pentode screens

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#1 pentode screens

Post by ed »

voltage on G2 as an input stage:

derived from divider on B+
or
regulated from B+
or
derived from resistor from anode voltage

there are many examples on t'internet of each, but which is a nono...
anybody got any preferences or thoughts or advice?

or just forget it and go for a cascoded triode?
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#2 Re: pentode screens

Post by Nick »

Well, the third makes it a triode. The other two, its worth remembering that g2 is a positively biased grid, so there will be current, so a low impedance supply will keep the voltage more constant. If you use a resistor chain, you may want to make sure the current is enough to keep the grid stationaly. Also consider taking the resistor string to the cathode and use a smaller cathode resistor. My favourite is none of the above and is using a resistor string to bias a mosfet as a source follower to feed the grid
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#3 Re: pentode screens

Post by ed »

food for thought, thanks.

I am thinking, honest. I was having trouble as Morgan was pulling me one way and Merlin another way and then Pete Millet put a different slight on it and what with all the talk of noise introduction and the creeping third harmonic I got quite dizzy......

and then there's the option of a cascode which is supposedly quieter...you didn't comment on the cascode...does that mean you don't rate it?
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#4 Re: pentode screens

Post by Nick »

No cascodes are just fine, but pentodes are good as well.
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#5 Re: pentode screens

Post by Mike H »

ed wrote: Fri Dec 06, 2019 9:51 pm and then there's the option of a cascode which is supposedly quieter...
This is one of those persistent myths, pentode noise mainly applies to RF usage. Not applicable in the AF band.
 
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#6 Re: pentode screens

Post by ed »

Yeah, when's a little noise bothered us fearless fools....

I think I'm attracted to a pentode, probably 12hg7, probably with something like a maida reg. Probably not with Pete Millet's version though...for some reason I'm thinking his boards are too pricey with the postage...he(Pete) seems to favour the Maida type reg as previously a lot of his circuits were using oa2....

with age comes skinnyflintyness

onward
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#7 Re: pentode screens

Post by Nick »

Yeah, when's a little noise bothered us fearless fools....
Yep, the d3a in my phono stage is quiet enough. As Mike says, you have to look at the noise bandwidth that the numbers are measured over.
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#8 Re: pentode screens

Post by Mike H »

ed wrote: Sat Dec 07, 2019 10:27 pm Yeah, when's a little noise bothered us fearless fools....

I think I'm attracted to a pentode, probably 12hg7, probably with something like a maida reg. Probably not with Pete Millet's version though...for some reason I'm thinking his boards are too pricey with the postage...he(Pete) seems to favour the Maida type reg as previously a lot of his circuits were using oa2....

with age comes skinnyflintyness

onward
Pentode has the advantge of being a Voltage controlled current source, meaning its mu (gain) isn't greatly influenced by the anode Voltage changing. Very similar behaviour to a FET, which we love to use as CSS's. :D

What's it for exactly? Audio amp or regulator?
 
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#9 Re: pentode screens

Post by ed »

Mike H wrote: Mon Dec 09, 2019 6:31 pm
Pentode has the advantge of being a Voltage controlled current source, meaning its mu (gain) isn't greatly influenced by the anode Voltage changing. Very similar behaviour to a FET, which we love to use as CSS's. :D

What's it for exactly? Audio amp or regulator?
audio amp.....I'm simplifying my sv572 to put it back in a box. I need a driver replacement for the aikido front end and this time I want a bit more gain to stretch the 572s muscles. Flat out the aikido could only tickle it...I also wanted to get rid of the 12v meanwells and the coleman regs and try replacing them with a smaller meanwell, such as the 7v version without a regulator. All very much simpler and smaller.

ironically I started the sv572 design with a 12hg7 driver but never actually built it..so better late than never.
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#10 Re: pentode screens

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You seem to be gung-ho at the moment about adding 60kHz noise to your lovely DHT's (?)
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#11 Re: pentode screens

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IslandPink wrote: Mon Dec 09, 2019 9:27 pm You seem to be gung-ho at the moment about adding 60kHz noise to your lovely DHT's (?)
I wouldn't exactly say gung ho......but what can you tell me about connecting meanwells directly to sv572 that's going to stop me trying....

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#12 Re: pentode screens

Post by IslandPink »

No - by all means try - but don't commit to any system changes that depend on it. I briefly tried SMPS direct into GK-71 part-way through the Lehane development work. It was clear it would be a short-term set-up.
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#13 Re: pentode screens

Post by izzy wizzy »

IslandPink wrote: Mon Dec 09, 2019 11:30 pm No - by all means try - but don't commit to any system changes that depend on it. I briefly tried SMPS direct into GK-71 part-way through the Lehane development work. It was clear it would be a short-term set-up.
I've gone a similar route. SMPS direct ain't so great even with some filtering. I didn't want to use regs for all kinds of reasons but glad now I have them.
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#14 Re: pentode screens

Post by ed »

Doh!
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#15 Re: pentode screens

Post by Dave the bass »

IslandPink wrote: Mon Dec 09, 2019 9:27 pm You seem to be gung-ho at the moment about adding 60kHz noise to your lovely DHT's (?)
:D

Pah! We're all getting older... why I doubt 'even I' can hear above 59.5kHz anymore. Bwahahah (etc)
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