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In Reply to: SET amp hum posted by JeffR on November 8, 2006 at 17:48:29:
I believe these amps use an AC heater supply. If that's truly the case, then you cannot completely eliminate the hum. It's not a defect, just part of the design. Many of Ron's amps came with AC heaters, the hum can be adjusted, but I've never completely eliminated it on my Moondogs.You could always modify the heater to run off of DC.
Follow Ups:
That being said, someone had customized my Moondogs when I bought them and had A/C on the heaters. They were humming. I changed them back to DC on the heaters and now they are pretty darn quiet. Almost no hum at all.
Cut-Throat
No, the Moondogs used an AC heater. I built mine from the kit and there is no rectification on the heater supply. It's just a tap off of the power transformer.If someone "customized" your amps to run with the AC heater, you were lied to. BTW - concensus is that the AC heater sounds better.
Well, can you tell me the purpose of "BR1" on my Moondog schematic?This was not here when I bought my Moondogs. (BTW - I was not lied to at all, I was just trying to tame the hum) So I added BR1, as outlined in my instructions and schematic.
Cut-Throat
Look at the green 2.5v leads. They lead (with a paralelled hum pot) directly to the heater of the 2A3 via pins 1 and 4. This is the AC heater supply I am referring to.The rectifed signal is used for the 6sn7s, but not the output tube.
I've got 2 schematics (separated by 2 years) for the Moondog and both show the rectified low level supply (BR1), so I am curious why someone thought removing it would be a good thing. Whenever I've seen reference to an AC heater, it always been discussed in context of the output tubes.
Yes, the 5.5VDC supply would be for the 6.3V tubes, not the 2.5V tube. But it is most assuredly a heater supply. 6SN7s have heaters, which is the proper terminology for the filament inside an indirectly heated tube that is separate from, but heats the cathode. DHTs like a 2A3 have a filament which also function as the cathode.AC heaters and insufficient B+ ripple reduction of driver and input stages can both cause hum that is often mistakenly blamed on AC filaments of DHTs.
There are too many different applications and gain structures possible to suggest only AC or only DC supplies as a definitive method for all cases. And there are a lot of different ways to build each.
Well, that's what I was told. Anyway mine came without BR1 installed. So I hooked it up and it tamed quite a bit of hum.BTW - If yours still hum, they can be made 'dead silent' - I use 104db speakers too. Try lifting the ground on your Moondogs with a cheater plug and see if the remaining hum disappears. If it does, you'll know you have a ground loop problem and can work on that to remove the remaining hum. That was the last source of hum on mine.
I was convinced that it was not ground loop hum until I tried it. But indeed it was, slight but it was there!
Cut-Throat
Yes, ac heaters make the situation more difficult, but it should not be an excuse. The amp I built has ac heating too. No hum.I would not recommend switching to dc heating as it messes up the sonics.
"I would not recommend switching to dc heating as it messes up the sonics."Don Allen is an advocate of DC heating.... He's converted several amplifiers from AC to DC heating, and I personally did not notice any degradation of sonics. And it got rid of the hum problems.
Or at least mine. In speaking with Don I think he prefers AC on 45 tubes but does use DC on 300b tubes. At least that is what I got out of talking with him.I know what your talking about when you mention how hes done amp converts to run DC in removing hum. Hes mentioned this in conversation. With Don its, as he would say, "whatever sounds good". No magic formula for any and all.
Lovin this 45 amp now, just amazing!
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.
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 !!
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.
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!
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.
> > 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.
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.
I'll talk to Don tomorrow. I thought he said he uses DC on the filaments, but my memory could be wrong.I also want to say I'm trying out some Fritz Hemptone tower speakers, which utilize those new hemp full-range drivers. Mated with the Don Allen modified Sansui receiver and Cakepan amp, I've may have struck sonic gold.... It's one of those rare occasions where I could sense the plaster in the concert hall, the booze on a singer's breath, the complex harmonic richness of a piano, the wooden body resonance of an acoustic guitar, the hair-raising complexity and precision of a world-class drummer, and the massed sound of acoustic instruments of a symphony orchestra..... (No, it does not sound as good as the Who concert.)
I like my banana fiber speakers better --- I use fostex speakers
and 6B4G's. I had to use simple DC on my SET amp -- the 6B4G's
humm way to much on AC. From what I have seen on the net, every
body sugests constant current sounds better than DC heating.
If I had rebuild my amp I am thinking of simple transistor/9.1 volt
zener ,regulator for 8.4 volts @ 1.1 amps and have a 2 1 ohm resitors
give me pesudo constant current to my 6B4G's. ( Note that is
6.2 volts @ 1 amp but still with in ratings of the tube )
I use about 60 MA but have a 3.4K output transformer, rather than
say a 2.5K one.
Todd,What mods did Don do on your Sansui? Which model?
Gary
It was the same unit he was using for a while- A Sansui TA-300. He got another unit and sold his to me. I spent about six months waiting for one to show up on eBay, and the one time it came up, I got "out-sniped". (I was one of two people who "snipe bid" on the unit, right at the end of the auction, but the other guy bid higher than I did.) Then Don offered me his unit when he got another TA-300 to work on. He told me he could do other models, and maybe Harman/Kardon units as well. I think the TA-300 has the same tuner as the TU-717.Don disabled the stock line stage and power amp sections of the unit, and put in the same line stage he uses in his dedicated line stage, transforming the solid-state receiver to a tube (actully hybrid) tuner-preamp. So all it would need is a power amp, and it's good to go. I got the unit because it's nice to have a good tuner and great line stage in one convenient package.
Thanks, Todd.
I did speak to him today. The filament supply depends on the individual unit. My particular amp was converted to DC filaments when Don transformed the amp from a 300B amp to a 45 amp. Don also told me he's not certain that DC filaments degrade sound.
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