The planar v72 from scratch
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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 !
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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Great idea
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 .
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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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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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...
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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interesting..
..how do you get grid bias to the first stage, with both cathode and input being at ground?
/Jakob E.
..how do you get grid bias to the first stage, with both cathode and input being at ground?
/Jakob E.
Because of grid current a 47K grid resistor can bias the grid at -0,5Vdc. More info on 6s17k-v:
https://gearspace.com/board/diy-ele...s/591890-wavebourns-tube-mic-pre-project.html
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.
https://gearspace.com/board/diy-ele...s/591890-wavebourns-tube-mic-pre-project.html
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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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)
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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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.
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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