[L0rdGwyn]

Some of the noise issues of the GOTL stem from using high gain output tubes. Unlike a cathode follower output, an SRPP has voltage gain, so using high gain output tubes like 6BX7 will increase the volume sensitivity as well as the noise floor. There is no step down output transformer, so what you end up with is an excessive amount of gain for headphones in that configuration. The amp was originally made for 6AS7G with its mu of 2 which is much more reasonable. Hope that makes sense, at work so cannot say more right now!

 

[Paladin79]

Some of the noise issues of the GOTL stem from using high gain output tubes. Unlike a cathode follower output, an SRPP has voltage gain, so using high gain output tubes like 6BX7 will increase the volume sensitivity as well as the noise floor. There is no step down output transformer, so what you end up with is an excessive amount of gain for headphones in that configuration. The amp was originally made for 6AS7G with its mu of 2 which is much more reasonable. Hope that makes sense, at work so cannot say more right now!

I am so used to not being able to comment on specific products that may be advertised here I tend to back away from such discussions. I got in trouble once for saying a specific headphone might be my second choice.

 

[Monsterzero]

Some of the noise issues of the GOTL stem from using high gain output tubes. Unlike a cathode follower output, an SRPP has voltage gain, so using high gain output tubes like 6BX7 will increase the volume sensitivity as well as the noise floor. There is no step down output transformer, so what you end up with is an excessive amount of gain for headphones in that configuration. The amp was originally made for 6AS7G with its mu of 2 which is much more reasonable. Hope that makes sense, at work so cannot say more right now!

I do not attempt to pretend that I understand 95% of what some of you guys talk about in the diy world, but I do understand that certain tubes have a higher gain factor than others and what the pitfalls of using these tubes can present, but when I say many tubes were unlistenable on the GOTL, I'm talking about using them with 6as7g and 6080 tubes, not just 6bx7s

 

[JazzVinyl]

I do not attempt to pretend that I understand 95% of what some of you guys talk about in the diy world, but I do understand that certain tubes have a higher gain factor than others and what the pitfalls of using these tubes can present, but when I say many tubes were unlistenable on the GOTL, I'm talking about using them with 6as7g and 6080 tubes, not just 6bx7s

I did not have "many" tubes that were un-listenable in the GOTL. That was quite rare for me. But it was not what you would call "dead silent" with no input applied.

Mine had a Lindahl transformer problem as well, it made a very annoying / loud buzzing noise when some combinations of low mu (6AS7/6080) power tubes were used. There was no cure except "not to use those tube combos". Never once did it with 6BX/BL7's...

 

[Monsterzero]

Mine had the Lundahl upgrade as well

 

[L0rdGwyn]

I am so used to not being able to comment on specific products that may be advertised here I tend to back away from such discussions. I got in trouble once for saying a specific headphone might be my second choice.

Ahh well I don't have MOT status, so for now I can speak my mind. The GOTL is a DIY amplifier no longer in production, I'm familiar with it as I've owned one and also done work on a few of them. If my MOT status were to change, I'd probably have to scrub my posts, I aimed some pretty harsh criticisms at a certain 300B amplifier not too long ago, that would probably have to go D:

I do not attempt to pretend that I understand 95% of what some of you guys talk about in the diy world, but I do understand that certain tubes have a higher gain factor than others and what the pitfalls of using these tubes can present, but when I say many tubes were unlistenable on the GOTL, I'm talking about using them with 6as7g and 6080 tubes, not just 6bx7s

Oh okay well if they were still noisy with low gain output tubes, then it was likely heater to cathode leakage, which would be an issue inherent to the input tubes themselves. The heater circuit in the Airmid has a DC elevation above ground, which makes that leakage a non issue.

 

[JazzVinyl]

Oh okay well if they were still noisy with low gain output tubes, then it was likely heater to cathode leakage, which would be an issue inherent to the input tubes themselves. The heater circuit in the Airmid has a DC elevation above ground, which makes that leakage a non issue.

Agree, that is THE way to go. I was able to cure many noise ills in the GOTL by externally heating the SN7 driver socket with clean DC.

 

[L0rdGwyn]

Agree, that is THE way to go. I was able to cure many noise ills in the GOTL by externally heating the SN7 driver socket with clean DC.

Regulated DC for heaters are definitely a good option when ultra low noise is needed (I use them in my phono stage and DAC), but this is actually AC heaters with a ground lift, the heater circuit is referenced to a positive voltage above ground. This basically makes the heater and cathode function as a reversed bias diode, so no noisy leakage currents from the heater will enter the tube's cathode. IME, for your average amplifier using indirectly-heated tubes, AC heaters are just fine and DC is a luxury, it's only when you are talking about small signal amplification (e.g., phono stage, DAC) with tubes that I would say DC heaters are a necessity.

 

[Xcalibur255]

I do not attempt to pretend that I understand 95% of what some of you guys talk about in the diy world, but I do understand that certain tubes have a higher gain factor than others and what the pitfalls of using these tubes can present, but when I say many tubes were unlistenable on the GOTL, I'm talking about using them with 6as7g and 6080 tubes, not just 6bx7s

To phrase a different way, one of the things Keenan is saying here is that some circuit designs are setup in such a way that the output tubes do not contribute any amplifying gain to the music signal at all. I believe his Airmid is like this, where as the GOTL on the other hand uses the output tubes in a way where they do contribute gain. Each style of circuit has its pros and cons but this is one of the reasons Airmid achieves lower noise and a blacker background over the GOTL. This is also a major reason why the original GOTL design is so fussy with the 5998, because the original first implementation of this amp was meant specifically to use the 6080/6AS7 and the 5998 tube has 2.5x higher gain which was bringing out lots of power supply noise you were not originally meant to hear.

Things did change over time of course, some of this is only really applicable to really old versions of the GOTL like mine.

 

[mordy]

To phrase a different way, one of the things Keenan is saying here is that some circuit designs are setup in such a way that the output tubes do not contribute any amplifying gain to the music signal at all. I believe his Airmid is like this, where as the GOTL on the other hand uses the output tubes in a way where they do contribute gain. Each style of circuit has its pros and cons but this is one of the reasons Airmid achieves lower noise and a blacker background over the GOTL. This is also a major reason why the original GOTL design is so fussy with the 5998, because the original first implementation of this amp was meant specifically to use the 6080/6AS7 and the 5998 tube has 2.5x higher gain which was bringing out lots of power supply noise you were not originally meant to hear.

Things did change over time of course, some of this is only really applicable to really old versions of the GOTL like mine.

As I recall, there was a GOTL option for a 5998 switch, but the people who ordered it did not notice much of a difference.

 

[JazzVinyl]

Regulated DC for heaters are definitely a good option when ultra low noise is needed (I use them in my phono stage and DAC), but this is actually AC heaters with a ground lift, the heater circuit is referenced to a positive voltage above ground. This basically makes the heater and cathode function as a reversed bias diode, so no noisy leakage currents from the heater will enter the tube's cathode. IME, for your average amplifier using indirectly-heated tubes, AC heaters are just fine and DC is a luxury, it's only when you are talking about small signal amplification (e.g., phono stage, DAC) with tubes that I would say DC heaters are a necessity.

Was just reading how the elevated ground lift works. Seems extremely easy, connect the transformers' CT to a positive DC voltage. Why is this not the standard practice?

 

[L0rdGwyn]

Was just reading how the elevated ground lift works. Seems extremely easy, connect the transformers' CT to a positive DC voltage. Why is this not the standard practice?

It is extremely easy and I have no idea!

 

[Xcalibur255]

As I recall, there was a GOTL option for a 5998 switch, but the people who ordered it did not notice much of a difference.

This was my idea actually (in the GOTL amp world at least, certainly not in the larger world of tube amps). I was teaching myself about how different tubes bias up differently in the same circuit and learned that the 5998 can and often does sound thin or lean in many circuits people use it in because it runs "cold" in those circuits. For example an original GOTL will bias a 6080 tube at 100mA plate current, but when you drop a 5998 in there it will bias up at only 60mA. The 5998 actually has higher max plate current and higher max dissipation ratings too so there is lots of room to go up with the plate current.

You can kind of go down a rabbit hole with this actually. It gets debated a lot whether to run a tube hard or not, and whether it changes the sound or not. I think it's at least partially dependent on the circuit implementation too so you probably can't apply conclusions generally across different amps. My ears have consistently told me over the years that at a certain point where the tube is running cold (well below its maximum bias ratings) it will begin to have a skeletal sound. Not the same thing as having thin or weak bass, it's more like the musical notes sound thin on the inside if that makes any sense.

So I convinced Glenn to rig up a switch that would change the biasing so the 5998 would draw more current and bias a bit "hotter" than it would with the 6080 bias value. Glenn was pretty conservative about it too, he didn't want somebody to be able to red plate a 6080 if the switch was in the wrong position so it doesn't change the bias all that much, only around 20mA I think. I know some people didn't think it made that much of a difference but I will always stand firm on my own opinion. My ears heard a fairly big difference, a very positive one. The 5998 to me sounds bolder and meatier and somehow more relaxed and less "in your face" at the same time with the higher bias. Like its speaking more confidently with a smoother voice instead of trying to shout with a thin screechy one. I'm trying to explain the sound via analogy here of course, the differences were not huge like the wording might make it seem, but I don't really have to listen that closely to hear it personally. It's always been a big enough change that it was easy to hear for me.

A different circuit that uses a CCS for biasing might turn all of this on its head too.

edit: cleaned up some stream of consciousness typing mistakes here and there.

 

[JazzVinyl]

The GOTL allowed a dizzying array of tubes to be used. The devout tube rollers' dream amp!

 

[Paladin79]

The GOTL allowed a dizzying array or tubes to be used. The devout tube rollers' dream amp!

lol