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Old 16th December 2012, 01:26 PM   #21
SY is offline SY  United States
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Exactly the same as if the plate were solid. It still acts as a triode. But the capacitance from g2 to g1 is generally lower than the capacitance from plate to g1, so you do better for bandwidth than the plate-connected triode.
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Old 17th December 2012, 07:39 PM   #22
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I guess the reason you're asking is noise. With triodes, to minimise noise you need to operate at the lowest possible anode current you are happy with. High gm helps too, but anode current is the main thing to think about.
That's not my experience with a triode-connected EF86, see Low noise, low microphony, low hum valves posts 19, 24 and 25.
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Old 17th December 2012, 08:43 PM   #23
Gluca is offline Gluca  Italy
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I believe radiotron agrees with you too on noise and current.

Btw ... would anyone be interested in a LCR group buy? Lundahl has 0.45H and 1.8H chokes available ... we just need a very basic PCB (to make soldering easier and for a better look) and source a couple caps and few resistors.

they go for something like 110euro per pair, wound on mu-metal core and shielded. maybe we can get a special price for higher quantities.
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Old 17th December 2012, 09:01 PM   #24
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Both, really. It's a pentode tube used as a triode with the screen as the plate. The plate is a Dunsel.
Hi SY,

I hope we both agree that in a conventional pentode circuit, where the screen grid is connected to some sort of voltage source (such as a decoupled voltage divider), most of the signal current flows through the anode and a small part through the screen grid.

In Frank Blöhbaum's circuit, the screen grid voltage is fixed by a voltage source and the current follower (common-base or common-gate stage). Applying the substitution theorem, as the screen grid voltage is fixed like it is in the conventional pentode circuit and the anode voltage is also similar, the signal current should still be mainly flowing through the anode and only a small part through the screen grid. The only difference is that the screen grid current passes through the current follower and then joins the anode current again, which eliminates partition noise.

The fact that you can get similar transconductance and noise with a triode-connected pentode with a current follower (cascode) on top of the whole thing does not mean that the circuits internally operate the same. According to Blöhbaum, a practical advantage of his circuit over triode connection plus cascode is that the power rating of the current follower can be smaller, as it only conducts a small part of the current.

Best regards,
Marcel
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Old 17th December 2012, 09:10 PM   #25
Merlinb is offline Merlinb  United Kingdom
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Quote:
That's not my experience with a triode-connected EF86, see Low noise, low microphony, low hum valves posts 19, 24 and 25.
It is important not to draw too many conclusions from only four samples. Some of your data is consistent with the theory, and the remainder may be due to experimental error or simply odd valve samples. Flicker noise is highly variable between samples.

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I believe radiotron agrees with you too on noise and current.
Radiotron cannot be trusted for noise at audio frequencies. It only quantifies noise at radio frequencies, and was written before a lot of the low frequency work was carried out.

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The fact that you can get similar transconductance and noise with a triode-connected pentode with a current follower (cascode) on top of the whole thing does not mean that the circuits internally operate the same.
SY is right. It doesn't mater that more current flows in the anode than in the screen, all that matters is the transconductance, and that is almost entirely determined by the screen grid. Hence you can disconnect the anode and the circuit still functions basically the same. Thinking about it the other way around, Blohbaum's circuit is a hybrid cascode, but where a whole lot of current is wasted by allowing it to flow through the anode, in parallel with the transistor. I guess you could argue this is an advantage of the circuit, since you can have little dissipation in the transistor but maintain a good slew rate.

Last edited by Merlinb; 17th December 2012 at 09:24 PM.
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Old 17th December 2012, 09:45 PM   #26
SY is offline SY  United States
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Originally Posted by MarcelvdG View Post
In Frank Blöhbaum's circuit, the screen grid voltage is fixed by a voltage source and the current follower (common-base or common-gate stage). Applying the substitution theorem, as the screen grid voltage is fixed like it is in the conventional pentode circuit...
It's also fixed like it is in a cascode, right?

Let's take an example (which is the one I experimented on), a D3a and the "Bloehbaum" connection done via an NPN transistor. With Vg2 = 140V, Vg1 = -1.2V, for a D3a Ip = 20mA, rp = 120k, gm = 30mA/V . Ip/Ig2 ~ 3.5, so screen current will be slightly uder 6mA. I use a 10k plate load, so that gives a gain of 300. The base of the transistor, an MPSA42, is held at AC ground. hfe is ~100. Impedance looking into the emitter is 10k/100 = 100R in series with 1/gm, which is negligible. The gm at g2 is 1/3.5 gm to the plate (the current splitting ratio) = 30/3.5 = 8.6mA/V, so tube's gain to the transistor's emitter is 8.6 x 0.1 = 0.86V/V. The transistor has gm = 35*Ic ~210, so its voltage gain to the anode is 2100, whereas the D3a's voltage gain to the anode load is 300. The transistor is apparently doing all the voltage amplification, and if you disconnect the plate and run 6mA to the screen, that's exactly what you see- the circuit performance hardly changes.
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Old 17th December 2012, 10:04 PM   #27
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But, but, but... If the G2 plus transistor are the same as a cascode, why isn't their contribution to output gm divided proportionally with their gm? (Which in a cascode is set by the "lower" device.) It just feels like an argument of special pleading.

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Chris
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Old 17th December 2012, 10:53 PM   #28
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Originally Posted by Chris Hornbeck View Post
But, but, but... If the G2 plus transistor are the same as a cascode, why isn't their contribution to output gm divided proportionally with their gm? (Which in a cascode is set by the "lower" device.) It just feels like an argument of special pleading.
The math is very straightforward. It doesn't take much voltage swing between base and emitter to get a large current swing. If we use the usual cascode gain approximation (gm1*Rp), gain with the 10k load would be 860, lower than the more involved calculation. We need to use the more involved equation because the collector impedance and bipolar transconductance are hugely higher than a triode used on top. My way of calculating is a bit simpler in this case, recognizing that the bipolar transistor only reacts to AC voltage between base (at AC ground) and emitter (driven by g2, which acts as the anode). That gain calculation is straight textbook.

Remember, the transistor's emitter is ONLY driven by the screen.
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Old 17th December 2012, 11:14 PM   #29
SY is offline SY  United States
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Chris, another way to look at it is that the voltage gain to the screen is slightly less than unity. So the transconductance splitting as far as the plate load is concerned in Frank's circuit is between the tube (gm ~ 30) and the transistor (gm ~210). It's not surprising then that the transistor dominates.
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Old 17th December 2012, 11:22 PM   #30
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Great explanation SY!
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