Over the years Ive built many indirectly and directly heated amplifiers, and always if some dht was in the system, it sounded better in a way I cant quite explain. A lot of people build amps with them, they even started to be made again for new amplifiers etc. From the builders standpoint it has only disadvantages - you need separate filament supply for each tube, well filtered, current regulated in the best case, everything that happens on the cathode, every small hum or noise gets amplified. They are more microphonic than indirectly heated tubes. Now I have DAC output stage with the 45 tube used as a preamp tube. (if anyone wants more details, its choke loaded with output capacitors). And the same thing happened.
Like I said I can not even properly describe the sound. It never is boring every track is interesting, I actually enjoy music now. I stopped thinking about how to better the schematic, I just dont know what to play next. The sound is big, like even at lower listening values. Like it comes from nothing in the air. The voices and treble are heavenly. Music is now actually touching me emotionally way more than before. Like the simplest description that Im always saying is magical emotional sound. That just is not quite there with other tubes.
My big question is WHY, what electrical factors are different than from indirectly heated amplifiers? I really want to find out why this happens. I wondered the same thing in electrolytic vs foil caps in the power supply, but the explanation here was quite simple, being way lower losses in the dielectric, vibration not being a problem cause the foil caps are sinked in oil. The tangens delta difference between electrolytic and foil cap is like 0.3 vs 0.001. Plus the foil caps are way better at filtering higher frequency noise etc. But with DHTs I still dont know. Why nobody measured this?
Its the exceptional linearity (the curves are perfectly spaced and also as flat as a ruler).
Or its some microphonic feedback from the speaker vibrations getting to input tubes?
Or its the special care that you must take while building a DHT amp compared to indirectly heated amp?
Or its a different cause? I still dont know, does anybody has any ideas?
Like I said I can not even properly describe the sound. It never is boring every track is interesting, I actually enjoy music now. I stopped thinking about how to better the schematic, I just dont know what to play next. The sound is big, like even at lower listening values. Like it comes from nothing in the air. The voices and treble are heavenly. Music is now actually touching me emotionally way more than before. Like the simplest description that Im always saying is magical emotional sound. That just is not quite there with other tubes.
My big question is WHY, what electrical factors are different than from indirectly heated amplifiers? I really want to find out why this happens. I wondered the same thing in electrolytic vs foil caps in the power supply, but the explanation here was quite simple, being way lower losses in the dielectric, vibration not being a problem cause the foil caps are sinked in oil. The tangens delta difference between electrolytic and foil cap is like 0.3 vs 0.001. Plus the foil caps are way better at filtering higher frequency noise etc. But with DHTs I still dont know. Why nobody measured this?
Its the exceptional linearity (the curves are perfectly spaced and also as flat as a ruler).
Or its some microphonic feedback from the speaker vibrations getting to input tubes?
Or its the special care that you must take while building a DHT amp compared to indirectly heated amp?
Or its a different cause? I still dont know, does anybody has any ideas?
Likely slight reverberations from microphonic resonances. Weak magnetic stray fields from the OT could be contributing too.
Try some headphones and then some Mu metal shielding around the tubes. The internal electric field variation between plate and cathode could also set off some weak mechanical resonances at high signal peaks. Someone mentioned they could hear the tubes "singing", (in addition to the OT singing )
Try some headphones and then some Mu metal shielding around the tubes. The internal electric field variation between plate and cathode could also set off some weak mechanical resonances at high signal peaks. Someone mentioned they could hear the tubes "singing", (in addition to the OT singing )
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I've always assumed it was the absence of the extra element, the "cathode" sleeve, combined with the extreme linearity. Why eliminating that extra element would result in greater immediacy and clarity, I couldn't tell you, and I doubt there's any way to objectively measure the effect. Another, and probably more measurable effect, is the elimination of feedback due to the low output impedance of directly-heated triodes.
I have found that once you start adding back things like feedback and electronic control circuits (auto-bias, complex regulated filament supplies) a lot of that magic starts to disappear.
I have found that once you start adding back things like feedback and electronic control circuits (auto-bias, complex regulated filament supplies) a lot of that magic starts to disappear.
I have no technical explanation, but I wholeheartedly agree that DHTs have a special sound - transparent, clear, nuanced, with very lifelike tonality on voices and acoustic instruments. When I first hooked up a 26 my jaw dropped quite literally. I think I must have uttered something like "so THIS is what I've been looking for all these years.....!". That was way back in 2008. The 26 is still in my amp, driving PSE 4P1Ls. All DHT. And thanks to Rod Coleman for being one of the big reasons why DHTs now sound so good.
As you apply more N FDBK, extraneous inputs, like Microphonics, will get removed. So its your choice of weak echo chamber or no EC.
So what led the tube manufacturers to "screw it up" for everyone? Save a few power supplies? Last I heard the latest Intel silicon requires 20 different voltage rails to operate. So you do what you have to to get what you want.
Edit; I forgot to ask, does the sound phenomena follow the rectifier tube type as well?
The DHT wire like cathode E field varies as 1/R^2 in close range, while a flat surface cathode field varies as 1/R. This effectively causes the grid to behave as if it is further away with the DHT case. So better linearity results with reduced "island" effect from grid wire proximity. The planar cathode doesn't behave like a violin string acoustically however, so this would be seen as a benefit, alongside the electrical isolation of the heater.
Higher gm results from putting the grid closer to the cathode, and gm was King after N FDBK came along. So tube linearity went out the window in favor of gm. With the exception of some frame grid tubes, like 12HL7, where ultra close grid wire pitch compensated for "island" effect.
Trying to make Triodes out of wire aligned grid1/grid2 beam __trodes makes things even worse for lineariy, because the grid1 and grid2 wire alignment puts the grid2 wires (now acting as plate electrostatically) in the shadows of the grid1 wires. The "island" effect (on the cathode) of the grid1 wires pushes their transfer curve up toward square law (from 1.5 law) with close g1 to cathode spacing, while the shadows which the grid2 wires live in are pushing toward lower than 1.5 power law. The mis-match in transfer power laws make for SQRT distortion.
The UnSet scheme is the best way to make linear Triodes from Beam __trodes. High power maintained, with optimum screen grid V, and best linearity from local N FDBK. But NO euphonic microphonics unless you use a DHP.
Higher gm results from putting the grid closer to the cathode, and gm was King after N FDBK came along. So tube linearity went out the window in favor of gm. With the exception of some frame grid tubes, like 12HL7, where ultra close grid wire pitch compensated for "island" effect.
Trying to make Triodes out of wire aligned grid1/grid2 beam __trodes makes things even worse for lineariy, because the grid1 and grid2 wire alignment puts the grid2 wires (now acting as plate electrostatically) in the shadows of the grid1 wires. The "island" effect (on the cathode) of the grid1 wires pushes their transfer curve up toward square law (from 1.5 law) with close g1 to cathode spacing, while the shadows which the grid2 wires live in are pushing toward lower than 1.5 power law. The mis-match in transfer power laws make for SQRT distortion.
The UnSet scheme is the best way to make linear Triodes from Beam __trodes. High power maintained, with optimum screen grid V, and best linearity from local N FDBK. But NO euphonic microphonics unless you use a DHP.
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In my experience, directly-heated rectifiers sound different from indirectly-heated ones. I get the impression of a bit more transparency, but it's subjective of course.
One of the few advances of the new models of DHT is elimination of these microphonics.
The EML 300B is an example. Anyone comparing the sound of these against the microphonic EHX and many other 300Bs will be able to judge for themselves; but IME microphonics are not at all euphonic.
In measurement terms, examing the triode curves of DHT versus a comparably-sized IDHT is valuable, easily illustrated with an example.
I have taken curves - both from Audiomatica.com for fairness. (Used with permission - thank you, Audiomatica)
DHT: VV30 (Vaic 300B structure) vs EL34.
The most obvious difference is the change in slope (ra) as Va increases.
With the VV30, the slope Va/Ia decreases by barely 50% across the most part of a typical load-line; in a similar space the Va/Ia slope of the EL34 more than triples.
The cause of this is related to another effect visible on the triode curves.
-- See how the EL34 grid loses control at high voltages and low current. This is accounted for by the larger diameter of the IDHT's emissive surface; a greater area of cathode surface is exposed to the anode through the grid spacing. The effect can be seen on lesser DHTs, where coarser grid wires and larger spacings are found in the grid design.
These effects combine to corrupt the regularity of the EL34 triode curves, and all other IDHT power tubes, to a greater or lesser degree.
It's no surprise to find that the differences are audible.
I have taken curves - both from Audiomatica.com for fairness. (Used with permission - thank you, Audiomatica)
DHT: VV30 (Vaic 300B structure) vs EL34.
The most obvious difference is the change in slope (ra) as Va increases.
With the VV30, the slope Va/Ia decreases by barely 50% across the most part of a typical load-line; in a similar space the Va/Ia slope of the EL34 more than triples.
The cause of this is related to another effect visible on the triode curves.
-- See how the EL34 grid loses control at high voltages and low current. This is accounted for by the larger diameter of the IDHT's emissive surface; a greater area of cathode surface is exposed to the anode through the grid spacing. The effect can be seen on lesser DHTs, where coarser grid wires and larger spacings are found in the grid design.
These effects combine to corrupt the regularity of the EL34 triode curves, and all other IDHT power tubes, to a greater or lesser degree.
It's no surprise to find that the differences are audible.
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Joined 2006
It might sound a bit exotic, the materials used for making the filament/cathode in a tube indeed could carry a strong correlation with the tube's sound signature. 😀😁
So the magic behind DHT tubes could possibly be by part due to :
In DHTs, the cathode itself is the tungsten-based filament while IDHTs have their nickel cathode coated with barium, strontium oxide (for enhancing the electrons emission efficiency).
Reference:
https://www.effectrode.com/knowledge-base/secrets-of-the-tube-alchemists/#:~:text=The cathode in a vacuum,layer coating the cathode's surface.
"So, all tubes are not the same, because all oxide-coated cathodes are not the same. In part, it’s why a Sylvania 12AX7 doesn’t sound exactly like a Brimar 12AX7, even though their technical specifications are the same on paper...... no wonder some manufacturers chose to keep their cathode coating formulas closely guarded secrets. "
So the magic behind DHT tubes could possibly be by part due to :
In DHTs, the cathode itself is the tungsten-based filament while IDHTs have their nickel cathode coated with barium, strontium oxide (for enhancing the electrons emission efficiency).
Reference:
https://www.effectrode.com/knowledge-base/secrets-of-the-tube-alchemists/#:~:text=The cathode in a vacuum,layer coating the cathode's surface.
"So, all tubes are not the same, because all oxide-coated cathodes are not the same. In part, it’s why a Sylvania 12AX7 doesn’t sound exactly like a Brimar 12AX7, even though their technical specifications are the same on paper...... no wonder some manufacturers chose to keep their cathode coating formulas closely guarded secrets. "
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Other people have already given good answers, but I think it is also important to point out that there is nothing stopping a indirectly heated cathode tube from being just as good if not better.
We now have a plethora of Russian tubes that are highly linear and can sound DHT like in the right context.
I have no doubt in my mind that with enough time and money you could re-engineer something like a 6a5 to be better than it's 2A3 counterpart.
We now have a plethora of Russian tubes that are highly linear and can sound DHT like in the right context.
I have no doubt in my mind that with enough time and money you could re-engineer something like a 6a5 to be better than it's 2A3 counterpart.
Large level microphonics are certainly annoying, but I suspect that low level undiscernible ones may add that mysterious "detail". It is harmonically related when it gets excited by the main signal, just like distortions, but not continuous like distortions, and delayed, almost like a subtle accompaniment.
Here is an oxide cathode 12HL7 in triode mode, Ultra fine grid pitch keeps it linear.
And here is a 6197 oxide cathode tube in pentode mode with near constant Ri across plate voltage
And here is a JJ KT77 oxide cathode tube with nice even plate curve spacing (and it has much better triode curves than the EL34 too)
I would say all these features are a tube element design factor and nothing to do with oxide cathodes. It's just that the old DHT designs had greater grid spacing usually, gm not being 1st priority at the time.
Here is an oxide cathode 12HL7 in triode mode, Ultra fine grid pitch keeps it linear.
And here is a 6197 oxide cathode tube in pentode mode with near constant Ri across plate voltage
And here is a JJ KT77 oxide cathode tube with nice even plate curve spacing (and it has much better triode curves than the EL34 too)
I would say all these features are a tube element design factor and nothing to do with oxide cathodes. It's just that the old DHT designs had greater grid spacing usually, gm not being 1st priority at the time.
In the last century several -several ten?- thousand tubes made. Now we only use a few dozen of them in Hifi/audio/DIY/tube equipments as "good" or "good sounding" ones, the rest disappeared.
It's typical as "natural selection" by Darwin, only the most "viable" (according to some aspect) survive.
If the one criterion comes to the fore ("the thoriated tungsten filament tubes usually sounds better than others"), these types using will be preferable, the designs, the testing, the product manufacturing disseminates it better, the "evolution" starts.
p.s.
The thoriated tungsten filament tubes usually sounds better than others. 😛
It's typical as "natural selection" by Darwin, only the most "viable" (according to some aspect) survive.
If the one criterion comes to the fore ("the thoriated tungsten filament tubes usually sounds better than others"), these types using will be preferable, the designs, the testing, the product manufacturing disseminates it better, the "evolution" starts.
p.s.
The thoriated tungsten filament tubes usually sounds better than others. 😛
I have a take on why DHTs sound good:
1. Highly linear, i.e., the curves are actually straight in a small signal area AND at larger swings.
2. Wide input voltage range, i.e., you can swing a huge signal into the input without hitting the edges where the curves bunch up. Another way to say this is DHTs are typically low gain devices.
1. Highly linear, i.e., the curves are actually straight in a small signal area AND at larger swings.
2. Wide input voltage range, i.e., you can swing a huge signal into the input without hitting the edges where the curves bunch up. Another way to say this is DHTs are typically low gain devices.