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cotdt 28th November 2012 11:56 PM

What is the theoretical advantage of direct heated triodes?
I use DHTs like 45 and 300B myself, and they indeed sound better than indirect triodes of the same power class (only EL84 is a match for DHTs in terms of transparency). The specs of DHTs look inferior, and they hum unless you use DC heaters, which adds complexity.

What is the theoretical advantage of direct heated triodes? The specs look inferior.

DUG 29th November 2012 12:10 AM

fewer parts...easier to build??

SY 29th November 2012 12:12 AM

Microphonics. People like that extra little "ting!" in their music.

Not to discount the nostalgia aspect, which undoubtedly plays a role. Disagree strongly about fewer parts- I'd argue that it's the opposite, which also attracts some people.

iko 29th November 2012 12:40 AM

Let's be honest here for a moment. The real reason why people love dhts is because they have hotter cathodes! :)

kmaier 29th November 2012 12:42 AM

The 45 is probably my favorite DHT, but only 2 watts or so output. I don't consider the specs inferior (but what is your comparison and/or benchmark?).

With a good quality 45, AC filaments can better 80dB s/n referenced to 1-watt (with a good design and proper layout and parts selection). The WE300b, no way... DC is required due to it's center-tapped filament.

Complexity varies with design topology and style choices of the "designer", while adding DC filament for a DHT will add another power supply section. I've done a couple clean sheet designs for the 45/2A3 and they're not complex... quite simple actually.

As for being microphonic, I'd say it depends heavily on your specific DHTs, both brand/internal structure and condition. Having a large collection of 45s and 2A3s, most of the better ST glass versions (Sylvania being preferred) are dead quiet and near mechanically immune, sans whacking directly on the tube. I have others (includes all early Globe types, some ST versions and new EML 45 solid plates) which are extremely sensitive. Playing a few trumpet notes (real trumpet, not a recording) within 10 feet literally blew the tubes about... audible when hooked to a speaker. A characteristic specific to either a bad or poor quality tube IMHO.

As with anything else, a clean simple design that has been implemented properly (parts selection, layout, construction and tube fitting) will have good measured performance, low distortion, wide bandwidth and a fairly neutral presentation. With proper parts election, they should run consistently and reliably for decades (sans a few tubes).

Regards, KM

bear 29th November 2012 12:51 AM

Used in ZFB amps all tubes have rather unique sonic signatures. DHTs certainly not an exception.


Doug Kim 29th November 2012 12:53 AM

If there's any scientific advantage, it (of course) will be from direct heating of cathod.

1. DHT's cathod gets order of magnitude higher currents (a few amps) than indirect ones (a few tens to hundreds mA);
2. And it gets slightly hotter than indirect triodes.

Both may act to give better emission characteristics of electrons from cathod, but I am not sure.

What else?

Bigun 29th November 2012 12:59 AM

the gigs up chaps, DHTs have been outed !

6L6 29th November 2012 01:04 AM


Originally Posted by cotdt (
What is the theoretical advantage of direct heated triodes?

When they were designed, the best minds in science and engineering were working on them, and they are designed for the audio range.

The designers had to work with materials and (much more important in my mind) geometry to get them distortion free.

wa2ise 29th November 2012 02:33 AM

I would have thought that the filament being heated with a few volts of AC would cause problems with 60Hz artifacts getting into the audio. Various nonlinearities not cancelling out even if you manage to balance the hum out.

There's some filament tubes intended for portable battery operated radios, like the 3S4 or 3V4, that were designed for DC on the filaments, and the filament pins were labeled as to which gets the positive filament supply and which gets the negative side.

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