Screen grid zeners; which?

[Wavebourn]

Originally posted by Johan Potgieter
Hi Simon,


Thanks for that reference. I am greatly temted to be naughty and reply with some of Eddie's own advice: Do not listen to all the guru's! No further comment.

Indeed! For example, when some guru suggests better regulation by dropping some voltage on a gas discharge tube in series with the load.

[Johan Potgieter]

Seems you and I missed something in our technical upbringing, Anatoliy.

[mach1]

Indeed! For example, when some guru suggests better regulation by dropping some voltage on a gas discharge tube in series with the load.

Yes, I found that analysis quite bizarre ....

[rdf]

For fun I ran a Spice analysis of an ideal version of Pearl's config against a true triode connection. An EL84 was driven directly by an ideal AC voltage source. Ideal DC voltage sources provided cathode bias and plate-screen bias. Distortion figures were compared between the plate-screen voltage set to an arbitrary low number (0.001 VDC) and a 31.5 VDC approximation of Pearl's circuit. The plate load was an 8kohm resistor, input set to 2 VAC 0-p. Pearl's circuit required a grid-cathode voltage of 6.42 VDC against the triode's 8 VDC to maintain the same 41.5ma IP and 250-251 VDC Vpk. Results below, conclusions none. =D

[Wavebourn]

Originally posted by Johan Potgieter
Seems you and I missed something in our technical upbringing, Anatoliy.

Conclusion: old f**ts must die like mammoths.

[wrenchone]

Interesting discussion - I'm planning a small SE triode amp using 6LR8 compactrons with the power pentode section triode connected. I was thinking of using the simple coward's approach with the screen nailed to the plate through a resistor, but I may try the Pearl approach to determine if I can hear a difference - I'd forgotten about it until now.

[Klimon]

Come on Johan, don't give me that kind of denigrating armchair-smartass bogus.

Interesting discussion - I'm planning a small SE triode amp using 6LR8 compactrons with the power pentode section triode connected. I was thinking of using the simple coward's approach with the screen nailed to the plate through a resistor, but I may try the Pearl approach to determine if I can hear a difference - I'd forgotten about it until now.

Promises to be interesting, it's not much work and at the very least you'll learn something.

Simon

[Johan Potgieter]

Originally posted by Klimon
Come on Johan, don't give me that kind of denigrating armchair-smartass bogus.

Simon, I seem to have offended you, and I apologise for that.

But would you kindly perhaps just quote where I was a smart-*** and what bogus you are referring to, so that I am at least not at a disadvantage? I think that is a fair request?

[All I can think of is my remark about Eddie, and that was criticism of his epistle, not you. For the record, there are several matters in that article that are plain nonsense, but I am not going to set myself up as a critic per se. That is not what this thread is about. You would have noticed that others agreed with me - are they also s-a's?]

[Johan Potgieter]

Back to the topic, and perhaps at my own expense (kindly pardon my resistance to go through the whole lengthy Grimwood and Eddie documents again):

The question crossed my mind before, but was unmentioned. Apart from a fixation with screen dissipation, what are the other claimed advantages again of this enhanced triode thing - not higher output?

One gains by allowing the anode to go higher up the voltage scale on positive signal peaks, but loses at the bottom because the screen has a low voltage limit before onset of 2nd harmonic distortion, above which the anode must now stop by the value of the built-in anode-screen voltage drop, instead of being able to approach its lower limit as in normal triode style.

[Wavebourn]

Johan;
it seems to me they claim an advantage of AC-coupled screen grid to anode against an approach with a resistor to drop excess of a voltage to meet specs of max G2 dissipation. Looks like guys invented a bicycle and found that the 2'nd grid is a grid, i.e. when biased positively in respect to cathode it draw a current, also it is a control grid as any one grid, so current variations cause control, as the result in case of high AC resistance transfer function will be changed.