The planar v72 from scratch

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Tubetec

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A 5mm pipe plumbing accesory might serve as a mounting for this tube at the edge of a circuit board ,either at the anode or the f2 connection .
Alternatively a flourescent tube clip could be made to fit the k,f1 connection ,
Incidentally, either of these clips in their various sizes make handy mountings for radial electrolytic caps too , it means your scaffold no longer has to bear the weight of the components , 6s17k-v is good to 500 G after all !
 

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solkatten

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A 5mm pipe plumbing accesory might serve as a mounting for this tube at the edge of a circuit board ,either at the anode or the f2 connection .
Alternatively a flourescent tube clip could be made to fit the k,f1 connection ,
Incidentally, either of these clips in their various sizes make handy mountings for radial electrolytic caps too , it means your scaffold no longer has to bear the weight of the components , 6s17k-v is good to 500 G after all !
Great idea👍
 

solkatten

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Putting things in place… That is one heavy channel of analog audio👍
 

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Tubetec

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5mm fuseclips at either end would probably be a neater way of socket mounting the 6s17k ,
I did see some of the planar valves on ebay , at a couple of dollars each there defiantely worth a look for audio .

I was trying to figure out if a self biased cathode follower with a directly connected fillament was possible , raised heater reference voltage to ground ,can the cathode resistor itself be the ground reference for the filament supply?

I'll add a sketch tomorrow for clarity .
 

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solkatten

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5mm fuseclips at either end would probably be a neater way of socket mounting the 6s17k ,
I did see some of the planar valves on ebay , at a couple of dollars each there defiantely worth a look for audio .

I was trying to figure out if a self biased cathode follower with a directly connected fillament was possible , raised heater reference voltage to ground ,can the cathode resistor itself be the ground reference for the filament supply?

I'll add a sketch tomorrow for clarity .
It is possible the hard part is probably max grid leak value

The filament supply could float on top of the cathode voltage, like in my cascade.
 

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solkatten

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I scrapped the fet+tube cascade, couldn´t get it quite enough. I think the noise was injected via the floating filament supply in the heather/cathode junction of the 6s17k-v... Anyway I had another design floating around that I will try instead. Just modding the same card still got the controlls + PSU…

The input stage is design around a grid bias 6s17k-v gain=40 dB. Because it got a grounded cathode I think this is the only way to implement varible feedback...

The line stage is 6n6p driving the LL1671 trafo. I will probably put the line stage at 100Vdc and at ca 25-30ma. Even with a small serial feedback resistor on the cathode the Rp will be at 2K5, so it is still in a good place to drive the transformer and gain will go down a notch. I will probably increase the grid stopper to 10K to avoid oscillation.

The only thing this has in common with the v72a is the feedback loop, but what the heck...
 

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solkatten

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Because of grid current a 47K grid resistor can bias the grid at -0,5Vdc. More info on 6s17k-v:

Another way is to use filament bias. You have 2ohm resistor from the shared cathode/filament connection to ground , that elevatets the cathod 0,5v. Then you need a ultra clean 6,3v+0,5v= 6,8vdc filament supply.
 
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solkatten

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Finished version...
 

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solkatten

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I ended up building this version of planar v72 but with LL1538 IPT. Its up and running. It don't have huge amount of gain but enough for a condenser mic.
THD is around 0,05% at 1KHz. With no feedback it has a slight roll off, with full feedback, it has 1-2dB rise in the top end... I guess the feedback also alters miller c in the input stage, but it could also be the altering source impedance driving the OPT causing the effect...

The freq response with a little feedback is nice and extended in this one, cudos to Lundahl (1538 and LL1671SE)
 
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solkatten

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Rev. Schematics. No dc on feedback pot. and compensation C over the FB resistor 👍
 

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Tubetec

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Blocking the dc on the pot sounds like a good plan as they tend to create crackley noises otherwise .
0.05 % thd seems like a respectable enough figure for a single ended pre amp where the harmonics are mostly lower order . There was a Jensen SE tube line output transformer Bill(CMRR) recomended with very low distortion , in 0.00x% THD territory.
 

solkatten

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Blocking the dc on the pot sounds like a good plan as they tend to create crackley noises otherwise .
0.05 % thd seems like a respectable enough figure for a single ended pre amp where the harmonics are mostly lower order . There was a Jensen SE tube line output transformer Bill(CMRR) recomended with very low distortion , in 0.00x% THD territory.
Yep Im ok with 0,05 % figure, mostly H2 just need to fix the high end ringing…
 

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