
From what amp did you get that PT?