pentode in triode mode SE vs a DHT SE like 2a3/300b

[Klimon]

Hi Tom,

Given my lack of technical background I can't argue with you on basis of maths; however there are different independent people that came up with similar circuits (see for example the thread "triode trick" in this forum)... if there isn't a problem, why would people see one and design a solution for it = defies all logic. The link I included IS to a commercial site but take a look in the forum where the technically-oriented sceptics can find more info on this topic. More importantly, the circuit has a more than subtle impact on sonics says my ears.... A pseude-triode probably behaves very much like a true triode, but I'm sure there's still a difference, and circuits ala bill Perkins / vaughn audio minimize the difference, so I believe.

Regards, Simon

[phn]

Originally posted by Klimon
Hi Tom,

A pseude-triode probably behaves very much like a true triode, but I'm sure there's still a difference, and circuits ala bill Perkins / vaughn audio minimize the difference, so I believe.

Regards, Simon

So you say a triode is not a triode even if it has all the characteristics of a triode? By that logic the Aristotelian concepts are out. Then exactly what makes a triode a triode?

[ilimzn]

It appears to me that much of this argument is a case of 'it does not follow', namely, if a directly heated triode is linear, it does NOT follow that if you want a linear triode, it must be directly heated.
Ditto WRT triode strapping - not to be confused with 'pseudo-triode' which is often what ultralinear and similar explicit means of providing plate feedback are called (remember that for a pentode, G2 is the equivalent of the triode plate). Triode strapped pentode/tetrode is a triode, but it does NOT follow that it is linear or, for that matter non-linear. It's just a different triode - one that was most probably NOT designed for best triode operation, or it would not have been a pentode to begin with. But, 'on occasion' the gods of electron flow have been kind and you get a pretty linear triode.
Also, comparing on 300B and 2A3 grounds involves some assumptions - in order to be efficient with practical loads, a triode has to have a fairly high gm and relatively low mu, i.e. low Rp. If one wants to compare 2A3, 300B etc with triode strapped tubes, then at least compare with most if not all the relevant parameters in mind.

[phn]

True. One reason DH power triodes are good is the combination of relatively low Rp and high gm. High mu DH triodes are mediocre at best. That's the reason novals tend to be the preferred gain/driver tubes for DH power triodes.

In the heyday of DHTs they couldn't make a tube with low Rp, high mu and gm. At least nobody did. The E55L/8233, when triode strapped, has a mu of 50 and Rp of 600 Ohms. And it's as linear as any tube. But the latter can be said of lots of noval pentodes.

[andrew_whitham]

Originally posted by phn


So you say a triode is not a triode even if it has all the characteristics of a triode? By that logic the Aristotelian concepts are out. Then exactly what makes a triode a triode?

looks like a triode, smells like a triode, distorts like a triode must be a triode?

I wonder if the confusion here arises from the idea that a pentode strapped as a triode doesnt sound like a mental 'reference triode'. this is not arguable. A triode strapped EL34 for instance wouldn't sound like 300B but then It wouldnt sound like a 6AS7 or trioded KT88 either

Klimon, I know youve tried the triode trick but bear with me here:

The circuits that try to minimise any lingering effects of the screen in triode operation, are making the pseudo-triode more like a true triode. But with the internal grid, cathode and anode spacings of the base pentode. Probably not the same as a true triode, thus the difference! Fundamentally its IS a different tube after all. Using a resistor won't suddenly turn it back into a pentode. Its just that Bill Perkins' circuit adresses some of the compromises inherent in it not being a triode in the first place.

OTOH, What about the compromises inherent in having a DHT heater supply floating around in the signal path!

Andy

[andrew_whitham]

must type faster......

[Salas]

Originally posted by salas
As for DHT vs Triode strapped beam tetrode or pentode, there are findings of an even more benign distortion amount and pattern in low gain DHT tubes. They certainly sound a bit more open and natural.

See low gain. Means low Rp. Less of N1/N2 etc, etc.

[Wavebourn]

Originally posted by salas


See low gain. Means low Rp. Less of N1/N2 etc, etc.

...and think of distortions added by more amplification needed to drive it...

[Klimon]

Klimon, I know youve tried the triode trick but bear with me here:

The circuits that try to minimise any lingering effects of the screen in triode operation, are making the pseudo-triode more like a true triode. But with the internal grid, cathode and anode spacings of the base pentode. Probably not the same as a true triode, thus the difference! Fundamentally its IS a different tube after all. Using a resistor won't suddenly turn it back into a pentode. Its just that Bill Perkins' circuit adresses some of the compromises inherent in it not being a triode in the first place.

Exactly my thoughts, although the words are different

A pseude-triode probably behaves very much like a true triode, but I'm sure there's still a difference, and circuits ala bill Perkins / vaughn audio minimize the difference, so I believe.

@ Phn:

So you say a triode is not a triode even if it has all the characteristics of a triode? By that logic the Aristotelian concepts are out. Then exactly what makes a triode a triode?

If you will I'ld say a pseudo-triode IS (electrically) a triode. If you permit some nuancation I would propose that although a pseudo-triode is very near a pure triode, there are downsides to the standard resistor strap method (downsides lessened by the extra Bill Perkins blablabla circuit) that should be acknowledged, and form a difference with pure triodes. I perceived this difference (as exemplified by my one amp, hardly pure science but I didn't invent the triode trick so it's not a one-man show) as being negative sound-wise, when compared with the extra circuit which minimises the difference.... There's no point in a "A or B" discussion, my stance is "A with a taste of lemon"; bless Aristotle.

Simon

[phn]

Originally posted by salas


See low gain. Means low Rp. Less of N1/N2 etc, etc.

The Western Electric 417A, 437A, EC8010, EC8020, triode-strapped E55L, D3a etc. are all high mu (40 to 70), high gm, low Rp and ultra-low distortion.

The 2A3, 300B, 205D, AD1, RE604, etc. are all great tubes. But they are only great because they stopped making and improving DH triodes.

Originally posted by Klimon


Exactly my thoughts, although the words are different




@ Phn:



If you will I'ld say a pseudo-triode IS (electrically) a triode. If you permit some nuancation I would propose that although a pseudo-triode is very near a pure triode, there are downsides to the standard resistor strap method (downsides lessened by the extra Bill Perkins blablabla circuit) that should be acknowledged, and form a difference with pure triodes. I perceived this difference (as exemplified by my one amp, hardly pure science but I didn't invent the triode trick so it's not a one-man show) as being negative sound-wise, when compared with the extra circuit which minimises the difference.... That's my point I think , bear with me and you'll find heaps of healthy logic; Aristotle would've been proud of me

Simon

So what is it that makes one chip a D/A converter and another chip an op-amp if the chips aren't defined by their characteristics?