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In Reply to: Don does use AC on 45 based amps posted by 2chJunkie on November 11, 2006 at 10:17:33:
Hi yall,Here is the deal folks, with a 45 or 2A3, you have a 2.5 VAC filament voltage and you CAN use AC, which DOES sound better than DC, on those tubes.
With a 300B you have a 5 VAC filament voltage, too high for AC, so you are FORCED to go with DC, a compromise. But then, a 300B tube always was and IS a compromise in many ways to a 2A3, so, the best SE amp you can build is not a 300B, but a 2A3, run at lower current, about 45 mA instead of 60 mA., and with AC heating on the 2A3. Its HARD, no, impossible, to beat a GOOD 2A3 amp folks.
If your speaker requires more power than a 2A3 can provide, junk em and get more efficient ones....smell de roses.
Jeff Medwin
Follow Ups:
One of the problems with dc on a filament is the unbalanced loading. That is, one side of the filament/cathode runs at higher current than the other. The dc across it shifts operating point. What you get is a compound tube that averages somewhere in the middle.Running ac is similar in that the two ends of the cathode swing hi/lo relative to the center. The average dissipation across the cathode, however, is constant. There are no so-called hot spots.
That is why they can sound different.
Yes, 5V 300B is 6dB more likely to hum than a 2A3. But still I think within reason if you design well. I once tried to make an ultrasonic sinewave balanced heater circuit (Royer type), but it failed. Turns out the circuit is only good for reactive loads.
jh
PS - Interestingly I ended up biasing my 2A3 at 50mA. I thought everyone ran them at 60.
Listen to 45 mA with 250 VDC p-k Jim. Run tubes at 5/8ths to 2/3rds dissipation for least stressed, most relaxed and free sound, NOT STRAINED sounding. Dennis Fraker's mantra it is !!Jeff Medwin
I went too low. I'm down at about 230V across the tube.jh
Jim, since there always has to be a potential difference at the two ends of a heater (and therefore the cathode), at any given point along its length the voltage with respect to the grid must also vary accordingly. As you have pointed out for DC, even with AC at any given moment the voltage will also be different and one could argue that it's even worse because it's modulating the cathode to grid voltage from point to point along the HC’s length, so not only should it cause hum but also modulate the operating point.To my mind the very fact that it's possible to null out hum in AC fed DHTs suggests that this potential difference isn't a problem in reality. HC to grid distances aren’t perfectly uniform and aren’t some filaments spiral shaped anyway? In other words the average is all that counts. And if indeed this was detrimental to sonics then we would all be better off with indirectly heated tubes anyway (not). So perhaps this micro analysis isn't really important in the scheme of things? Perhaps all we are doing is trying to find theories to support our individual beliefs as to why AC is better than DC or vice versa, but arriving at incorrect (and impossible to prove) conclusions? Just a theory.
Rgds,
Naz
I am not a expert in design, but I do know that on my Morrow 300B amps changing from AC to DC removed the hum and improved the sound. I also have DC on my Don Allen 45 amps which has the best sound of any 45 amp I have ever heard. My guess is that it has a lot to do with how the amp is designed.
> > With a 300B you have a 5 VAC filament voltage, too high for AC, so you are FORCED to go with DC, a compromise < <Rubbish Jeff!!! Why is it too high for AC? And again ... not to mention your other comments!
Naz
DUUHH,Because 5 VAC is two times higher than 2.5 VAC, and because 300Bs are sonic junk compared to 2A3s if you don't need the extra power of a 300B on your load.
"In the 1940s and 1950s EVERYBODY in audio who "knew their stuff" knew NOT to use a 300B." My mentor, born 1925, always told me that, and certainly wouldn't let me build with a 300B.
Nothing has changed, except we forget good lessons from the past. In 2006, change "EVERYBODY" to "most" informed people. The King is not wearing any clothes Naz.
Jeff Medwin
> > Because 5 VAC is two times higher than 2.5 VACAm I missing something, what has this to do with > > "With a 300B you have a 5 VAC filament voltage, too high for AC, so you are FORCED to go with DC, a compromise" < < ??? You CAN use AC for 300Bs, period!
As far as your sweeping statement goes "300Bs are sonic junk in comparison to 2A3s" ... careful, you might upset well respected Thorsten who incidentally also seems to hold the same view about 845s in comparison to 300Bs! Who's right ... nothing personal but on this point, neither of you IMHO and that's not a put down on 2A3s. Each of these tubes can sound wonderful.
Naz
I think the spacing of the 2A3 and the trannie in the photo of that prototype are layed-out a bit too close to each other, such that fields from the trannie will bend electron fields inside the output tube. I usually like two inches or more spacing.Jeff Medwin
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