[Isaacc7]

Operating points are important boys and girls. I have been running the 7c5 tubes at 250v 30ma with the idea of taking it easy on the tubes. While I noticed improvements when I switched inputs, output impdance, and to ultra linear, I still wasn’t getting the sound I thought the tubes could give me. It was always a bit on the lightweight side lacking heft in the midrange. Went back and looked at the data sheet. Moved to 285v 35ma and what a difference. It filled in the lower midrange and added a touch of depth. Now we’re cooking! Currently using Tungsol 6c8g inputs. I’ll try triode again tonight and see if my triode/ultra linear hot take holds up.

 

[UntilThen]

Where are the pictures Isaacc ?

 

[Isaacc7]

Where are the pictures Isaacc ?

I will pictures from time to time. Here’s the current configuration; Sylvania JAN 7c5 Tung Sol 6c8g.

 

[UntilThen]

It’s a purposeful looking amp and the light is blue.

 

[Isaacc7]

It’s a purposeful looking amp and the light is blue.

To the eye it is a cool white. It really is a serious looking amp. I laugh because right now I’m getting all of 5 watts in class A. AB1 goes up to 13 or so but still a lot less than the amp looks lol.

 

[Isaacc7]

Huh. Apparently I completely misunderstood how ultra linear works. I thought the screen would always see less voltage than the plate by 43%. Wrong. Need to set the plate voltage such that it doesn’t exceed the screen voltage. Glad I noticed this before going to higher power tubes. And lower power too I guess. I’m just glad lol.

 

[OctavianH]

Operating points are important boys and girls. I have been running the 7c5 tubes at 250v 30ma with the idea of taking it easy on the tubes. While I noticed improvements when I switched inputs, output impdance, and to ultra linear, I still wasn’t getting the sound I thought the tubes could give me. It was always a bit on the lightweight side lacking heft in the midrange. Went back and looked at the data sheet. Moved to 285v 35ma and what a difference. It filled in the lower midrange and added a touch of depth. Now we’re cooking! Currently using Tungsol 6c8g inputs. I’ll try triode again tonight and see if my triode/ultra linear hot take holds up.

And now comes the 1M$ question: how do you determine the optimal operating point for a tube type based on the datasheet? OK, I know, you'll tell me "on the linear zone" of the graph, but can you elaborate on this? I mean looking at this graph of 7C5 I marked around 250V->285V and could not see a lot of difference.

So, what parameters are you taking into consideration? The idea is to go towards max ratings?

PS. I know this is not a thread for this but, you started.

 

[baronbeehive]

And now comes the 1M$ question: how do you determine the optimal operating point for a tube type based on the datasheet? OK, I know, you'll tell me "on the linear zone" of the graph, but can you elaborate on this? I mean looking at this graph of 7C5 I marked around 250V->285V and could not see a lot of difference.

You've pretty much got it. The linear points are where the lines are fairly equally spaced and horizontal, hence linear, the non linear points are where they are more irregularly spaced and curved looking, hence non linear. Any point could be selected in this region for least distortion but you have to take into account the max current of the tubes for example so it's necessary to stay within the range of the tube for best performance. There are other factors such as voltage swing but I found this doesn't make as much difference as the best bias point in terms of linearity. The anode and cathode values give the "best" operating point in the "linear zone", but often it's a case of experimenting.

It would be interesting to know, for example, where the operating points in Issaac7 were to produce the difference he found.

 

[OctavianH]

I tried to mark with those red lines 250V and 285V he mentioned and both are in the linear zone.

 

[baronbeehive]

I tried to mark with those red lines 250V and 275V6 he mentioned and both are in the linear zone.

Oh yes, I didn't notice that.

This shows the 2 operating points. The one at 250v is in a slightly more "compressed" region where the lines are closer together, the one at 285v is slightly up towards a less compressed region where the lines are more separated, but both fairly equal in linearity. The one at 285v with the bias point at -15v also has room to swing from -5v to -25 whereas the point at approx -17v can only swing between -9v and -25v approx. There are other factors which could affect it, such as whether the plate current and voltage are optimal for the tube, without going into a long discussion.

My view, if it was my amp, would be for a optimal bias point around -6v just looking at the graph, but plate current might be too high. But he should consult Sonic about the amp's operation as I'm sure he has done.

 

[Isaacc7]

I tried to mark with those red lines 250V and 275V6 he mentioned and both are in the linear zone.

Wow, I ended up writing more than I thought I would about this. TL;DR different combos in the linear zone sound different and run up against limitations of the hardware.

Yeah, both of those voltages are in the linear zone and if you were building from scratch either could be used in theory. There are a few other things to keep in mind though. The first is an equipment issue. This particular amp has a regulated, variable B+ voltage. It can range from 250v-400v. The trick is that the amp always produces 400v and it is up to the regulator to eat the extra current if I dial down the voltage. There are limits to what the regulator can handle, Tomas says the max it can handle for reliable service is 20w. That means that if I set the voltage to 250v the max I should set the bias current is 30ma. Bumping up the B+ even a little frees up the regulator which allows it to either run cooler or I could bias the tubes a little hotter. It turns out that setting tubes at 250v 30ma maxes out the regulator at 20w. Setting the tubes to 275v 35ma actually drops the load of the regulator to 17.5 watts. Running cooler is better for the electronics long term.

That brings me to the other thing. The characteristic curves show how a tube is supposed to react as voltage swings back and forth across the range. They are useful in that they can show you the limits of the tube and where it will generate harmonic(?) distortion. The trick is that a tube does not operate in a… uh, vacuum (ugh). The whole audio circuit, including the speakers, is a dynamic electrical system. I’m getting outside my wheelhouse here but I see plenty of discussions of DIY builders talking about dialing in a tube’s settings based off of system performance. Different transformers, different kinds of feedback, different power supplies, etc. will all play a role in what kind of distortion (harmonic, IM, crossover) and how much of it there is at any given operating point. And all of that is evaluated by looking at equipment while running the amp into a resistive load. That is a good approximation of how headphones work in the circuit but a reactive load like speakers adds a whole other level of complexity to how the circuit behaves.

That is my long winded way of saying that while there may be a range of settings that the tube itself might have a “linear” response it does does not predict how the system will sound. If I were making the amp from scratch I would be able to balance all this stuff out. My amp is a (literal) black box so I have to figure out which settings output the best sound. I have a feeling that each set of output tubes will have a sweet spot of operating point and mode (triode or ultra linear) for my speakers.

After all these years of messing with audio I feel like I have an appreciation of both the engineering required to make things work and the hopeless complexity of the interaction of electricity inside equipment, it’s interaction with speakers, and our brains. All I can do is twiddle knobs till my ape brain likes what it hears.

 

[Isaacc7]

Oh yes, I didn't notice that.



This shows the 2 operating points. The one at 250v is in a slightly more "compressed" region where the lines are closer together, the one at 285v is slightly up towards a less compressed region where the lines are more separated, but both fairly equal in linearity. The one at 285v with the bias point at -15v also has room to swing from -5v to -25 whereas the point at approx -17v can only swing between -9v and -25v approx. There are other factors which could affect it, such as whether the plate current and voltage are optimal for the tube, without going into a long discussion.

My view, if it was my amp, would be for an optimal bias point around -6v just looking at the graph, but plate current might be too high. But he should consult Sonic about the amp's operation as I'm sure he has done.

Pretty sure this graph is for pentode operation, the curves look different for triode and ultra linear. One of the limitations of going by the curves is that you don’t always have curves to reference! I have been using this website to figure out starting points https://www.vtadiy.com/loadline-calculators/power-stage-calculator/. I have also been looking at the data sheets for the tubes themselves To determine typical and maximum operating points. When I start going further afield with tubes like the 6ar6 and el38 I will rely on the hive mind and max ratings to help set the parameters.

 

[OctavianH]

Wow, I ended up writing more than I thought I would about this. TL;DR different combos in the linear zone sound different and run up against limitations of the hardware.

Yeah, both of those voltages are in the linear zone and if you were building from scratch either could be used in theory. There are a few other things to keep in mind though. The first is an equipment issue.

Now I understood, many thanks for your detailed reply. I did not knew all those specific construction limitations but it is fascinating. You are lucky to have such a nice amp but it is also, in the end, your merit to ask for something like this. Your journey is also very rewarding for all of us because, at least in my case, I have a lot to learn. I have no idea if other amp with variable operating point exists.

Pretty sure this graph is for pentode operation, the curves look different for triode and ultra linear.

This is true, I did not knew in which graph to look for ultralinear, but I wanted to understand if it is a way to predict if, for example, I have tube type A, B or C I have not tried before, based on the datasheet and some graphs and knowing the operating point that tube will have in my amp, tube B would be the perfect match and then go towards B instead of trying all A/B/C. This was somehow my purpose when I asked. This subject is very important for me, while I usually search for different tube types I have not used before and take advantaje of the flexibility of Eternity to try them in my amp, if I can narrow the option list and go forward to the best sounding ones, I would be much in advantage.

And now, if everyone listens to vinyl on expensive systems and post pictures here, I'll put a CD today:

Very nice booklet, vinyl does not have one right?

 

[baronbeehive]

.... All I can do is twiddle knobs till my ape brain likes what it hears.

That's what I was trying to say lol.

 

[baronbeehive]

I have no idea if other amp with variable operating point exists.

I haven't heard of one... except mine lol. But that is hard wired not adjustable any more.
.

Very nice booklet, vinyl does not have one right?

No, I remember the old fashioned sleeve notes though.

Next question, where do you get those Megadeth beer mugs?