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Let's take two separate facts:
1. Most knowledgable amp builders say that AC heating of the output tubes sounds much better than DC.
2. Many commercially built SET amps use DC heating.
Would DC heating be an "automatic disqualification" for you? My guess is that many commercial amps are built using DC because the builder wants to play it safe when it comes to the amount of hum the amp produces. Comments? Thanks!
Gerry
Follow Ups:
I assume you have to mail order and can't hear the amp. Yes, DC is a play it safe design, but not a deal breaker IMO.
As I described in the thread just below, Craig Uthus of Eddie Current converted my Moth S2A3 from DC to ultra-high-frequency AC (40k Hz) on the heaters and it sounds way better--more organic and palpable with greater extension and low-end drive. And it's now absolutely dead quiet. With 99db coaxes, I can turn the stepped attenuator wide open and put my ear in the horn and hear no sound at all, as though the amp is off.
Edits: 07/24/11
Only if it had sufficient room inside the chassis to convert to ultrasonic heating. I can't stand the hum from mains-driven filaments, nor do I care for the flat sound (for lack of a better verbal description) of DC. Ultrasonic heating is the way to go.
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Buy Chinese. Bury freedom.
DC done right whoops the crap out of AC...who the heck wants their 105db horns humming like a busy housewife during those quiet passages and in between songs? There seems to be NO WAY to avoid hum with AC heating... especially with directly heated tubes...if anyone has any ideas, I'm all ears...
slownlo
I did buy an amp that`s DC heated, the Coincident Frankenstein MKII 300b monoblocks. This amplifier is quiet, very transparent and has superb tone. It must simply be a matter of design and implimentation that determines the sound quaility regardless of filament heating choice
Some of the better SET I have owned used DC. It can be done right ac is not the only way. I do not enjoy amps with hi noise levels. But I have heard owned great sounding SETs with either. I have owned over 50 SET amps alone not counting the many many other types. So I do have a lot of experience with sound quality's of different SET amplifiers. I do know that audio designers can be a highly biased lot ;) There way or the highway.
Hi!
Here is my view:
point 1. There are also knowledgable amplifier builders who prefer DC filaments in terms of sound over AC! I'm one of them, at least for tubes like 300B, 10Y, 801A, 211, 845. With 45, 46 or 2A3 I use AC.
point 2. commercial amps need to be as hum free as possible to avoid complaints.
Doing DC filaments right is quite expensive. Choke input supplies have been mentioned. Very good dedicated filament regulators like Rod's boards yield good results. I still prefer the full passive approach, dual chokes per filament in a LCL configuration. As for example in my latest 211 amp (see link below).
Best regards
Thomas
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http://vinylsavor.blogspot.com/
Yes, us commercial designers are is inclined to minimize the hum. AC heaters are OK for those who use speakers of just barely enough efficiency, say 90-96dB for 300B amps. If the sensitivity is higher, the hum will be distracting to many listeners.
I of course agree with Tre' and Stuben about how to do it. I use Schottky diodes and a dual bobbin split choke (whose leakage inductance serves as a common mode choke) with a hum pot to retain symmetry. No capacitors at all. And a shield winding between the filament winding and the rest of the power transformer. Not many talk about that of late - I borrowed the idea from Lynn Olsen.
Like many questions, the answer is "it depends". In this case it depends on the output tube, speaker sensitivity and (most important) the technique used to accomplish it. I believe the general consensus is that DC can sound just as good "if done right".
Paul - this is off-topic and NOT the reason I started this thread, but since you are here... Would it be possible to add attenuators to a pair of Paramount V1.1 amps? I'm thinking Goldpoint, DACT or even the Slagle Autoformer module. Has anyone done this?
Obviously room underneath the chassis is one concern. How about gain and impedance issues? Of course the idea for this is to be able to run a (low output impedance) digital source directly into the amps. Thanks!
Gerry
"I believe the general consensus is that DC can sound just as good 'if done right'".
I can't claim to have tried this with many tubes, but I don't believe the 211 can ever sound as open with DC as it does with AC filaments. At one time, I experimented with a switching supply that provided squarewave AC through a high-isolation, low-leakage transformer. I rectified and LCR filtered the floating output, then used it to power the 211 filaments with no semiconductors or ground paths in-between. The sound was no better than any of the other DC sources I tried (conventional unregulated linear supply and regulated supplies of several types). My conclusion was that for these tubes - and probably certain other types - there's no substitute for AC filaments. Note that companies like Bottlehead have much more experience with smaller tubes like the 300B and 2A3 than I do. If they say those can be successfully powered with DC, I tend to believe it. Even with those tube types though, the question remains as to whether other manufacturers that you might buy from will get the formula right.
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Buy Chinese. Bury freedom.
Yes, though of course you'd have to drill a new hole. Start a thread on the Bottlehead forum for more details.
DC heating CAN sound just as good with the right care taken in the design. With that said, you have to know what the builder/manufacture has provided in the heating design.
I built a pair of mono blocks and heated one with AC and then proceeded to build a DC supply for the other and compared until I was happy that the DC heating was not inferior. This took very quiet diodes, a hefty choke,a combination of cap filtering and a common mode choke on the back end. I used a scope and my ear until I was happy with the results.
This also depends on the output valve used. A 2a3 with a 2.5 vac supply will be more palatable as far as 120 HZ hum. A valve using 5 vac or higher will be more of a hummer even after the null pot is applied.
If you're looking at a 2a3 or 45 amp, personally, I would go with AC heating. If you're looking at a , 845, 300B or a 50, VT-52 (not commercial but wonderful)..I would assess the design. You're throwing a bunch of money around regardless so make the building / manufacturer prove their design is possible.
Stuben
automatic disqualify..........unless {obviously} is a preamp.
Speculation for the third question.
Brute force technique is necessary for a dead quiet SET.
Involving several parameters.
Cost would be my first guess.
The Mind has No Firewall~ U.S. Army War College.
I think DC gets a bum rap.
DC done right does not cause bad sound in my experience.
First the DC filament power supply filter should be a critical choke input with plenty of capacitance to really drop the ripple.
I use LCRC.
Second the filament voltage should be floating to the filament, not grounded.
I have seen many DC filament supplies that have the negative side grounded.
That's a big mistake. It guarantees that the last cap in the filter WILL be in the signal path.
By floating the filament, the audio signal should flow equally through the last cap in the supply in opposite directions which should be the same as no flow at all, taking that cap out of the signal path.
This is what I do with my cathode biased DHT output tubes.
Using AC, I null out the hum using a 10 turn wire wound pot then without changing the hum pot, I replace the AC with DC.
Doing this, the only difference I hear between AC and DC heating is the lack of hum.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi,
Is the resistor simply a Rdrop value for purpose of dialing in exact heater voltage or a larger value?
TIA
Opus
Yes, it's there to drop (adjust) the voltage and it helps with filtering.Without the resistor it would be an LC filter. With the resistor it's a LCRC filter.
Without that resistor, but with the same amount of capacitance and adjusting the AC voltage feeding the bridge down to end up with the same amount of dcv, the ripple would be .04 volts peak to peak instead of .0012 volts peak to peak.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/26/11 07/26/11 07/26/11 07/26/11 07/26/11 07/26/11
DC works well with a choke input supply - that's the important bit. I use a 280mH choke, 30,000uF or so, and then follow that with a Rod Coleman designed filament supply with a gyrator and a current source.
Few if any commercial SET amps would use a choke input, but that's the way to go if you are building and are serious about a clean supply.
Andy
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