It is an average anode current due to intermodulation. It has nothing to do with cathode power and temperature. It is like adding AC bias. Do you like intermodulation of your music with 120 (or 100 Hz) AC? Fine, but I don't.
Similarly, some people believe that PP amps don't need so good B+ filtration like SE amps need. It is wrong as well. Yes, in pauses when both tubes draw the same current, hum is balanced in output transformer. But when music plays, this hum is being added to the music, and intermodulates as well, since output tubes are non-linear. Do you like it? I don't.
The inverse result if the positive of DC heater is grounded, so I could say that the valve is overpowered...
Underpowered/overpowered due to DC heaters are both BS. No offence.
What about of a lot of harmonics? Have you some experimental evidence or some detailed calculation?
Last edited:
Headphone amps are the worst case scenario because there is no listening room that masks some (musical) signal. In other words hearing resolution is best and the DC heater supply will be the winner hands down.....
Link to info and raw plots and jig photos relating to post #128 measurement data. The photo of the underside of the chassis shows a LED replacing the cathode resistor - the next set of tests I did.
12AX7 heater leakage test info.zip
12AX7 heater cathode resistance (Rhk) characteristics for 10 tubes are shown in Fig.4 in Hum article.pdf with test rig details in Ref [6] of that article Heater-cathode conduction plots.pdf
12AX7 heater leakage test info.zip
12AX7 heater cathode resistance (Rhk) characteristics for 10 tubes are shown in Fig.4 in Hum article.pdf with test rig details in Ref [6] of that article Heater-cathode conduction plots.pdf
I prefer DC, but let's be honest for a big EL34 power amp, who wants to build an 8 amp DC power supply just for the heaters? Not me. So I used DC for my preamp and will likely use it for a headphone amp. DC also works out really well when you use a goofy transformer that doesn't have a 6.3 or 12V winding, but does have a 15V. Rectify, resistors to drop the voltage a bit, then a regulator.
Yeah, but if I were to say change from 12AU7 tubes to 12BH7, I wouldn't wan't my heater voltage to change that much due to the change in load. It's very much situation dependent.
I have not had an EL34 that had hum when using AC filaments. I measure hum from the 8 Ohm tap into 8 Ohm load. Using indirect heated cathode output tubes, I have had hum as low as 100uV.
AC filaments on a DHT does cause hum, and does cause intermodulation. I most often use DC filaments there.
AC filaments on a DHT does cause hum, and does cause intermodulation. I most often use DC filaments there.
I've used the open ones, but usually they are enclosed in a metal case. One of my systems uses an ATX supply for a computer. It also powers boost converters fro B+.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.