What is it about direct heated triodes?

Loheswaran

Well-Known Member
Dec 19, 2014
432
99
258
As per the thread starter why are these so loved? What are their strengths and weaknesses?
 

cjfrbw

Well-Known Member
Apr 20, 2010
3,323
1,313
1,730
Pleasanton, CA
Many tubes (and transistors) are natural compressors, especially the indirectly heated small triodes and pentodes. This doesn't mean they can't sound good, because compression, tube or otherwise, can sound very good and some people even think it is an enhancement.

I have an unproven (but possibly provable) theory that lower powered directly heated triodes, usually with oxide cathodes, create the illusion of dynamic expansion, especially in the upper midrange. This gives them an impression of open ease, and enhances fluidity, imaging and tonal space. The odd inverse penalty is that the effect is greater and the sound more delicate with lower cathode voltage DHTs, which can make the sound perhaps wispy. Higher cathode voltage DHTs can have more substance, but may lose a bit of the dancing in air spookiness. I suppose you could also add the ineffable 'charm' factor.

Even though the DHTs are some of the most linear amplifiers, perhaps the illusion of dynamic expansion (aside from issues of harmonics and harmonic distortions) can be unevenly applied over the frequency range, and could be considered a coloration.

Ancient mossy mavens insist on AC powered cathodes for the best sound, in spite of the noise, but I don't find much to object to with DC powered cathodes. There are even some of the mossies who think that some audible noise floor is necessary as dither for the best sound quality, usually around these AC cathodes. This would fly into the face of many audiophiles who despise noise of any kind, and want the blackest and most noise free backgrounds.

Transistors seem to be either dynamically neutral or compressive. With rare exceptions (i.e. SIT/Vfet) they never or seldom if ever get that same fluidity and ease. I have a couple of VFET amps that come close, with the addition of certain solid state virtues.

Higher power DHT with thiorated filaments are a bit different. My theory is that they amplify fairly cleanly up to the first watt, and then add increasing harmonics with power demand. The neural effect is pleasurable and also seems to enhance spatial/tonal cognition.

These are not hills I choose to die on, and are perhaps nothing more than poetic whimsy over years of using and loving DHT sound quality. I won't get into righteous polemics over them, as they are matters of taste as much as anything.

Drawbacks, of course, are microphonics, noise, and the harmonic spray of the tubes.
I'm depraved enough to like some of the noise, microphonics and harmonics, so horses for courses.
 

Atmasphere

Industry Expert
May 4, 2010
2,336
1,837
1,760
St. Paul, MN
www.atma-sphere.com
In a nutshell, linearity.

The weakness of course is that they have a directly heated cathode, so special care has to be taken in the circuit to prevent hum- such as a DC supply for the filament. Most of the DHTs are fairly high impedance- for example the type 45 likes to see about a 5K load if you plan to get much power out of it.

Even though the DHTs are some of the most linear amplifiers, perhaps the illusion of dynamic expansion (aside from issues of harmonics and harmonic distortions) can be unevenly applied over the frequency range, and could be considered a coloration.

The quality of 'dynamics' is actually the simple fact that many people try to use SETs with speakers of insufficient efficiency. If the amp is making about 20-25% of full power, the higher ordered harmonics start to show up on musical transients. Since the ear uses the higher ordered harmonics to sense sound pressure, the result is that an SET used in this manner sounds 'dynamic' but its really distortion interacting with our physiology.

This is why many SET owners think that a few watts is all they need because it plays plenty 'loud' but if you use a sound pressure level meter in the room you find out that its not playing as loud as thought, or that the owner thinks that sound pressure is really how 'loud' sounds. But if the system is low in distortion, there will be a natural tendency to turn the volume up higher, because the natural loudness cues of the music itself will determine how loud it sounds.
 

cjfrbw

Well-Known Member
Apr 20, 2010
3,323
1,313
1,730
Pleasanton, CA
Treasured 'feature' or reviled distortion? I guess you pick your poison and run with it.

Audiophiles have a statistical histogram of hearing apparatus, neural nets, and pleasure buttons so standardizing for taste and perception is always a moving target.
 

Atmasphere

Industry Expert
May 4, 2010
2,336
1,837
1,760
St. Paul, MN
www.atma-sphere.com
Well the problem is that once you know its distortion that causes the amp to sound 'dynamic' then it sort of wrecks it for you- its easier to hear. The dynamics should come from the recording, not the amp; in that regard its certainly not neutral. BTW, the fact that the ear/brain system uses the higher ordered harmonics to sense sound pressure is something that in common with anyone. Its not a variable from person to person. The idea that SETs are 'far more dynamic than their low power would suggest' shows up in many reviews of SETs and comments about them online. Its simply because the speaker used with them just doesn't have enough efficiency.
 

cjfrbw

Well-Known Member
Apr 20, 2010
3,323
1,313
1,730
Pleasanton, CA
Thorium cathode DHT's used in transmitting tubes can have very high anode dissipations up to hundreds of watts.

However, yes, the 'traditional' DHT tubes are telephony/audio tubes of lower power ratings from a fraction of a watt output to 10 watts or so, and can be used as drivers or amplification tubes (SET).
 

Loheswaran

Well-Known Member
Dec 19, 2014
432
99
258
Thorium cathode DHT's used in transmitting tubes can have very high anode dissipations up to hundreds of watts.

However, yes, the 'traditional' DHT tubes are telephony/audio tubes of lower power ratings from a fraction of a watt output to 10 watts or so, and can be used as drivers or amplification tubes (SET).

I am so gonna have to read up on some electronic circuitry...

BTW is that your pet parrot - Love it
 

Loheswaran

Well-Known Member
Dec 19, 2014
432
99
258
Dear All

Can i just say I really appreciate the well-considered answers and contributions on this thread.

It sounds as if these are not going to be hard to fit into my system unless I have a speaker change - which frankly - I'm not keen on doing...

I just need/fancy a change and I can't think of the right way to go - i want a tad more warmth and tonal colour to what i have and need to drive 4 ohm 88db speakers that are in a nearfield...
 

Atmasphere

Industry Expert
May 4, 2010
2,336
1,837
1,760
St. Paul, MN
www.atma-sphere.com
Do these DHT's tend to be lower powered in general?
The problem is the output transformer- the bigger you build it the less bandwidth you get. 7 watts is often the practical upper limit where you can get 20Hz to 20KHz reliably. But to really reproduce audio correctly (in other words avoid phase shift) you need to go from 2Hz to 200KHz (which is something our amps can do). Phase shift will go to 10x the low end cutoff and a 10th of the high frequency cutoff. So if you can only make it to 20Hz before the amp rolls off, you'll have phase shift up to 200Hz. Over that range you'll hear less impact, as that is how the ear translates the phase shift over a spectrum like that.
 
  • Like
Reactions: bonzo75

Loheswaran

Well-Known Member
Dec 19, 2014
432
99
258
The problem is the output transformer- the bigger you build it the less bandwidth you get. 7 watts is often the practical upper limit where you can get 20Hz to 20KHz reliably. But to really reproduce audio correctly (in other words avoid phase shift) you need to go from 2Hz to 200KHz (which is something our amps can do). Phase shift will go to 10x the low end cutoff and a 10th of the high frequency cutoff. So if you can only make it to 20Hz before the amp rolls off, you'll have phase shift up to 200Hz. Over that range you'll hear less impact, as that is how the ear translates the phase shift over a spectrum like that.

Thanks for the explanation - for an electric luddite like me.

Does anyone sell your amps in the UK
 

microstrip

VIP/Donor
May 30, 2010
20,806
4,698
2,790
Portugal
We have two main technical reasons for different sound in DHT and indirectly heated triodes - noise and microphony. DHT usually use different emitting materials than indirect cathodes, resulting in different types of noise. DHT have a long thin filamentary cathode and indirect heated triodes have a more robust planar structure that surrounds the heating filament, being less microphonic - see https://onlinelibrary.wiley.com/doi/abs/10.1002/j.1538-7305.1934.tb00681.x. Remember that there is a variable electrical field between the tube electrodes that creates forces insides tubes - they have intrinsic electrical microphony. Although we can easily listen to semiconductor intrinsic microphony (mainly due to thermal effects) , as tube electrodes are in vacuum we can't listen to their electrical microphony!
 
  • Like
Reactions: analogsa

Phantom-Audio

Well-Known Member
Apr 23, 2017
323
365
195
Jakarta
www.phantom-audio.com
If you need Sales of Atma-Sphere in the UK I can assist with this.
 
  • Like
Reactions: Atmasphere

About us

  • What’s Best Forum is THE forum for high end audio, product reviews, advice and sharing experiences on the best of everything else. This is THE place where audiophiles and audio companies discuss vintage, contemporary and new audio products, music servers, music streamers, computer audio, digital-to-analog converters, turntables, phono stages, cartridges, reel-to-reel tape machines, speakers, headphones and tube and solid-state amplification. Founded in 2010 What’s Best Forum invites intelligent and courteous people of all interests and backgrounds to describe and discuss the best of everything. From beginners to life-long hobbyists to industry professionals, we enjoy learning about new things and meeting new people, and participating in spirited debates.

Quick Navigation

User Menu

Steve Williams
Site Founder | Site Owner | Administrator
Ron Resnick
Site Co-Owner | Administrator
Julian (The Fixer)
Website Build | Marketing Managersing