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Old 20th May 2009, 01:57 PM   #1
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Default How do you make a DHT amp REALLY HUM FREE????

I just made a DHT amp for a friend - I couldn't hear any hum on my system but he has Lowther horns - like 104db efficiency - and he says hum is audible.

I don't have any horns to check for hum and I've never had a challenge like this before - to make an entirely hum-free amp with DHTs.

I'll use 2a3 or 300b or something like that, but I'd also like to use two stages of DHT before that, like 01A into 71a for instance.

SO:

What precautions should I be looking at right from the start so I can design it.

- all DC filaments?
- separate power supply?
- care with transformers like interstages, or cap couple?
- what's an acceptable ripple in the power supply - how many chokes, glow tubes etc?
- one star earth or what?

Anything else you guys have learned from building amps for high efficiency horns?

Andy
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Old 20th May 2009, 04:10 PM   #2
kmaier is offline kmaier  United States
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Default tough call

Andy,

Hum-free is somewhat relative based on other related equipment and of course the sensitivity of your speakers. I designed a fairly compact chassis (10x6x2) several years ago and with selected tubes I can measure S/N ratios close to 90dB referenced to 1-watt output. This is with all AC filaments and a simple pi-filter with a vacuum-tube rectifier. 80dB is a very quiet amp however in a normal listening environment.

What was the output hum level on your amplifier? Did you measure it?

As you noted, there are several important design and engineering points. The physical layout, parts selection (iron especially), wiring (eliminating ground loops ,etc.), tubes, etc. all play an important role. I used Hashimoto iron exclusively in the design and it's exceptionally well-shielded. I used a 45 triode for the output stage and a single 5814A for the input/driver. If you choose unshielded iron, you'll need to exercise more caution on the physical layout and use a much larger chassis to space things out more. The usual good construction techniques will apply, like twisting the AC voltage pairs.

As for DC filaments, it depends on your tube selection.... not just type but in some cases the brand and internal topology. Case in point, the 2A3 is available in multiple NOS internal topologies and several new manufactured ones also have different internal topologies. For the original RCA single-plate, you will not have much luck without a DC filament supply. This is the result of the filament structure being a center-tapped design which is fed voltage from the center for one point and the two ends connected together for the second point (WE300B is the same). I get much lower hum levels with the later dual-section 2A3 as the filament structure is a simple "M" for each section, albeit not as low as a good 45.

DC filaments eliminate this but I personally prefer an AC filament for the 45 and 2A3. I think the "other Kevin" (kevinkr) noted earlier in a post that the 71A is succeptable to picking up hum, and not from the filament. If you go the route of all DHTs, DC filaments might be your only option to get the hum levels acceptable. Shielding the smaller tubes might also be required.

The power supply section can be extremely quite using high-quality chokes and filter caps. I used a 15H choke on the 45 amp and a 10H choke on the 2A3 version. With F-W rectification, PS ripple is very, very low and 120Hz (for the US), not 60Hz which would be from the filament supply. The output noise at idle is only from the output tube filament and it's not really a pure sine-wave (somewhat complex and different between many tubes of the same type, etc.) due to the tolerances of the filament wire, the oxide coating as well as the rest of the internals and alignment. They're never perfect, so I test and match sets of tubes for each SET pair to ensure matched gain, power, distortion and noise.

I would suggest building a breadboard first to design your circuit and then do the engineering to put it into a chassis that works with the iron and other parts. Hope this helps.

Regards, KM
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