Grid Current

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Paul Barker
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#46

Post by Paul Barker »

You won't leave it alone will you :shock:

NO, forget about 360 degrees. It has no advantages and plenty of disadvantages. I don't know what carp you've read about it but burn it.
Last edited by Paul Barker on Sat Jan 26, 2013 5:07 pm, edited 1 time in total.
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pre65
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#47

Post by pre65 »

Paul Barker wrote:You won't leave it alone will you :shock:

NO, forget about 360 degrees. It has no advantages and plenty of disadvantages.

:wink: I'm off out now. :)
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#48

Post by Nick »

GM70 works well in A2, Colins monoblocks are set up to take advantage of +ve grid excursion, Thats why it will push out 40+ W

There are published +ve curves, so its clearly designed with that in mind,
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#49

Post by pre65 »

So, with A2 cathode drive, and (as an example) the driver valve supplying 100ma to the output valve cathode.

Does the power supply have to maintain 100ma twice ? (ie for the driver valve and the output valve)

Or is it the same 100ma going round in a circle ?:?
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#50

Post by Paul Barker »

Twice. This is because the current comes from the minus part of the power supply goes through the cathode and completes it's journey at the plus side of the power supply.

the two stages are effectively in parallel, it is only the ac which is passed across from one to the other. In our case across the Interstage.

so this is costly in power supply, yes.

But remember in a direct coupled driver where the grid is driven the two valves are also in parallel, only the AC is coupled. the DC current for the grid however in that isntance continues on to the power supply plus through the driver.

The driver does not actually source grid current, the cathode of the output valve sources it, the driver provides the journey to it to the final destination.

If only the person who first imagined how electrons flow had got his direction right :roll:
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#51

Post by Paul Barker »

the real issue for you is that to balance this interstage the driver has to run at the same current as the output valve. so for example in the gm70 for optimal power you require 300mA per channel.

I would build the 211 myself.

It seems you may not have any 211's so I advise the 801a version for you.

as said already I had to imagine something suitable for the interstage and work backwards, in my book the 211 fits hand in glove.

Most of my designs originate with what I have then I work out what suits most optimally what I have.
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#52

Post by pre65 »

What I'm really trying to get my head round concerns these two quotes by Nick,

"Yep, or you put the secondary of a interstage in the cathode. The big plus is there is no transition from A1 to A2"

"Yes, thats the point, because it all flows through the cathode there is no A2 change"

and the practical examples you (Paul) have given where this does not appear to be the case. :?

Or, am I misunderstanding what Nick says ?

EDIT.

I'm thinking I am now more clear about what Nick said, and that I DID misunderstand. :)
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#53

Post by Nick »

pre65 wrote:What I'm really trying to get my head round concerns these two quotes by Nick,

"Yep, or you put the secondary of a interstage in the cathode. The big plus is there is no transition from A1 to A2"

"Yes, thats the point, because it all flows through the cathode there is no A2 change"

and the practical examples you (Paul) have given where this does not appear to be the case. :?

Or, am I misunderstanding what Nick says ?

EDIT.

I'm thinking I am now more clear about what Nick said, and that I DID misunderstand. :)
What I said was was from the perspective of the driving valve. The output valve will still pass grid current when the grid is +ve of the cathode, and -ve when not, but this change will be far less obvious to the driving valve than if it was driving the grid.
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#54

Post by Paul Barker »

Yes Nick has put it perfectly.

What he was seeing as the major benefit was that the driver is not imbalanced by sinking the grid current to the B+ on behalf of the output valve.

In Nick's idea only the output valve has grid current to deal with.

In that respect it must surely be advantageous.

But we are left dealing with other issues instead and so we now find ourselves with two choices. Specify our own interstage for a particular job or compromise our operating conditions to the interstage we have to hand.

you and I have the same interstage.

I would seriously build the 801a if I were you. I imagine it will be an excellent sound, though limited to 4 watts. But it would put Nick's idea into practice for evaluation.
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#55

Post by Paul Barker »

Phil I can send you the Russian Pentode and loctal bases, you must have some 801a's do you?

If you need any power supply bits I will have a look.

I am away on a British Gas induction course (though I have worked for them before their memory has had a reboot, the guys I knew have gone and young blood have replaced them) from tonight for a week, so slight delay.
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#56

Post by pre65 »

Thanks for the offer Paul.

I've got 4 of the 4P1L pentodes and bases so that's OK. Not sure about the 801, I like the idea of a 9W and 400v GM-70. :wink:
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#57

Post by Paul Barker »

OK.
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#58

Post by IslandPink »

I'm up to speed with it all now, having read the first page :oops: and thanks to Paul and Mike for clarifying the bit about the secondary inversion.
It does look like a very nice idea, and the advantage of having the big driver valve ( 300B or 2x 4P1L or whatever ) as a cathode-follower makes the job of the voltage amplifier a lot easier as it doesn't see a lot of input capacitance into the driver . You just need a high-ish gain valve, maybe an ECC40 or something , or less depending on how much your pre-amp puts out .
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#59

Post by Nick »

I would think it would suit the 572b well. Must do something with the pair i have one day.
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#60

Post by Paul Barker »

Here is another which fits the resistance of the IT

813 Triode

100mA 20v, 380V B+ 5k load 8.4 Watts. That is with Grid current quite low at 12mA. Peak to Peak drive voltage swing 60 Volts. Don't forget to externally connect g3 to negative pole of filament or better still below ground through a small battery. Back in the day we were all experimenting with the vrious connections holding g3 below ground a few volts was found "sweet". not possible on many valves except the likes of this and genuine EL34's.

813 sounds much less refined and silky than the 811 but you could turn that on it's head and say 811 sounds much less raunchy than the 813.

A quite different presentation, also considerably different to GM70.

I would quite like to give 813 another outing.

GG may produce an entirely different sound character anyway.

B+ is right in the manageable regions.
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