New SE With Triode Mode KT120/150

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Nick
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#106 Re: New SE With Triode Mode KT120/150

Post by Nick »

You can put a solid state ccs in series with the choke, the choke gives you the ac swing the ccs gives you better isolation and frequency response.
But if the CCS prevents the current from changing, how do you get any di through the choke to produce any dv?
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#107 Re: New SE With Triode Mode KT120/150

Post by JamesD »

"Gyrate" the CCS as its signal related di?

Gets most of the PSRR advantage and a high impedance anchor for the choke which is the opposite from the normal situation so not sure how it would really work - probably needs "Siming" to see whats going on - assuming it does work...
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#108 Re: New SE With Triode Mode KT120/150

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I would have thought a cap multiplier or just a regulator would be better.
a high impedance anchor for the choke
Is that what it needs? I would have thought a low impedance anchor would be what it needs.
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#109 Re: New SE With Triode Mode KT120/150

Post by JamesD »

I'm not sure - hence the comment that it needs simulation to explore what might happen - conventionally the top of the choke is a low impedance point to ground so signal variations get shunted away. The gyrator bottom end attempts to maintain a constant voltage so that anchors the top of the choke letting the bottom of the choke swing where ever it needs too - so I think it works but not convinced that it will yet as the top of the choke current paths have different characteristics and I don't immediately understand how that might affect the choke...
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#110 Re: New SE With Triode Mode KT120/150

Post by Nick »

But by maintaining a constant voltage does the gyrator not look like a low impedance path to ground looking into the gyrator output port? If you inject or remove current from that point it will maintain its voltage hence looking like a low impedance.
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#111 Re: New SE With Triode Mode KT120/150

Post by Cressy Snr »

Nick wrote: Tue Jul 07, 2020 5:43 pm I would have thought a cap multiplier or just a regulator would be better.
Once the new amp is built, I will indeed have a simple shunt regulator on the output of the pre/driver stage power supply. This is in the usual form of series resistor feeding a VR tube stack. I know it’s old school and is beaten by solid-state, but I like pink and orange glowy things.
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#112 Re: New SE With Triode Mode KT120/150

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Nick said:
But by maintaining a constant voltage does the gyrator not look like a low impedance path to ground looking into the gyrator output port? If you inject or remove current from that point it will maintain its voltage hence looking like a low impedance.
Interesting view point it maintains PSRR by being high impedance as seen looking into the gyrator 'top' port so its not a true low impedance path to ground but it maintains a virtual ground by way of maintaining the voltage at its output port... its a virtual earth because this can be any voltage and its voltage variation it removes - one might consider that as ac earth rather than dc earth so it does mimic a low impedance path to ground without providing one ... or does it - need to do some more tracking of where the ac currents currents flow to and from...

J-C would be pleased that we're having a Norton based discussion rather than the Thevenin one we usually have with valves :-)
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#113 Re: New SE With Triode Mode KT120/150

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one might consider that as ac earth rather than dc earth so it does mimic a low impedance path to ground without providing one
Of course this is the advantage of a shunt reg in that it can cope with a excess of voltage by actually dumping to ground instead of a series reg only being able to wait for the load to consume the excess current.
Interesting view point it maintains PSRR by being high impedance as seen looking into the gyrator 'top' port so its not a true low impedance path to ground
Agreed, but we sort of don't care what happens in the mystery place before the regulator.
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#114 Re: New SE With Triode Mode KT120/150

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I never thought of the shunt reg dumping the excess voltage to ground! Nice way to think of it! I always went with the Norton view of a variable current path to ground - absorbing any excess current or supplying required current to maintain the required B+ ... Neat!
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#115 Re: New SE With Triode Mode KT120/150

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I’ve done as much as I can without having to dismantle the existing valve amp:
C14D8FA9-D3B7-4DAE-A60A-70C8473D3D2E.jpeg
Selector switch is wired to the three inputs and a pair of wires is taken off to feed the volume pot.
Next step is to remove all the iron from the existing amp. The larger mains transformer needs the paint removing from the end bells. before it can be considered decent enough to sit on the new chassis.

It’s going to be interesting trying to fit five chokes under the chassis. Why do I start these things?

Now a question for the panel. 150H and a plate resistance of 40K, gives a -3dB point of 11Hz on the input stage, eleven times the ideal, and it’s going to have to do, as I suspect there will be no room to double up on them. I’m worried about loose bass, especially with vinyl arm cartridge resonances being right in the 11Hz area.
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#116 Re: New SE With Triode Mode KT120/150

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I’m worried about loose bass, especially with vinyl arm cartridge resonances being right in the 11Hz area.
Why? Having a -3dB at that point will reduce the signal from the arm. Its not as if the LR filter is going to have a peak, its just a first order filter. Just make sure any cathode bipass cap is large enough to avoid getting in on the act.
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#117 Re: New SE With Triode Mode KT120/150

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JamesD wrote: Wed Jul 08, 2020 5:03 pm I never thought of the shunt reg dumping the excess voltage to ground! Nice way to think of it! I always went with the Norton view of a variable current path to ground - absorbing any excess current or supplying required current to maintain the required B+ ... Neat!
May be a result of the way my brain visualises what’s going on (though not in any way visual). After all, the idea of it being Norton or Thevenin is just a abstraction for the benefit of the analysis, in reality its neither and just is what it is (IYSWIM).
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#118 Re: New SE With Triode Mode KT120/150

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Nick wrote: Wed Jul 08, 2020 5:31 pm
I’m worried about loose bass, especially with vinyl arm cartridge resonances being right in the 11Hz area.
Why? Having a -3dB at that point will reduce the signal from the arm. Its not as if the LR filter is going to have a peak, its just a first order filter. Just make sure any cathode bipass cap is large enough to avoid getting in on the act.
Or you can use the cathode cap to get in on the act and extend the -3dB, but then you do have to be careful to avoid a peak.
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#119 Re: New SE With Triode Mode KT120/150

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11Hz is fine as the choke f3 point and in particular its fine with the arm resonance being at the same frequency - these two do not directly interact and any resonance effects in the signal would be reduced... this only applies to the resonance fundamental of course...

It will cause some phase change in the audio band but not enough to worry about - so why do we say 1-3Hz is ideal for the F3 of stage coupling? Its mainly because these things do not occur in isolation but each F3 point in an interstage coupling, in a cathode bypass and in the OPT are working in cascade to produce a system F3 that is much higher in frequency and steeper in roll off than each single filter point so the general rule has become to aim for a F3 of 1-3Hz for coupling, 3-10Hz for cathode bypass and 10-20 Hz for the OPT as a practical guidline that attempts to manage the cost or 1Hz for each stage if going for the 'best' result...

Hope that helps!
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#120 Re: New SE With Triode Mode KT120/150

Post by Cressy Snr »

Nick wrote: Wed Jul 08, 2020 5:31 pm
I’m worried about loose bass, especially with vinyl arm cartridge resonances being right in the 11Hz area.
Why? Having a -3dB at that point will reduce the signal from the arm. Its not as if the LR filter is going to have a peak, its just a first order filter. Just make sure any cathode bipass cap is large enough to avoid getting in on the act.
Cheers Nick/James,

I’ll try to keep everything else down in very low single figures.
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