TUBE EXPERIMENTATIONS | by Mr. Miniwatt

[Mr.Miniwatt]

Looking at the datasheet of 6AU6 during the morning coffee, and also reading some remarks on other forums where people claim this is a sharp cut-off pentode not suitable for audio, I realize I am not quite using efficiently this tube at 180V/4mA. What I learned during the years is that these tubes perform better when used around the typical operation from the datasheet. So if we look here, we see the following:



Now, if I would try to avoid forcing the tube at maximum plate dissipation, even if this seems to be 3.5W, I guess a better way to use this would be maybe around 200Va and 10mA. I guess if you are around 2/3 of the max plate dissipation you are fine.

This reminds me that my amplifier is limited, my next one if I will ever own one will have variable plate voltage on both stages and also a way to set very different currents. For example to be able to use EL34 at maybe 400Va and 60mA on output and 6AU6 at 200Va and 10mA on input, then to switch to EL86 at 200Va and maybe again 60mA while putting input stage at 100Va and 10mA for E88CC. Sounds crazy, I know, but this is the amplifier which will allow to me to elevate my experiments. Of course, this adds a lot of complexity and nobody is crazy to build me that, maybe I'll build it myself when I retire. And why not, gyrator loaded plates, fixed grid bias or CCS and who knows what more. For sure solid state and tube rectification switchable. When using big boys to switch to solid state and when wanting a more creamy old style of sound to switch to tube rectification and use old european 4 volt rectifiers like AZ1. In the end the tubes I love the most are old european pentodes from the Philips red series. It is also true that the tube socket I hate the most is the european side contact one. Love and hate relationship, since we are in the st. Valentin weekend. LOL

PS. Many associate 6AU6 with EF86. I never tried EF86 but I tried some "TV tubes" like EF41, EF80, EF85, EF183, EF184 and some of them were not so bad. All go towards a more sterile sound, but the detail retrieval is quite ok. Of course, others are much better so only crazy people like me found these experiments to be valuable.

PS2. Forgot that I need 4V and 6.3V heaters for output and 6.3V and 12.6V heaters for input. One of the reasons is that I have this tube, Philips FDD20 13V heaters:


They say this is close to a 6N7, so double triode common cathode and should work fine with 12.6V heaters. I can try to use it with external power supplied adapters but well, where would be the beauty?

 

[Mr.Miniwatt]

After several hours with Telam EL86 and Mullard 6AU6, I had the feeling that I lost some resolution and transparency compared to Philips 4654 and the Siemens E88CC. I guess both of the tubes are inferior to the counterpart in the previous combo. And now another question arised in my mind: let's try something I quite know and liked in the past, just to see how I see the 6AU6 compared to it. Maybe I am biased by novelty and the 6AU6 is not as good as some of my old favourites. So I put a Brimar 6SL7 in the input stage, more exactly this tube:



6SL7 was in the past one of my favourites, so I used extensively many of them in different circumstances. Brimar and Fivre 6SL7 were some of the ones I was most of the times choosing. I have several versions of Brimar 6SL7 (including CV1985) but this one from above is my favourite.



On the amp, works as a charm, at 3mA to be at a decent marging below the 1W/section rating. I plan to leave this for a while here and see how Mullard 6AU6 seems compared to it. A first impression would be that it's close in sound quality. Of course, differences of gain and each tube's "house sound" but in rest, that Mullard 6AU6 pair can sit at the table with 6SL7 if you would ask me. Of course, 6SL7 is an idea more muscular, I have a feeling of more bass and maybe an idea more dynamic, but not a night and day difference. And yes, both are below E88CC in resolution, clarity and so on. I am quite disappointed about the 6SL7, I thought I will be more impressed about it after a long pause.

 

[Mr.Miniwatt]

Tube question of the morning: What's the difference between Telefunken EC806S and Telefunken E86C, both gold pins?

I have here a Telefunken EC806S:



I also have here a Telefunken EC86 gold pin:



And why not, a Telefunken EC86 non gold pin:



If I look at the getters indeed, EC86 have 1 rod while the EC806S seems to be fixed by 2 of them. And also some differences on the 3rd mica on all of them.

This question raised to my mind when I saw on interned a few minutes ago a photo of a Telefunken branded "EC806S/E86C". Too many versions. So do we have 4? EC86 non gold pin, EC86 gold pin, E86C gold pin and EC806S?

I will borrow a photo from here to show the "EC806S/E86C" one:



This one seems to be similar with my EC806S and different from the E86C and EC86. And now I see that if I look closer, that "second" rod is not touching the getter, which seems slimmer and larger than the getters of the others.

More analysis, now that I put these 3 photos in front of me, tubes being stored. The EC806S has U-code meaning it was manufactured in Ulm factory while the EC86 non gold pin version has B-code referring to the factory located in Berlin. Ok, now what? That's all the difference? And the EC86 gold pin has the first letter deleted from the date code.

 

[Mr.Miniwatt]

I will need to investigate more with these Telefunken EC86 gold pins. But in the meantime I will continue my tour of "cheap" alternatives to input stage and this time I went to Tungsram 6J6. This tube is interesting, it has a foil getter, I guess they call it "dimpled foil getter" in fact.



Anyway, on the amp I was remembering these were more towards neutral last time I checked them, but today I hear a lot of bass when paired with the Telam EL86. I am a little bit intrigued, I guess I need to listen more to these. Not bad sounding but a little bit dull, I guess highs are recessed or something.



H7 as a date code, printed on vertical, I guess according to here it's March 1961.

 

[Mr.Miniwatt]

I wanted to remain in the same price range, but experience some more styles of sound. So I went to one of my favourite output types called EL81. EL81 has a quite wide variety of manufacturers and many of them sound good. I tried Philips X-code (Sittard) which are my favourites because of the rich dense presentation, but also B-code (Mullard, Blackburn) and Tesla versions. All of them were nice, and besides these the french Mazda 6DR6 works in the same adapters. This time one of the cheapest, Siemens EL81 which I suspect is in fact manufactured by Tesla:



I left the same Tungsram 6J6 on the input stage.



EL81 is more musical and romantic than the EL86, but the Siemens EL81 version is the most neutral of the ones I have around. So not much romantism today, soundwise these go towards a more clean and analytical side. EL81 is ignored by many and still sells extremely cheap for the sound they provide. UltraSonic Studios Oblivion, the first model from the same builder who made my amplifier used EL81 in the output stage and 6AV/AT6 in the input stage (there were 2 versions, one with 6AT6 and one with 6AV6). Single ended input stage with balanced output stage, so 4 x EL81.

Some words from the builder about Oblivion below:

The topology is fully balanced, open loop (no gNFB) amp with parafeed transformer output. The input stage is single ended, followed by CCS loaded source followers that drive the output tube grids. The grid drivers relieve the input tube of any drive duty. Tubes do only voltage amplification. Tubes do what they do best and FETs do what they do best, supply current.
The LTP output stage is perfectly balanced by using a very powerful CCS in the tail. All tube anodes are gyrator loaded. Both B+ and B- supplies are regulated. This amp can drive most headphones from very low impedance up to high impedance with ease. It’s also capable of driving speakers.
The speaker outputs are also balanced, meaning the black terminal is not ground but is carrying signal in anti-phase to the red terminal. This is a great way to drive a speaker coil.
2,3W into 8ohms, 1W into 20 ohms, 420mW into 50 ohms, 220mW into 100 ohms, 110mW into 200 ohms, 80mW into 300 ohms.

I never heard Oblivion, but it was a predecessor of Eternity. In my amplifier EL81 sounds spectacular and it is a very budget friendly choice for an output stage. It is true that I consider EL86 superior, but from time to time I return with pleasure to EL81. Interestingly enough, I consider both EL81 and EL86 superior to the famous EL84.

And even if I will not use these today, the Mazda 6DR6, a very beautiful tube.

 

[Mr.Miniwatt]

After a lot of experiments, back to the classics. Philips 4689 and Siemens E88CC Aϕ revision with grey shield.



While I consider the 4654 metal base the reference, I have to admit I like to sweeten a little bit the E88CC with the 4689. While it sounds more musical and an idea more fun tuned to me, the 4689 metal base remains in big lines at the same sound quality level like the 4654 metal base.