[OctavianH]

I have the same adapter for my Tung Sol 6F8G black glass round plates. The adapters were from Poland - no connections with Felliks Audio. Very well made adapters. I had them for many years now. Here I'm using it in my Wa22 and it's quiet. No noise at all.

Yes, the adapters are top class. There is a guy in Poland called on Ebay piotrdoor and he is making these adapters. I have bought from him several pairs of 6F8G->6SN7 and I have also EL32->6V6 and EL38->6V6. I am not sure why he is not very interested anymore in making more adapters, I've asked him for others. Anyway, these are the best adapters I have.

Later edit: These adapters appeared also on some speaker amplifiers made by Feliks Audio. On the old F.A. website I was able to see some of them, but now they upgraded the website and removed those amplifiers.

 

[OctavianH]

Gaining a little bit of confidence from the measurements of Chris, I started today to measure my double triodes mentioned yesterday. All are quite good matched, except 2 special cases, but luckily for me, I found at least one from each type very well matched.

So, more fun with DuoVac!

My values were around -6.5V to -7V for the operating point we want to use in our amps.

And the winners are:

Now, comparing these with the values proposed by Tomas, who set the -6.3V as the bias for 6SN7, I would say mine were higher, meaning that these tubes might exceed 5mA on the amplifier with his bias proposal according to my measurements. I've seen that he prefers to run them "strong". Looking at the measuremens of @hpamdr I see his piece of 7N7 needed -6.3V like Tomas said. So, I have to see if I am doing something wrong here.

 

[hpamdr]

Gaining a little bit of confidence from the measurements of Chris, I started today to measure my double triodes mentioned yesterday. All are quite good matched, except 2 special cases, but luckily for me, I found at least one from each type very well matched.

So, more fun with DuoVac!



My values were around -6.5V to -7V for the operating point we want to use in our amps.



And the winners are:



Now, comparing these with the values proposed by Tomas, which set the -6.3V as the bias for 6SN7, I would say mine were higher, meaning that these tubes might exceed 5mA on the amplifier with his bias proposal according to my measurements. I've seen that he prefers to run them "strong". Looking at the measuremens of @hpamdr I see his piece of 7N7 needed -6.3V like Tomas said. So, I have to see if I am doing something wrong here.

Measure is really fun, but a bit less when you discover that what you got long time ago is not exactly what was described....
Keep in mind that i buy many tubes not NOS but a bit used normally above 80%
I have others 7N7 and VT-231 I measured above 6.5 and 6N7G above 7V. (I will also see if i need to re-calibrate the tracer)
I've almost convinced my friend that i need to keep current uTracer and that he need a better one

 

[OctavianH]

Measure is really fun, but a bit less when you discover that what you got long time ago is not exactly what was described....
Keep in mind that i buy many tubes not NOS but a bit used normally above 80%
I have others 7N7 and VT-231 I measured above 6.5 and 6N7G above 7V. (I will also see if i need to re-calibrate the tracer)
I've almost convinced my friend that i need to keep current uTracer and that he need a better one

In this case I would say we need to revisit the 6SN7 bias to -6.5V to be safe. And that you should change your job to sales/marketing because you convince a lot of people to buy stuff, and many of them are happy. Look at me with my DuoVac. You were born to be a successful salesman!

 

[hpamdr]

In this case I would say we need to revisit the 6SN7 bias to -6.5V to be safe. And that you should change your job to sales/marketing because you convince a lot of people to buy stuff, and many of them are happy. Look at me with my DuoVac. You were born to be a successful salesman!

Not really
To be safe, we have a known target but should under bias and fine tune to the best with tube on socket !

 

[OctavianH]

Not really
To be safe, we have a known target but should under bias and fine tune to the best with tube on socket !

I have that "fixed bias" switch but I think I'll ask Tomas to set it for input at -6.6V to be able to use my L63 without problems. For weaker tubes I think I'll use the variable bias and lower it a little bit. I'll think about this. The best would be to be able to adjust both variable and fixed bias trimmers. But I am not sure this is possible.

@SonicTrance What do you think?

 

[SonicTrance]

I have that "fixed bias" switch but I think I'll ask Tomas to set it for input at -6.6V to be able to use my L63 without problems. For weaker tubes I think I'll use the variable bias and lower it a little bit. I'll think about this. The best would be to be able to adjust both variable and fixed bias trimmers. But I am not sure this is possible.

@SonicTrance What do you think?

I can set it at -6.6V no problem. The fixed bias is also adjustable but with a trimmer on the inside. I can guide you if you ever want to adjust it.

 

[OctavianH]

I can set it at -6.6V no problem. The fixed bias is also adjustable but with a trimmer on the inside. I can guide you if you ever want to adjust it.

Yep, according to my measuremens my best medium values for fixed bias are Input=-6.6V and Output=-21V (for 6SN7/KT66 and all equivalents).
I measured what currents will have several tubes when -21V is set as bias and KT66 are fine (at the top limit) but the 6L6 are under 15mA.
For mostly 6SN7/7N7/6F8G the -6.6V is fine. It is not clear to me how on Levi's amplifier KT66 and 7581A/6L6 worked well at the same bias.

If I will ever want to adjust it, what panel I have to unmount? Is there any way to make it more accessible?

 

[SonicTrance]

Yep, according to my measuremens my best medium values for fixed bias are Input=-6.6V and Output=-21V (for 6SN7/KT66 and all equivalents).

Alright, I'll set your output "stock bias" to -21V also.

It is not clear to me how on Levi's amplifier KT66 and 7581A/6L6 worked well at the same bias.

The 7581A did run slightly cooler than the KT66 at the same bias point. The difference was so little I did not think it was worth a own spot. There will always be differences from tube to tube of course.

If I will ever want to adjust it, what panel I have to unmount?

Unscrew 4 screws and take off bottom panel.

Is there any way to make it more accessible?

I've just finished that stage and the bias switches are all wired up. It was very time consuming so I'd say no to that
What's the point of having a fixed bias switch if you want it easy adjustable? Just use the adjustable one at the top plate?

 

[OctavianH]

I've just finished that stage and the bias switches are all wired up. It was very time consuming so I'd say no to that
What's the point of having a fixed bias switch if you want it easy adjustable? Just use the adjustable one at the top plate?

Yep, you're right. If 4 screws are all it has to be done, I'll ask you if needed (otherwise I'll adjust other trimmer and destroy your work LOL).
-21V seems fine to me for the tubes I have. As you see, the "strongest" GEC KT66 goes up to 23.6mA but all others are below. So it seems fine for me. If you think we are too close to 24mA and there is a risk for the fuse to blow, set it to -21.2V or -21.4V and it should be also fine. I let you decide. Of course, I am interested to know if at -21V on your tests the KT66 work fine.

 

[SonicTrance]

Yep, you're right. If 4 screws are all it has to be done, I'll ask you if needed (otherwise I'll adjust other trimmer and destroy your work LOL).
-21V seems fine to me for the tubes I have. As you see, the "strongest" GEC KT66 goes up to 23.6mA but all others are below. So it seems fine for me. If you think we are too close to 24mA and there is a risk for the fuse to blow, set it to -21.2V or -21.4V and it should be also fine. I let you decide. Of course, I am interested to know if at -21V on your tests the KT66 work fine.

Ok great! =)

Maybe tonight I can test the output stage for the first time. I'll let you know!

 

[OctavianH]

Maybe tonight I can test the output stage for the first time. I'll let you know!

Yep, in the meantime I try to determine myself to stop listening to my pair of KT88 so I can measure it again but it sounds so good.

The reason for my need to remeasure these KT88 is that last time I found a grid bias of -21.6V. This cannot be fine, since KT88 have to dwell around -25V according to the datasheets. If -21.6V is fine, it means I can use GEC KT66 and this pair of Psvane KT88T2 with the fixed grid bias, which is something which doesn't sound fine to me.
Another thing I might do is to measure my other pair of KT88 from Shuguang and let them turned on for around 10 minutes to be sure they are warmed enough. These KT88 seem slow starters when measured (same for 6550A).

 

[SonicTrance]

The reason for my need to remeasure these KT88 is that last time I found a grid bias of -21.6V. This cannot be fine, since KT88 have to dwell around -25V according to the datasheets.

Doesn't seem right. KT88 should draw around 40mA @ -20Vg, 200Va.

 

[SonicTrance]

@OctavianH
The Gold Lion KT66 pulls 20mA @ -23.5Vg, 200Va. That's very unexpected. You still want the stock bias @ -21V?

 

[OctavianH]

@OctavianH
The Gold Lion KT66 pulls 20mA @ -23.5Vg, 200Va. That's very unexpected. You still want the stock bias @ -21V?

Yep. There's something "fishy" with those KT66. According to the datasheet the Vg should be lower.

Later edit:

Check this