The planar v72 from scratch

[solkatten]

Yep Im ok with 0,05 % figure, mostly H2 just need to fix the high end ringing…

I must say Im impressed with clearity and details that the 6s17-v brings. Its very non microphonic and silence… with a -12 dB pad on the output gain controll work fine

 

[gyraf]

Do we know anything about it's service life expectancy? That's where a good deal of the mil-spec tubes fall short, because they were made for utmost reliability but for a very short lifespan..

/Jakob E.

 

[solkatten]

Do we know anything about it's service life expectancy? That's where a good deal of the mil-spec tubes fall short, because they were made for utmost reliability but for a very short lifespan..

/Jakob E.

5000 hours in the nose cone of soviet Nukes up to 500g of acceleration

 

[solkatten]

Added a pentode line stage to this design to add some flavour… tri/ul/pen switch. HP and LP filter…

 

[solkatten]

Here is the input stage + LP and HP. Just a bit of fixed feedback, to kick down gain a notch, keeping THD low and input impedance high...
The pentode line stage (E81L) is a modded version of Jonte Knif Therapy pre... To build this stage was the reason I bought the LL1671 trafo in the first place...

On the input I going to try my home brewed 1:4,5 toroidal IPT...

https://www.nostubeshop.com/documents/E81L therapy_pre.jpg

 

[solkatten]

Its up and running...
I settle on the grid leak scheme, for optimal noise performance... 22K x2 substitutes the 6k8 x2, wich delivers the feedback and provide the grid leak bias for the 6s17k-v at 2,5ma. Cathode is now grounded. Theres DC on the vol. pot so I haft to have seperate C for feedback and coupling to the next stage... And now there is a slight roll off above 10K, I probably haft to remove the zobel network on the IPT after the transformer change...

Edit: The slight roll of above 10K is due to miller c in the line stage (E81L) in triode mode, in UL and pentode mode freq. response is flat like a panncake...

The E81L idle at 15ma at ca 180V.

 

[jhaell]

Really neat work! Super interesting to follow.
Would like to hear it!

 

[Tubetec]

I found a spec sheet for the 6s17k and it has translations to English ,

 

[gyraf]

"..Service life guarantee: 2000 hr. "

 

[Tubetec]

With the right mounting arrangement the tube could be swapped out quickly and easily , handy for selecting from a batch also .
2000 hours use on the back of a mobile missile launch platform is probably extended somewhat in a studio situation ,which is a very easy life in comparison . Tube looks very linear with lower plate voltages and moderate current . that might extend the life another bit .

 

[solkatten]

The only unknown is how much cooling aid the original tube sockets gave. So it is probably wise to operate the tubes at moderate current and voltage for long service life... But the cost of a tube is 1 euro, its a cheap experiment..

 

[Tubetec]

Heres the only example of a disc seal triode mounting I could find , very much like the fuse clips I suggested , with what looks like ceramic as the base material .

Heres an article on Microwave valves by the BBC ,

https://www.bbceng.info/additions/2016/No. 8 - Microwave Valves.pdf

 

[solkatten]

Other planar triodes seems to have sockets like this...

 

[Tubetec]

Larger output tubes run forced air cooling with a fan/ducting/fin arrangement .
6s17k has an anode rating of 2w maximum , with 60-70 volts @5 mA dissipation would be a good bit less .

 

[Eskiclown]

5mm fuseclips at either end would probably be a neater way of socket mounting the 6s17k

As I think this fuse clip socket idea is great (thank you Tubetec !), I have experimented a bit.

I've ordered 5mm fuse clips (these ones) for both the anode and filament2 pins. And 10mm fuse clips (these ones) for the cathode/filament1 connection.

Without modification, the 5mm fuse clips are way too loose for the 4mm pins. But by chance, the clips I've bought have a small tab that acts as an end stop. By bending this tab to the inside of the clip, you can make the inner diameter smaller : the contact is not perfect (because the 6s17k pin is not entirely in contact with the clip) but the bent tab pushes the 6s17k pin against half of the fuse clip - plus the tab contact itself. Good enough for me.

For the 10mm fuse clip, its inner diameter is perfect for the cathode/filament1 connection. But the model I have bought is too wide (couldn't find one that is narrow enough). So I split the clip in two with a Dremel tool. Two clips out of one !

Still need to find a solution for the grid connection. The pipe clamp idea (or something comparable) would be ideal as it would keep the tube from popping out of the socket. Just need to find the right part.