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TubeCad Error re: LTP?

24.27.41.140
Posted on June 10, 2021 at 23:09:26
Triode_Kingdom
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Posts: 10042
Location: Central Texas
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 The image below is from John Broskie's TubeCad magazine. Am I correct in thinking this is an error? Doesn't the CCS force equal signal currents, thus equal but opposite output amplitudes? I'm assuming this is a "perfect" CCS.












 

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RE: Coupling Cap, posted on June 17, 2021 at 12:23:45
Triode_Kingdom
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Location: Central Texas
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 What are we looking for?



 

the section with the grid grounded *NEVER has more gain, posted on June 17, 2021 at 12:52:07
Ralph
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 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.

 

RE: the section with the grid grounded *NEVER has more gain, posted on June 17, 2021 at 14:19:59
elblanco
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 GG has more gain. Not in this case due to less input. But

 

RE: Coupling Cap, posted on June 17, 2021 at 14:24:02
elblanco
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 balance or lack thereof. cap should be ~8uf for Fc decade below 20hz, right?

 

RE: the section with the grid grounded *NEVER has more gain, posted on June 17, 2021 at 15:00:43
PakProtector
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 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.

 

RE: PI Balance, posted on June 17, 2021 at 16:03:07
AmadeusMozart
 Looks good,, just be aware that different tubes and/or output transformer are highly likely to upset the feedback loop and the frequency compensation in the driver. (A lot of) testing will be require to ensure stability over different frequency ranges and output levels.

Personally I would be keen to add some resistors in series with the screens.

 

RE: PI Balance, posted on June 17, 2021 at 18:56:53
AmadeusMozart




















Two schematics from the RCA 26 manual, basically identical except for using a different output tube (and adjusting the B+ voltage).

To continue on my earler remark of having to adjust feedback loop and frequency compensation: Check the values of the feedback capacitor and the addition of frequency compensation in the phase splitter.

In the second schematic I like the way that the bias voltage is derived - an automatic adjustment when B+ comes on and there cannot be a run away because of a fault in the bias circuit.

This is what you have to figure out when not using the same components - especially transformers are troublesome, just any transformer which supposedly has the same impedance can result in total different behavior. This is due to interwinding capacitance and leakage inductance between primary and secondary. personally I would never ever consider trying to build a clone of the Williamson amplifier as it is only marginally stable and needed a very specially constructed OPT.

AM

 

RE: LTP VS Differential Amplifier, posted on June 18, 2021 at 01:55:53
PakProtector
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 Couple of things, the grounded grid amplification factor of mu+1 leaves it requiring that side to have higher gain even with balanced resistors.

That never happens.

So the basic issue seems to be that treating it as a grounded cathode and a grounded grid seems to be improper. I don't see treating it that way when the diff amp model actually seems to give the correct output predictions with grid No.2 signal counted as zero.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: PI Balance, posted on June 18, 2021 at 06:02:41
PakProtector
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Posts: 12363
Joined: May 14, 2002
 For all the worry about balance, a lot of the stuff I have built of late runs a pentode LTP. Screen current is not going through the loads, but most certainly is passing through a cascode MOSFET current regulator at the cathodes. Phase-to-phase balance is not to be despised...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: PI Balance, posted on June 18, 2021 at 07:47:56
Triode_Kingdom
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 That's the schematic of the HK Citation V, just an example of the PI balance circuit in an LTP (and the same topology as the amp I'm building). I'm using HK transformers, but not from the Citation. Screens in each channel will be driven by a VR tube in series with B+, similar to the Leslie 122 amplifier. This will set the screens at 105V below B+, so just about any 6L6 variant can be used.

















 

RE: LTP VS Differential Amplifier, posted on June 18, 2021 at 08:11:53
cpotl
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 "So the basic issue seems to be that treating it as a grounded cathode and a grounded grid seems to be improper. I don't see treating it that way when the diff amp model actually seems to give the correct output predictions with grid No.2 signal counted as zero."

In Crowhurst's Fig. 626 it is manifest that V1 is driving into the cathode of V2, and since the grid of V2 is grounded, that means that V2 is by definition a grounded-grid amplifier. I don't see that there is any room for that statement to be incorrect.

It may be that one could find some other words to describe the same situation, but they must, logically speaking, be equivalent words describing exactly the same situation.

In any case, whatever words one uses to describe the circuit, there can really only be one set of correct equations that can be written down for the small-signal analysis, and so only one correct prediction for the two signal outputs. The physics, after all, is what it is, and all that is really being done is to apply Kirchoff's law to the currents at each node, employ Ohm's law to relate potential differences across resistors to the currents flowing through them, and employ the small-signal analysis of how the tubes behave under small perturbations around the quiescent conditions. All of these are universally agreed.

There shouldn't really be room for there to be any differences of opinion about what the correct system of equations to describe the circuit should be. And it is a very simply circuit. I think that analysis of Crowhurst's, after correcting the obvious algebraic mistake, is valid.

By the way, what do you mean when you say "the diff amp model actually seems to give the correct output predictions with grid No.2 signal counted as zero"? Correct as judged how?

 

Bias etc., posted on June 18, 2021 at 08:44:45
Triode_Kingdom
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Location: Central Texas
Joined: September 24, 2006
 "In the second schematic I like the way that the bias voltage is derived - an automatic adjustment when B+ comes on and there cannot be a run away because of a fault in the bias circuit."

I do like the fact that bias tracks B+ regarding variations due to line voltage. However, this circuit brings the same liabilities to an AB1 amplifier as self bias. Namely, increased amplifier output and anode current causes bias voltage to increase, and that moves the amplifier closer to Class B. Small amplifiers operating near Class A may not suffer the effects, but large amplifiers biased in true AB1 will show crossover distortion at high output levels. This distortion will persist into quiet musical passages due to the time constant of the bias circuit, degrading fidelity. In addition, this circuit is essentially a voltage divider with the negative bias voltage being subtracted from available B+. That may not be a disadvantage, depending on the transformer used, but it has to be taken into account during initial design.

"I would never ever consider trying to build a clone of the Williamson amplifier as it is only marginally stable and needed a very specially constructed OPT."

The most significant issue with the Williamson IMO is the inclusion of too many stages and too many coupling caps. The Citation V topology is much cleaner, due in part to the simplification of the gain structure made possible by driving the outputs in pentode mode, rather than triode. Only two coupling caps are necessary, and there is significant headroom in every stage ahead of the outputs.

 

RE: LTP VS Differential Amplifier, posted on June 18, 2021 at 10:10:22
PakProtector
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 With a grid grounded in a diff amp, that section is not presenting a gain of mu+1 as it would if the circuit could be treated as a grounded cathode and a grounded grid. The balance is dependent on the magnitude of the tail load.

The usual implementation of a diff amp third stage in a Williamson is going to behave quite differently vs one with a hard, unsaturated CCS in its tail. The signal at the cathode node will be larger, and the plate output will be different.

If it were acting like a separate grounded grid stage, an LTP built around a pair of 6C19Pi running a hard CCS would show higher gain in the section with a grounded grid since mu/mu+1 is near the minimum practically possible. IOW, if treating the grounded grid as separate were proper, a minimum mu circuit would show the grounded grid section with higher gain. That never happens.

As you said, examining the circuit as a whole is required, and it will not show that the grounded grid side has a delivered amplification factor any different from the other half. The interaction between mu, gm, plate loads and the cathode load show the imbalance generating causes.

Without the equations in front of me, increases to mu and gm will both shrink the imbalance. Differences in the plate loads can work towards correcting this imbalance, and increasing Rk shrinks imbalance...
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: LTP VS Differential Amplifier, posted on June 18, 2021 at 11:31:42
cpotl
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Posts: 1002
Location: Texas
Joined: December 6, 2009
 "As you said, examining the circuit as a whole is required, and it will not show that the grounded grid side has a delivered amplification factor any different from the other half. The interaction between mu, gm, plate loads and the cathode load show the imbalance generating causes."

One needs to be sure that disagreements are not merely about how the words are being used. In terms of Crowhurst's equations, if the amplification factors are called A1 and A2, and defined to be the changes in the plate voltages of tube 1 or tube 2 respectively, divided by the input voltage on the grid of tube 1, then they can be read off from his equations (92) and (94) respectively (since his Ep1 and Ep2 are defined to be the changes in plate voltages on tube 1 and tube 2 in response to putting a 1 volt input on the grid of tube 1). That is to say,

A1 = Ep1 (in eqn (92)), and A2 = Ep2 (in eqn (94)).

(Actually, there is a further typo, in his eqn (92); there should be a plus sign after Rp1 in the denominator of his expression.)

Using the corrected eqn (96) that I had previously given, these equations then say it all. That is, all the signal voltages, namely Ep1 at plate 1, Ep2 at plate 2, and Eck at the common cathode point, are all expressed in terms of the input voltage on grid 1, the resistors RL1, RL2, Rk, and the characteristics mu1 and Rp1 for tube 1 and mu2 and Rp2 for tube 2.

Crowhurst is, of course, precisely worrying about the fact that tube 1 is driving into the impedance corresponding to the paralleling of the cathode resistor Rk and the input impedance of the grounded grid amplifier that it is driving. That is the basis of his calculation. So he has taken everything properly into account.

By the way, if we assume RL1 = RL2 = RL, and assume identical tubes so that Rp1 = Rp2 = Rp and mu1 = mu2 = mu, the resulting formula for A1/A2 agrees exactly with the one Ralph gave a few days ago, namely

A1/A2= 1 + (RL + Rp)/((1+mu) Rk).

If the tubes are taken to be different in their characteristics (unequal mu and Rp), but the two anode loads are still set equal, RL1 = RL2 = RL, then the ratio of the amplification factors is given by

A1/A2 = 1 + (RL + Rp2)/((1+mu2) Rk).

So the mu and plate resistance of tube 1 do not enter in the expression for the ratio of the amplification factors.

 

RE: LTP VS Differential Amplifier, posted on June 19, 2021 at 04:26:56
PakProtector
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 What in the hell is eqn 92 and 94?...and feel free to put up the corrected version.

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: LTP VS Differential Amplifier, posted on June 19, 2021 at 05:33:34
cpotl
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Location: Texas
Joined: December 6, 2009
 "What in the hell is eqn 92 and 94?"

Equations in the book by Crowhurst that has been under discussion in this thread. I put the relevant pages in a post on this thread a few days ago, with my corrections indicated on the 4th of the posted pages.

 

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