Ot may be linearity vs your other tube.
If it's EL34 that has a wiggle a low voltage.
A 6922 may be a better tube , a triode , to compare it with.
If it's EL34 that has a wiggle a low voltage.
A 6922 may be a better tube , a triode , to compare it with.
For the same heating power, the is not much difference in gm. Actually, the PX25A has 7.5mA/V - more than the 5mA/V of the KT66 in triode.
The real priority of the time was MORE POWER - which the Kinkless Tetrodes delivered.
But QUANTITY and QUALITY are different things; and you had to choose then, as you do now.
The IDHT has inefficient heating, which causes problems in itself - hotter running for the grid, increasing the risk of secondary emission: another source of degradation of the sound.
KT77 triode curves (MEASURED - Audiomatica) shows > 3:1 degradation in ra over a typical load line.
The KT77 is not especially well liked for sound either.
When we are comparing DHT and IDHT designs, it should be understood that we are comparing Power valves. Of course we can find IDHTs that offer high gm and good performance at relatively low voltages. But when we are comparing Power Valves, measurement at high voltage and current is needed.
Just try to find an indirectly heated Power Valve with measured curves as good as a PX25A or a 300B at 500V - 700V or higher..... and that's before we ask you to match majestic transmitters like the 845.
The reason that POWER IDHTs are always have worse TRIODE curves has already been stated: Loss of grid control at high voltages. This degrades the regularity of curve spacing, slope/ra and linearity.
I'll post a snippet of primary source material for this explanation, if any one is actually interested.
I always suspected that the very regular fall off of the harmonic structure is a big part of what makes them sound so good, especially in SE mode. That harmonic structure is either inaudible to the human ear, or at least not objectionable.
We have all read the comments about DHTs being euphonic (maybe some are) but I find them very neutral and clean when done right. It’s all the other amps that have a grunge we have gotten used to. When you hear an amp without it, that darn well ought to be euphoric! 😛
What it is physically about the DHT that makes it so linear and with such a non-offensive harmonic structure is an excellent question. It’s pleasant to read some of the good answers in this thread.
We have all read the comments about DHTs being euphonic (maybe some are) but I find them very neutral and clean when done right. It’s all the other amps that have a grunge we have gotten used to. When you hear an amp without it, that darn well ought to be euphoric! 😛
What it is physically about the DHT that makes it so linear and with such a non-offensive harmonic structure is an excellent question. It’s pleasant to read some of the good answers in this thread.
My old friend said tube audio - DHT has more distortion than digital, but it's a harmonic and musical distortion that sounds in the ear.
Each DHT tube has a unique carater. DHT SET amp with Vac filament gives something more.
Each DHT tube has a unique carater. DHT SET amp with Vac filament gives something more.
Well there is a big difference using sand or not using sand and how is implemented both, not only one way to go to Rome or there is several ways to skin a cat...
After reading this thread are we any closer to why dht supposidly sounds better.nope.
40 years ago i had a spanking new physics degree and was certain stuff like this was just about to get demystified. Any minute.
Not dhts per se, but at least easy stuff like Shinkoh resistors. Just crickets and handwaving ever since. Or "scientific" denial.
The Universe is not easy to decode 🙂
Btw, the OP started the thread with an example of a small signal amplifier where differences are much harder to explain. Not even sure a dht has less harmonic distortion than a good idht at 2v, yet the perceived sound differences between a 10Y and a 6N30P, both of them buffered, are like night and day.
Not dhts per se, but at least easy stuff like Shinkoh resistors. Just crickets and handwaving ever since. Or "scientific" denial.
The Universe is not easy to decode 🙂
Btw, the OP started the thread with an example of a small signal amplifier where differences are much harder to explain. Not even sure a dht has less harmonic distortion than a good idht at 2v, yet the perceived sound differences between a 10Y and a 6N30P, both of them buffered, are like night and day.
the advantageous field pattern of the filamentary cathode over the sleeve cathode, noted by @smoking-amp is worth consideration for small-signal comparisons.
Also:
High gm triodes often have to run the grid at a low-voltage bias (for low anode voltages); here the 6N30P has very little headroom before the risk of grid current sets in. The close proximity of grid to cathode makes the grid sure to get hotter; and temperature is the primary risk for grid emission.
At higher anode voltages ra rapidly increases, so it is difficult to get the best operating point.
I like these high gm triodes for making one stage do the work of two; but I use cathode degeneration to limit the first problem and shunt cascode to address the ra stability, and still give very high gain.
Can someone please explain why triode curves sometimes come out like shown here in the link. Is it some kind of secondary emission or lack of control? I've seen the phenomena in a few examples of old DHT and IDHT.
https://thumbnail.image.shashinkan....202312100000/?scid=wi_blg_shashinkan_thumb001
https://thumbnail.image.shashinkan....202312100000/?scid=wi_blg_shashinkan_thumb001
The "hit the point where all the curves bunch up on the right" not good enough? The quest for gm above all else in tube design not good enough? Higher gm; less stages needed; less expensive to build; cost is "only" how it sounds.
Definitely interested. 🙂
And I have been thinking about a "figure of merit" for output triodes without feedback - Pout @ 5% 2nd harmonic perhaps?
Instead of looking at DHT's as a step up, what if we look at them as the base line and see the IDHT as a step down and then ask why they are.
I understand the above explanation of the cathode differences per se. but wonder if it isn't possible there's more to the heater cathode interface than we usually plan for.
ie. When there's an issue with hum we know it can suffice to bias the heater positive, biasing the virtual diode between the two into saturation, but are there other relationships we usually don't think about.
Put one of Rod's regs on an IDHT and in some circuits , the difference between that and a plain constant current reg or a voltage reg can be heard.
It was common knowledge twenty or so years ago that current regs on heaters were better than voltage. Accepted for what it was, you didn't find many people asking why.
I understand the above explanation of the cathode differences per se. but wonder if it isn't possible there's more to the heater cathode interface than we usually plan for.
ie. When there's an issue with hum we know it can suffice to bias the heater positive, biasing the virtual diode between the two into saturation, but are there other relationships we usually don't think about.
Put one of Rod's regs on an IDHT and in some circuits , the difference between that and a plain constant current reg or a voltage reg can be heard.
It was common knowledge twenty or so years ago that current regs on heaters were better than voltage. Accepted for what it was, you didn't find many people asking why.