On Saturday, 8 December 2012 12:49:38 UTC+11, Alex Pogossov wrote:
"John L Stewart" wrote in message ... Steve Bench published an excellant cure for the AC DHT filament hum problem more than 10 years ago. See it at this link- http://diyaudioprojects.com/mirror/m...ch/humbal.html Have to say I'm not in favor of running the filaments on DC. Just another complication to get in the way of reliability.. Altho in some instances we gotta do to. Cheers, Joh Though steve's method of hum compensation by injecting rectified (double mains frequency) signal works, he is not (completely) right about the mechanism of the hum second harmonic generation. He attributes the 120Hz hum to the filament temperature modulation. Though it might contribute in smaller tubes, the main cause is different -- it is transconductance nonlinearity, the law of 3/2. If at the pek of the AC filament voltage, say one end of the perfectly AC balanced filament goes down by -3V and the second end goes up by +3V, the emission current from the first end increases by a LARGER AMOUNT than emission decreases on the second edge -- it is due to the nonlinear current-vs-grid voltage characteristic (law of 3/2). So at the peaks of the filament supply total emission is always higher. Thus we have a frequency doubling effect.. This effect will be most pronounced if the tube is driven to cut-off. According to Steve, at cut-off point there would be no plate current, regardless of temperature modulation. In reality, if one end of the filament goes negative, this end starts conducting (emitting), and the other end is still not conducting (deeper into cutoff). When the filament polarity reverses, the ends swap roles. So at zero instantaneous filament voltage the trube is not conducting, but at either peak one or the other end of the filament causes the plate current to flow. In a way it is a frequency doubler using "anode-bend" effect. To check which effect (temperature modulation or "anode-bend rectification") contributes most, one needs to check the phase of the 2F hum. If it is in phase with the mains -- it is anode bend effect, if it is delayed, it is temperature modulation. Temperature is 90 degrees lagging behind because of the thermal mass (inertia) of the filament. Regards, Alex
Maybe your'e right about this, and in SE amps with say one OP tube and with horn speakers of sensitivity 95dB/W/M, maybe hum could be heard if you don't use a hum nulling pot to at least minimise it. In PP amps with 2 x OP tubes and two heater supplies, perhaps DH cathode hum is more easily nulled and CMRR tends to supress hum across OPT primary because its applied by both tubes with same phase.
I won't be mucking about adding 2H signals any time soon, and I WILL CONTINUE with use of DC heated cathodes to avoid any hint of modulation of audio with any hum. Its reliable for audiophiles who hate any adjustments and who invariably adjust things wrongly if its possible to do so. Meanwhile IDC = SILENCE when amp is turned on and warmed up, even without any GNFB. Its not been universally popular because its not damn well traditional, and sure wasn't done in 1928 when 300Bs came out to power the new fangled sound systems that ended the silent movie era. No body minded if there was hum, but many actors and actresses had terrible voices so they lost their jobs, and to to get a job youse 'ad ta speak proppa and cleer like, and loud with good self projecting quality because general bandwidth of any audio was so bloomin' attrocious, and no better than 78 records could offer. But by 1938, things were better, and certainly by 1958, although just how good sound was depended a bit in how many prints of prints and copies were made. I have heard some absolutely terrible digitized old movies put onto DVDs and played by the old local Australian Film and Sound Archives ppl in their outdoor screenings of things like the 1939 Thief Of Bagdad in nice COLOR, but shame about the sound. I recall going to local cinemas in Sydney in 1950s where they had GE gear and a huge horn speaker. I don't recall much hum, but sound was definately "tubey" and although there was distortion, sound seemed remarkably clear and firm. Some film tracks of 1940s movies were exceptionally good despite the SNR.
But I digress.
There's always going to be lousy audio quality spoilt by noise somewhere, and over last 18 years I have chased it out of amps constantly, as part of my living, and much to everyone's delight. Simple for me with DH tubes means a decent CRC rectifier, usually using Si diode bridge and two 15,000uF caps plus whatever R is needed to drop say 10Vdc to 5Vdc for a 300V or whatever. 845 present a good challenge because heaters are 3.3Amps at +10V, and an LC filter or CLC filter makes sense, depending on the available Vac winding and the taps so Vdc can be trimmed. Luckily, 15,000uF with high ripple ratings are cheap and can be paralleled easily. The luxury of this wasn't around in 1930s, but car batteries were, and so if you wanted to heat a 300 silently, you could use a 6.3V car battery with series R to trim voltage. Hafta gofor drive later to charge the battery maybe, and of course you would hafta, because after wooing GF with sweet 1930's smoozik from 78s, you'd hafta drive the GF home and then her chaparone, and you'd discuss what's the next best step might be to get married, because otherwize a bloke'd afta go without a root.
But I digress. I recall ppl were very very nice to each other before the wedding.
Patrick Turner.