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In Reply to: RE: LTP VS Differential Amplifier posted by Triode_Kingdom on June 16, 2021 at 06:59:46
So with one grid driven the balance is delivered by the magnitude of the cathode resistor( be it a resistor or a ccs). With two grids driven, the balance is STILL driven by Rk. The Rk still delivers balance when both are driven...a stage like used in a Williamson will have poorer balance than one rigged with either a longer resistor, or a current reg.
Nowhere does that equation presented by Ralph which I believe to represent the ratio between common cathode amplification factor and grounded grid amplification factor( delivered in circuit that is ) come into play. The second half does not have a mu+1 vs mu delta.
The LTP is no longer a separate set of common cathode and grounded grid amplifiers. They're tied together and the gain equations applied to them separately does not apply that way. The drive voltage is entirely a function of Rk when run SE:PP and the amplification does not magically change if you put some signal into the other grid.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Follow Ups:
Is the way he depicts the two tubes (ignoring for now the cathode resistor).
We can clearly see that the first tube is driving the 2nd via its cathode.
We also know that the greater the mu of a cathode follower, the less signal voltage loss there will be on the cathode.
So in the scenario where only one tube is driven by the grid, we can see that the other tube will always have less output, as a function of its mu.
The better the CCS becomes, the more this difference is reduced. If both sides are driven by opposite signals, it becomes no concern. Since our circuits are all balanced anyway that is why we use matched resistors.
Put another way, if you're driving this circuit single-ended, there will be a slight difference at the outputs, especially if your CCS is questionable.
If the tube with no signal on its grid actually has more gain, it would work to compensate for an imperfect tail load. The circuit works the other way...
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
If the tube with no signal on its grid actually has more gain, it would work to compensate for an imperfect tail load.
Emphasis added.
It might help. But at that point IMO/IME its all about slop.
The grounded grid has more gain than a grounded cathode.And yet the section with the grid grounded *NEVER has more gain.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 06/16/21
The point here is that the input side will have voltage loss delivering the signal to the other half that does not have its grid driven.
GG has more gain. Not in this case due to less input. But
So then, with a large Rk, where is this input going?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I remember some time back simming the input section of the EAR 509 cct to get my head around it. It uses DC coupled cascaded LTPs; equal anode resistors. The first has a very long tail to a neg supply.
The outputs of the first were ever so slightly different but the second evened that up so the result was the same from the 2nd anodes.
Best not discuss what happens after those stages ;)
I have built with cascaded LTP's too. They do a very fine job IME if the gain is needed. Since I like a medium-mu linestage, two-stage amps have done quite well. The latest are LTP's for balanced drive to the active cross-over.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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