[mordy]

I noticed that you are running your tubes at 3.33v for the heaters. I had seen that number for some of the “equivalent” tubes of the ktz41 but the ktz41 had always been listed at 4v. I had initially run mine at 4.3 volts figuring that was within the tolerance level. Then lowered it down to 4v. Now I realize that 4v is actually still beyond the 10% tolerance usually allowed for heaters if the real value is 3.33. On a whim I set mine to 3.33 and it does seem to have conquered the weirdness I had been hearing. Now I’m getting closer to what I was expecting!

I wonder why there is variation of heater voltages in supposedly equivalent tubes. Now I’ll need to reevaluate my tube combos

Are there any studies on how the plate voltage effects the sound?

 

[CAJames]

Are there any studies on how the plate voltage effects the sound?

Do you mean plate voltage or heater voltage? I think either is tube and system dependent, but in speaker amps where you can control the plate voltage with voltage regulator tubes it seems like the general trend is the higher the plate voltage the tighter and more dynamic the sound, lower is softer and a little more defuse. FWIW/YMMV and all that.

 

[Isaacc7]

Are there any studies on how the plate voltage effects the sound?

Plate voltage has a direct influence on distortion/power. What is considered optimal will depend on the circuit and what you like.

In this case I was apparently running the heaters too hot. Normally I wouldn’t think this would make much difference but it seems like it has in this case. It could be in my head of course, longer listening and maybe some experimenting could help clear that up.

Is it possible that running heaters too hot could impact performance? The heat triggers electron flow, could there be too much?

 

[g0ldl10n]

I noticed that you are running your tubes at 3.33v for the heaters. I had seen that number for some of the “equivalent” tubes of the ktz41 but the ktz41 had always been listed at 4v. I had initially run mine at 4.3 volts figuring that was within the tolerance level. Then lowered it down to 4v. Now I realize that 4v is actually still beyond the 10% tolerance usually allowed for heaters if the real value is 3.33. On a whim I set mine to 3.33 and it does seem to have conquered the weirdness I had been hearing. Now I’m getting closer to what I was expecting!

I wonder why there is variation of heater voltages in supposedly equivalent tubes. Now I’ll need to reevaluate my tube combos

Well, I am running them at 3.33V because the CV1698 datasheet states thats the nominal filament voltage.

However, the KTZ41 datasheet shows nominal filament voltage of 4V.

But if you find that they run better for you at 3.33V, then stay there, nothing wrong with that I would think.

 

[Isaacc7]

Well, I am running them at 3.33V because the CV1698 datasheet states thats the nominal filament voltage.




However, the KTZ41 datasheet shows nominal filament voltage of 4V.



But if you find that they run better for you at 3.33V, then stay there, nothing wrong with that I would think.

Heh, maybe I should look into why things can sound better by “starving” the heater. I have heard of that before actually, it rings a dim bell.

 

[Isaacc7]

Heh, maybe I should look into why things can sound better by “starving” the heater. I have heard of that before actually, it rings a dim bell.

Hmm. My quick search about this didn’t bring up anything that would explain what I think I’m hearing. My next hypothesis is that maybe there is something about how these particular tubes are made that makes them sensitive to how hot the heater gets. Maybe the heater also heats up another part of the tube that impacts sound?

In any case I’m really digging the sound I’m getting with them for the first time. I will try to do some more experimenting this weekend to see if it’s just in my head.

 

[A2029]

Heh, maybe I should look into why things can sound better by “starving” the heater. I have heard of that before actually, it rings a dim bell.

Hmm. My quick search about this didn’t bring up anything that would explain what I think I’m hearing. My next hypothesis is that maybe there is something about how these particular tubes are made that makes them sensitive to how hot the heater gets. Maybe the heater also heats up another part of the tube that impacts sound?

In any case I’m really digging the sound I’m getting with them for the first time. I will try to do some more experimenting this weekend to see if it’s just in my head.

In IDHT tubes:
The heater heats up the cathode, and the cathode "boils off" electrons that fly across the interelectrode space from the cathode to anode. The cathode is specially coated with metals that help with boiling off the electrons. Sometimes this proprietary coating varies by manufacturer - which could account for some of the difference in tube tone from one manufacturer to another. Sufficient heating of the cathode is needed to provide a "space charge" to prevent/neutralize stray gas ions from bombarding the cathode with high velocity and eroding the cathode coating. The better the tube vacuum, and better the getter in a tube, the less stray gas ions present to bombard the cathode and cause cathode coating damage.

A voltage that is too high on the heater of an IDHT tube can cause overheating of the cathode and cause the active barium to separate from the oxide cathode coating, harming emission performance over time. However, in the short term a higher heater voltage can increase cathode coating activation and create a very strong space charge and higher cathode emissions as electrons are more excited to boil off the cathode. Too high of a voltage is anecdotally found to lead to a "harder", sharper, more solid state, and less relaxed sound.

A voltage that is too low on the heater results in poor cathode performance as well, as the cathode oxide coating is not able to reach optimal temperature for its particular chemistry to sustain sufficient emission, and cathode poisoning occurs from stray gases due to a lack of a strong enough space charge. Eventually the cathode oxide layer gets sputtered away from the cathode and onto the grid, and the tube loses emission over time. Too low heater voltage on an IDHT tube anecdotally results in a "weaker" tonal profile - having lower punch, bite, speed, etc., but being more relaxed/smooth. As the space charge gets too weak there is also an increase in shot noise due to space charge smoothing no longer being as effective.

There is a goldilocks zone for IDHT tubes of plus-minus 10% high/low of the datasheet value on the heater where there will be optimal emission characteristics. This can be stretched out to around plus-minus 15% or so before there is perceptibly noticeable changes to the tone of the tube. Running on the low side of voltage (e.g. -5-10%) can help extend cathode life in some tubes without much detriment to the tube tone. I usually run the tube heater at approximately the datasheet value as the manufacturer's intended and designed the tubes for (maybe 2-3% low) - the best trade off IMO between high emission and long life of the cathode coating, plus good tone.

In DHT tubes:
Similar to IDHT tubes, cathode life of the DHT filament (which is also the cathode in DHT tubes) is proportional to the voltage that it is run at. Running slightly lower voltage on the DHT filament can increase DHT filament/cathode life by reducing the decarburation rate (reducing the rate at which the surface cathodic layer is depleted from the DHT filament). One interesting thing that occurs with DHT tubes however is that when dropping filament voltage significantly (filament starving), THD at the same output voltage also drops (see here for the data: https://diyaudioprojects.com/mirror/members.aol.com/sbench102/dht.html).

 

[Xcalibur255]

An excellent write-up by one of the most knowledgeable people out there. This goes a long way toward explaining why proper biasing in an amp is such an artform and can be so tricky to get just right.

 

[HardstyleLoco96]

Hello, I need a bit of help and some recs. I bought myself a Ampapa A1 preamp. I'm new to tubes even though I been in the audio scene for a while. I'll be connecting the A1 to my SMSL SU-10 and SMSL SH-9. So mainly just changing the sound from the A1 but still having heaps of power from the SH-9. Anyways, what tubes would you guys rec for a nice airy soundstage while still having grunt and power in the sound. Keep in my mind, this is a budget pre amp, but I'd like to get the most out of it

 

[Isaacc7]

In IDHT tubes:
The heater heats up the cathode, and the cathode "boils off" electrons that fly across the interelectrode space from the cathode to anode. The cathode is specially coated with metals that help with boiling off the electrons. Sometimes this proprietary coating varies by manufacturer - which could account for some of the difference in tube tone from one manufacturer to another. Sufficient heating of the cathode is needed to provide a "space charge" to prevent/neutralize stray gas ions from bombarding the cathode with high velocity and eroding the cathode coating. The better the tube vacuum, and better the getter in a tube, the less stray gas ions present to bombard the cathode and cause cathode coating damage.

A voltage that is too high on the heater of an IDHT tube can cause overheating of the cathode and cause the active barium to separate from the oxide cathode coating, harming emission performance over time. However, in the short term a higher heater voltage can increase cathode coating activation and create a very strong space charge and higher cathode emissions as electrons are more excited to boil off the cathode. Too high of a voltage is anecdotally found to lead to a "harder", sharper, more solid state, and less relaxed sound.

A voltage that is too low on the heater results in poor cathode performance as well, as the cathode oxide coating is not able to reach optimal temperature for its particular chemistry to sustain sufficient emission, and cathode poisoning occurs from stray gases due to a lack of a strong enough space charge. Eventually the cathode oxide layer gets sputtered away from the cathode and onto the grid, and the tube loses emission over time. Too low heater voltage on an IDHT tube anecdotally results in a "weaker" tonal profile - having lower punch, bite, speed, etc., but being more relaxed/smooth. As the space charge gets too weak there is also an increase in shot noise due to space charge smoothing no longer being as effective.

There is a goldilocks zone for IDHT tubes of plus-minus 10% high/low of the datasheet value on the heater where there will be optimal emission characteristics. This can be stretched out to around plus-minus 15% or so before there is perceptibly noticeable changes to the tone of the tube. Running on the low side of voltage (e.g. -5-10%) can help extend cathode life in some tubes without much detriment to the tube tone. I usually run the tube heater at approximately the datasheet value as the manufacturer's intended and designed the tubes for (maybe 2-3% low) - the best trade off IMO between high emission and long life of the cathode coating, plus good tone.

In DHT tubes:
Similar to IDHT tubes, cathode life of the DHT filament (which is also the cathode in DHT tubes) is proportional to the voltage that it is run at. Running slightly lower voltage on the DHT filament can increase DHT filament/cathode life by reducing the decarburation rate (reducing the rate at which the surface cathodic layer is depleted from the DHT filament). One interesting thing that occurs with DHT tubes however is that when dropping filament voltage significantly (filament starving), THD at the same output voltage also drops (see here for the data: https://diyaudioprojects.com/mirror/members.aol.com/sbench102/dht.html).

That’s what I found in my googling as well. With indirectly heated tubes the heater really shouldn’t make a difference to the sound in the short term. That leaves me with two options; either this particular tube has other heat related issues or it’s just another case of an audiophile‘s unreliable memory

 

[Isaacc7]

An excellent write-up by one of the most knowledgeable people out there. This goes a long way toward explaining why proper biasing in an amp is such an artform and can be so tricky to get just right.

Biasing tubes can make a big difference in the sound. My particular amp allows me to screw up in all sorts of ways. I can vary the b+ from 250v-400v and also have a wide range of biasing voltage. There is also a triode/ultralinear switch and the ability to switch between 4 and 8 ohm outputs. All that and I use outboard supplies for the heaters. So this particular issue isn’t about biasing, just setting the proper heater voltage. That isn’t supposed to be difficult or even make any difference to the sound. It seems like it does in this case though.

It occurs to me that one variable I haven’t taken into consideration is that the adapter I’m using was made with dropping resistors with the idea it could be used with a 6.3v supply. I should look into it a bit more to make sure I’m setting it properly with that in mind. Still, even if it was off I wouldn't expect to hear a difference.

 

[Xcalibur255]

My 45 is a manual bias design, so I have direct experience with how bias current affects the sound. I once told the builder I could hear the difference between 34, 35, and 36 mA on the 45s and he told me I was full of it. I was telling the truth though, I can hear it.

I wish I could adjust the b+ voltage in some way on this amp, because it's a little on the hot side I have discovered over time. This amp can only take a single type of rectifier too, the 6BY5, so there aren't any options on that side of things unfortunately.

 

[Isaacc7]

My 45 is a manual bias design, so I have direct experience with how bias current affects the sound. I once told the builder I could hear the difference between 34, 35, and 36 mA on the 45s and he told me I was full of it. I was telling the truth though, I can hear it.

I wish I could adjust the b+ voltage in some way on this amp, because it's a little on the hot side I have discovered over time. This amp can only take a single type of rectifier too, the 6BY5, so there aren't any options on that side of things unfortunately.

Depending on the power supply, the b+ can vary with the line voltage. Haven’t measured it here but apparently it is noticeably higher than spec in a lot of places. There is a dealer near me that specializes in vintage gear, WE in particular, and he has to use bucking transformers so that the power transformers and the internal circuits aren’t run too hot. My amp’s meters don’t have fine enough markings to see single ma differences. I doubt I’d be able to hear anything that fine.

Speaking of which, I adjusted the heater voltage on the ktz41 while listening last night and… nothing. Sounds exactly the same at both voltages. So there we go, it was just my brain changing the sound based oin what I saw. Happens all the time in audio.

 

[LobalWarming]

Depending on the power supply, the b+ can vary with the line voltage. Haven’t measured it here but apparently it is noticeably higher than spec in a lot of places. There is a dealer near me that specializes in vintage gear, WE in particular, and he has to use bucking transformers so that the power transformers and the internal circuits aren’t run too hot. My amp’s meters don’t have fine enough markings to see single ma differences. I doubt I’d be able to hear anything that fine.

Speaking of which, I adjusted the heater voltage on the ktz41 while listening last night and… nothing. Sounds exactly the same at both voltages. So there we go, it was just my brain changing the sound based oin what I saw. Happens all the time in audio.

Brain-biasing. The final frontier in audio. Do they make a bias meter for that?

 

[Isaacc7]

Brain-biasing. The final frontier in audio. Do they make a bias meter for that?

Oh man, that would absolutely wreck the audiophile industry! The longer I do this the more I understand that I am a much bigger variable than the equipment. My mood, alertness, and who knows what else makes a bigger difference than the toe in of my speakers or which output tubes I‘m using. Hell, I have a great time with the radio in my car sometimes!