GK-71 - economical build.

andrew Ivimey · Post by **andrew Ivimey** » Tue Aug 20, 2013 9:29 pm

Nadbness .... ooops can't even blame cyrillic script but I love it. My blah blah 509s had a plasma flame but nothing as wild as these!

jack · Post by **jack** » Wed Aug 21, 2013 8:53 am

andrew Ivimey wrote:Nadbness .... ooops can't even blame cyrillic script but I love it. My blah blah 509s had a plasma flame but nothing as wild as these!

VTTCs with staccato modulation, for some reason, produce a very different plasma discharge to most solid-state TCs (SSTCs ot DRSSTCs) or SRSG (traditional synchronised rotating spark gap) coils - more like a modern QCW TC (the latest designs).

Built one a few years ago with an 833 - I love the smell of ozone in the morning...

simon · Post by **simon** » Wed Aug 21, 2013 10:55 am

I didn't understand any of those acronyms. And if I had I don't think I'd have known what they meant. Cool vid though.

jack · Post by **jack** » Wed Aug 21, 2013 11:55 am

simon wrote:I didn't understand any of those acronyms. And if I had I don't think I'd have known what they meant. Cool vid though.

VTTC - Vacuum Tube Tesla Coil
SSTC - Solid State Tesla Coil (quite basic & simple)
DRSSTC - Doubly Resonant Tesla Coil (more advanced version of SSTC when both primary & secondary coils are tuned & driven - requires current sensing feedback from the primary - can be made HUGE)
QCW - Quasi Continuous Wave - a type of DRSSTC with the addition of a main bus supply that is servo-driven, effectively an extremely powerful arbitrary waveform generator - can generate sparks many times the length of the secondary coil. QCW is the current state of the art in solid-state Tesla Coils
SRSG - Synchronous Rotary Spark Gap Tesla Coil - pretty much the original Tesla Coils design - uses an NST (Neon Sign Transformer), a capacitor and a spark gap driven by a synchronous motor.

A plasma speaker is essentially an RF Tesla Coil running at several MHz with a class E driver which is audio modulated.

pre65 · Post by **pre65** » Wed Aug 21, 2013 12:04 pm

You'd need all of the above to make my bwain spark.

simon · Post by **simon** » Wed Aug 21, 2013 6:13 pm

Thanks for the explanation Nick, and I was right

pre65 · Post by **pre65** » Thu Aug 29, 2013 2:41 pm

Hot news.

I now have a pair of GK-71.

I've just started the ACF pre, so might see how that goes before starting on the power amps.

pre65 · Post by **pre65** » Fri Aug 30, 2013 12:40 pm

I'm having a think in a quiet moment (I get them occasionally) about driving the GK-71.

Rather than copy Mark and use 4P1L I wondered whether 12HG7 as a pentode would be worth a try.

Just to be different.

IslandPink · Post by **IslandPink** » Fri Aug 30, 2013 5:45 pm

Certainly possible .
You'll get plenty of gain , maybe too much !
Something like 30 or 35mA with a 6k load would be good .

What do you think of the GK-71 then, for £12 each ?

pre65 · Post by **pre65** » Fri Aug 30, 2013 7:16 pm

IslandPink wrote:Certainly possible .
You'll get plenty of gain , maybe too much !
Something like 30 or 35mA with a 6k load would be good .

What do you think of the GK-71 then, for £12 each ?

Not bad.

Pete Millett used a 4K load on his 813 amp.

pre65 · Post by **pre65** » Wed Oct 09, 2013 5:34 pm

Shock horror.

I've decided my dual mono PSU is not up to the job of supplying the necessary oomph for GK71, so I've started on something different. My chokes are limited in ma so the PSU will split after the rectifier (hybrid bridge) into two sections.

Still not sure if I can get to Owston in November, but I'd like to have something new to bring if I do get the chance.

pre65 · Post by **pre65** » Fri Oct 11, 2013 1:27 pm

Would a driver valve with an amplification factor of 35 be enough to drive a GK71 ?

IslandPink · Post by **IslandPink** » Fri Oct 11, 2013 2:53 pm

Dunno, depends what it is

Maybe I can work towards a better answer -
(i) Is 35 enough gain for your system ?
(ii) What is the B+ going to be for the GK-71 , and are you flexible about the supply voltage for the driver ?

pre65 · Post by **pre65** » Fri Oct 11, 2013 2:59 pm

I was thinking of my 6N7 (or whatever the CV number is - CV1285 I think).

http://www.r-type.org/exhib/aac0112.htm

Twin triode with a common cathode. Probably with a CCS on the anode as I'm plum out of anode chokes.

Some many years ago our very own Paul Barker reckoned they would be ace for driving GM-70.

IslandPink · Post by **IslandPink** » Fri Oct 11, 2013 5:59 pm

I'm not sure how you'd use this one, but I'm sure Paul , Nick or James could comment better . It seems to be aimed at Class B usage, and the data sheet shows quite low current for Class A operating points . I'm concerned about grid current on the input for the higher current class B type operating points . It'll need to run at maybe 25mA to drive a GK71 properly , although I still have no idea what your B+ is going to be, or why you can't answer that question

- it will tell me what sort of current and voltage swing you are aiming to acheive into the grid of the GK-71 .

audio-talk

GK-71 - economical build.

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