Ed

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Paul Barker
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#16 Re: Ed

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Nick wrote: Tue Mar 07, 2023 9:02 am
ps. you don't have to have the Zout ( ie. the anode resistor ) very low, if you have enough current in the driver.
Not sure I understand that, the Zout is the Zout irrespective of the current. Though in practice high current will tend towards a lower value anode resistor (unless you have a lot of B+ to deal with).

B+ = 250, Va = 150, Ia = 10ma, RA = 10k
B+ = 1000, Va = 150, Ia = 10ma, RA = 85k

In both cases you have 10ma, but in one case the Zout will be 10k the other 85k (well a little less in both cases as the Ra of the pentode is not infinite). Of course you do need enough current to drive into the capacitance and grid leak of the driven valve but as long as you have enough current I would have thought the Zout would be the thing that was controlling slew at high amplitides.
Thanks Nick. I would be thinking like that too. Theres no issues about me having to use the anode resistor to get me down to the driver B+ shunt regulation is my go to from way back. Ive known since the beginning of time the pentode output impedance is dominated by anode load resistor.
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#17 Re: Ed

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Nick wrote: Tue Mar 07, 2023 9:02 am
ps. you don't have to have the Zout ( ie. the anode resistor ) very low, if you have enough current in the driver.
Not sure I understand that, the Zout is the Zout irrespective of the current. Though in practice high current will tend towards a lower value anode resistor (unless you have a lot of B+ to deal with).

B+ = 250, Va = 150, Ia = 10ma, RA = 10k
B+ = 1000, Va = 150, Ia = 10ma, RA = 85k

In both cases you have 10ma, but in one case the Zout will be 10k the other 85k (well a little less in both cases as the Ra of the pentode is not infinite). Of course you do need enough current to drive into the capacitance and grid leak of the driven valve but as long as you have enough current I would have thought the Zout would be the thing that was controlling slew at high amplitides.
Yes, I understand the numbers. I was commenting from a starting point of having worked with a number of options driving the ( awkward) 300B valve, including lower current triodes as well as various pentodes. Also I remember the discussions we had with James about slew limiting and driving current.
It seemed to me that after trying out a number of options, that current was more useful than low Zout. ( I say this in the context of what people usually do with a 300B). No doubt there is a significant limit to how high (Zout) you can go, and it might be somewhere just above what worked with the C3m, which was around 11k on the anode. But for sure, 11k Zout and 17ma sounds a lot better than 2.5k Zout and 8ma.
The slew-limiting test case is going to be for high amplitudes at high frequencies, we agree ?

By far the worst driver I used was the WE91B circuit ( initially without Thorsten's feedback mods ) which had a 3ma driver with a 91k anode load.

When you are working with a pentode driver, I find you end up with a tricky balancing act between B+, anode R, gain and dissipation.

ps. the 4P1L used as i suggested gives about 32x gain with a cathode bypass cap, and 22x without it. Might not be enough for typical sources unless your speakers are efficient.
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#18 Re: Ed

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Maybe I’ll take the 22 gain and go from that to a px4 AE bifilar IT coupled to Ed. Poor mans Sukuma style. Really looking for the sonic signature of the pentode first, amplify the poor gain (but unbypassed cathode resistor always sounds better) to me, aslong as you can add a gain stage, with px4 this time.

Speakers are another problem, hoping celestion 8” field coil speakers are fairly sensetive. Cant address more than one item at a time, may take years. Ill get state pension on 1st September but will have to work on and still not have time for projects. But nice to think about at this stage.
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Nick
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#19 Re: Ed

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But for sure, 11k Zout and 17ma sounds a lot better than 2.5k Zout and 8ma.
Probably, but I would need to run the numbers for the particular circumstance to say a certain y/n
The slew-limiting test case is going to be for high amplitudes at high frequencies, we agree ?
Yes, though a 1kHz square wave will meet that condition, so saying high frequencies is not as simple as it sounds. DIM is possibly relevant here. Also, consider that the Zin of the driven valve will be in parallel with the anode resistance, so the load line will rotate with frequency.
When you are working with a pentode driver, I find you end up with a tricky balancing act between B+, anode R, gain and dissipation.
Yes, that's partly why I used a resistor and CCS in parallel with the ref phono. You can use a lowish B+, have the CCS handle say 75% of the current to keep the available current high which allows you to use a high value parallel resistor but keep dissipation in that low as its only dealing with the remaining 25% of current, so you can either use a high value resistor to get what gain you want, or use a low value resistor to get a low Zout but still have high current available.

So you could have 2.5k and 17ma to use your example
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#20 Re: Ed

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Nick wrote: Tue Mar 07, 2023 9:58 pm Yes, though a 1kHz square wave will meet that condition, so saying high frequencies is not as simple as it sounds
It's OK you can trust me, i'm a physicist.
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#21 Re: Ed

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IslandPink wrote: Tue Mar 07, 2023 11:38 pm
Nick wrote: Tue Mar 07, 2023 9:58 pm Yes, though a 1kHz square wave will meet that condition, so saying high frequencies is not as simple as it sounds
It's OK you can trust me, i'm a physicist.
Exactly. You probably still think sin(X) = X
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#22 Re: Ed

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Well, it is for lens tilts - I'm an engineer as well !
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#23 Re: Ed

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I'm an engineer as well
Well, in that case sin(X) = 0
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