G2 voltage spec

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Paul Barker
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#16 Re: G2 voltage spec

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interesting.
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#17 Re: G2 voltage spec

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Bottle head used it I think. Designed for low noise, pretty sure going back Peter Q showed me a stash of them, but might have been another similar qualities tube Ive completely firgotten the four didget number representing it. i forget a lot of what i once knew. more in the ball park of the 5687 but seems by maths less worthy of exploration but Peter had a glint in his eye. That was a big hint, but its now lost on me! I have a couple only 12ay7 and cant remember brand or if gold pins, doubt it. Might be years before I bump into them again, if I live long enough.
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Nick
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#18 Re: G2 voltage spec

Post by Nick »

They are a useful valve. More gain and more linear than ecc82/81 though lower current of course.
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#19 Re: G2 voltage spec

Post by JamesD »

Going back to Paul's original question about triode connected pentode with G2 connected to the anode and respecting the pentode connected G2 specified voltage limit. This does not apply when the pentode is triode connected. The only voltage limit is the usual pentode anode limits in the triode connected case.

The G2 limit in the pentode case is due to pentode voltage swing below the G2 voltage exposing the G2 grid to anode current instead of the actual anode and this can cause the G2 grid to overheat. Keeping G2 voltage low restricts this effect.

The GK-71 has been used in triode mode up to full anode voltage with no problems - there are a couple of Russian sites showing this but I can't find them quickly to link to. I try and post them later.

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#20 Re: G2 voltage spec

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James to the rescue, feels like a
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#21 Re: G2 voltage spec

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Yes, but (and it why I didn't answer earlier as I don't know) If the restriction is g2 dissipation limits (as I am sure it is), then why connecting g2 to the anode as is shown in the 6550 example Paul posted would that reduce the g2 current? And if it doesn't reduce g2 current, then would the dissipation not be the same.
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#22 Re: G2 voltage spec

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If I try one I’d best check current straight away. But maybe the amount of electrons from cathode land on g2 determines current, many more land on plate.
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#23 Re: G2 voltage spec

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Paul Barker wrote: Mon Feb 21, 2022 4:34 pm If I try one I’d best check current straight away. But maybe the amount of electrons from cathode land on g2 determines current, many more land on plate.
Yes, of course. But I don't understand why the voltage on g2 being the same as A makes a difference, the electrons still need to pass g2 to get to A.
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#24 Re: G2 voltage spec

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Have to wait and test it. Otherwise g2 off plate on to ground so negative to autobias cathode. As with beams.

It’s not as if there isn’t enough power from the plate.
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#25 Re: G2 voltage spec

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I’m logged to prat around with the 801a amp, so I’ll use the 833 chassis and Mount both types of base and use variable fixed bias, monitored by ammeters AOT chosen off the different characteristic curves, apart from which just parallel the heaters and transformers connections, interstage and output. Then just tween bias and plug in the valve you’re listening to.

Sounds like a plan.

Anyone starting a book on which of the three options will sound best. Gk71, copper gm70, graphite gm70.

Might actually come out in that order gk71 1st. But haven’t checked the curves for power output of gk71 as such. I know where I’ll be operating the gm70 I’ll get 35 watts.
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#26 Re: G2 voltage spec

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I think its not just potential difference and impedance that determines the G2 current flow but the way the space charge distributes itself and the momentum of the electrons i.e. they are travelling fast enough within the space charge to move far enough past G2 ( and probably G3) to be captured by the anode,,, otherwise they would just distribute themselves according to the relative structure impedances i.e. Rp and Rg2 where Rg2 is 4 to 5 times Rp - typically anyway and certainly for audio valves.

No one has mentioned G3 (yet) and how that can influence things... for instance PT15 likes +40V on G3 and is more linear and has greater potential voltage swing when positively biased... And when taken to the anode rather than the cathode the triode curves change...

Never did think I would understand valves completely...
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#27 Re: G2 voltage spec

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Oh shit there’s a vintage iron anode brass base gm70 on eBay right now fair price . Who can afford it and bring it on test day?

Can’t see how to link it, this is what you’re searching for.

Image
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#28 Re: G2 voltage spec

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I'll bet on GM70 copper plate, GK-71 triode connected, GM70 graphite...

Just to be different.... I really like the GK-71 triode connected - its a bargin and still seems readily available..
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#29 Re: G2 voltage spec

Post by IslandPink »

The interest in the GK-71 for me is that you should be able to get a decent amount of power by 700V, even with auto bias.
I ran it at 460V B+ and it sounded really nice, but only about 3W .
Do you have the triode curves ?
GK-71 triode curves.GIF
GK-71 triode curves.GIF (177.59 KiB) Viewed 3507 times
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#30 Re: G2 voltage spec

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Yes, but thanks for thinking of it.

At 1kv to 1.2kv it should sing.

I actually have a 2.5kv variable lab supply Ed kindly gave me but it’s 100w 1kv @ 100 mA is the absolute max. Now it’s voltage and current regulated. So might sound nice. So I’ve ordered another cheap that’s faulty tripping fuses, hopefully that current leak is easily found I.e a bad lytic or a dead short Tant. Fingers crossed! Then I can monoblock output valves at 1kv 90mA so as not to operate power supplies quite at max. Might only cost me a few caps, I’ll only change the wonky one or two as I have to be tight. The rest I’ll reform for now.

As a pair though for my mad scientists lab I’ll be in my element, infinitely variable b+ down to 5v from 2k5 if necessary, don’t worry I won’t bridge them.

Quite handy to have a good one might help me find where spares or repair one has failed. I might beef up the heat sinks as I’d be pushing them close to max.
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