GK-71 op points

If they glow, this is the place to be
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IslandPink
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#106

Post by IslandPink »

Well this is all very jolly, but I don't think anyone answered my question - perhaps you don't know :shock:
Andrew's suggestion to warm the filament(ary cathodes) is fine but is a luxury option that requires more parts, real estate and a switch of some sort, as far as I can see.
The situation I will be running, at some point fairly soon, will have a slow-start B+ from a damper-diode-supplied shunt-reg power supply , and a quick-start filament(ary cathode) for the GK-71 provided by a SMPS .
The high-current pulse in the GK-71's therefore takes place before the B+ has even got off the ground .

I don't see this as a problem ...
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pre65
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#107

Post by pre65 »

You could use a thermistor in the filamentary supply to smooth the surge.
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#108

Post by ed »

JamesD wrote:Hint on the twisting for those that wish to look it up still...

Its what your right hand is for...

at least according to John Fleming...

Sorry! Couldn't resist it... :D

Andrew - please don't ever change - we treasure you!

James
swear I havn't looked it up ... yet, but my memory says left hand......right hand is what Philip is referring to.
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#109

Post by IslandPink »

Miller Larson effect it seems.
Filament bends in the earth's magnetic field , so it's not to do with B+ .
I got the impression from the discussions on Ham radio archives that it's probably not important for small transmitters - one guy said it wasn't worth worrying about on small things like eg. a 3-500 . Given the 3-500 has 14.5 amp heater and must draw eg. 80+ amps on start-up, makes me wonder if it's a non-problem on GM-70/GK-71 .
I could fit a thermistor , but it'll drop the voltage ( with my SMPS supply fixed at 19.3V ) somewhat depending on how much cold resistance you hope to get. I might get some CL-30's and CL-40's just in case.
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#110

Post by JamesD »

Ed,

Damn! I can't even remember which it is anymore! Memory going... poor old boy....

Thank goodness for the internet!

For reference - Left hand rule --> Motor (mechanical force). Right hand rule --> Generator (electrical force).

J
Last edited by JamesD on Fri Aug 02, 2013 2:05 pm, edited 1 time in total.
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#111

Post by JamesD »

Mark,

interesting! I had assumed it was the B+ field but, of course, the earth magnetic field is part of the system too...

J
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#112

Post by pre65 »

I'm tempted to have a go at GK-71. :wink: The urge to build something is growing.

Would the driver valve I used for my GM70 amp (12GN7a as a triode) be suitable for a GK71 ?

Also, the choke load for the 12GN7a was 100H, would a 500H choke likely be preferable ?

I've got a HT PSU and the components I used to heat GM70 so it should not be too expensive. :D
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#113

Post by Paul Barker »

pre65 wrote:
Would the driver valve I used for my GM70 amp (12GN7a as a triode) be suitable for a GK71 ?
Yes.
pre65 wrote: Also, the choke load for the 12GN7a was 100H, would a 500H choke likely be preferable ?
I don't have the triode data. But more inductance is never wasted aslong as there isn't a compromise eslewhere. you will benefit from better phase angle of bass and suffer no detremental bass effects. But to wind for so many henry and mainmtain good HF performance is a skill. Also what DC current does the 12gn7 run at and can the 500h choke handle it?
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#114

Post by pre65 »

Paul Barker wrote: can the 500h choke handle it?
I'm waiting for a reply from Tim in HK. :)
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#115

Post by Paul Barker »

pre65 wrote:
Paul Barker wrote: can the 500h choke handle it?
I'm waiting for a reply from Tim in HK. :)
One small thought Phil, it is hard to make a single core high inductance transformer without exceeding satisfactory capacitance specs. but when you use two seperate core chokes in series you double the inductance and half the capacitance. (whereas when you make twice the amount of turns on the same core you get 4 x the inductance and 2 X the capacitance).

So in effect in your situation a better result may be to use two 100h chokes not a single 500h one, as the indcutance goal is already usually reached unless by 200h (unless your valve is ainapropriate) you are after a grid choke.

Or I could have just said without so much detail, if the valve is suitable for LC coupling it shouldn't need 500h, if it needs 500h rather than fight the technical problems winding such a choke to handle DC (the grid choke which handles no dc is easy to wind to 500h and that is barely enough) go for a different valve or a different coupling.
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#116

Post by pre65 »

Thanks Paul. Perhaps a single 200H is the most cost effective option.
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#117

Post by IslandPink »

I'm assuming you're going to run the 12GN7 at a decent current, say 25 to 30mA, Phil ?
You'll need that to drive the GK-71 cleanly.
In that case I'd expect the anode resistance of the 12GN7 to be something like 1.5k, so 200H should be plenty. I used 55H on a 45 as driver, that was good .

Personally I'm going to use either 4P1L or EL86 as pentodes and get more gain . Should be fine if the GK71 is running in A1 , James says they are well-behaved as far as grid current goes, below 0V .
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#118

Post by pre65 »

In fact I do have a few 4P1L in stock, so that is probably a better option.
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#119

Post by pre65 »

I've gone and purchased a pair of GK-71 on Ebay. 8)

EDIT.

I've had a message from Boris, seems there is a problem with one valve (of my pair) so I've opted for a full refund. BUGGER. :(

So looks like I will have to pay a bit more to a different seller.
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#120

Post by IslandPink »

Ooh, disappointing when you're keen to get them quickly !
Try Oleg in Odessa . ( Alexer1 )
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