Zout versus RL(a-a)

[ricardogerassi]

I was looking at Genalex's KT-88 data sheet and I stumbled on the following: on the data sheet for a fixed bias U/L AB1 amplifier the following figures are shown (for a given +B value):

RL(a-a): 4.5 kohms
Zout: 7 kohms

When choosing an O/P transformer should I choose the primary impedance based on a plate to plate load (RL a-a) or Zout ?

Sorry if that's a silly question.

Thanks y'all

Ricardo
BRAZIL

 

[805amp]

In this case you should choose a transformer with a 4.5k primary impedance.

 

[HollowState]

Ricardo, use the plate to plate rating. 4000 to 4500 ohms is a very typical value for KT88s and 6550s. You could use a slightly higher impedance and get a little less power output coupled with a little less distortion. But you may not actually hear the difference easily. Typical plate voltage is around 500v.

 

[6A3sUMMER]

I would guess if the Zout is 7k ohms, the damping factor will be quite low, unless you apply Negative Feedback.

Either use UL and Negative Feedback, or use Triode mode. (or Beam Power mode and Negative Feedback).

Otherwise, use speakers that work well with low damping factors.

 

[Wavebourn]

Pentodes are almost current sources controlled by voltage. Local feedback such as UL decreases output resistance, but it still acts as a current source, if no more feedback is applied. There is almost no damping factor from straight pentode output. Output voltage will follow speaker impedance curve, some people claim that such drive decreases distortions of speakers, while makes frequency response worse.

Nominal load is the different story. It means optimal load for the tube, when ratio power/distortions is optimal.

 

[DF96]

Zout is the output impedance; it is a fact, although it will vary between samples and depends on bias. RL(a-a) is the recommended load impedance; it is a suggestion, based on a compromise between power and distortion. These two are unconnected, and almost always different numbers.

 

[rongon]

Re: Pentodes with local NFB:

Since local NFB decreases the apparent anode resistance of the output pentode to very low levels (800 ohms?), does that pentode with local NFB still act as a current source, or has it now been transformed into something akin to a triode (as is so often claimed)?

Sorry if this is too far off-topic.
--

 

[artosalo]

Pentode remains as current source with NFB too, but NFB "control" the drive voltage of the pentode as the voltage at the output varies and the result is more like voltage source looking stage.

 

[Wavebourn]

"Triode is a pentode with vertical curves, pentode is a triode with horizontal curves".

Nothing is perfect, neither voltage sources, nor current sources. Everything is in the middle. Triode looks more like voltage controlled resistance, while pentode looks more like voltage controlled current source. Even triodes can be either "left handed", controlled by negative voltage, or "right handed", controlled by positive voltage. Even the same triode in left-handed mode has "triode" curves, but in right-handed mode it has pentode curves.

 

[rongon]

The reason I was asking was because I'm still trying to figure out how a 6L6 with plate-grid negative feedback yields curves like a triode, and with low output resistance. If that's actually true, then it should be possible to use a 6L6 in pentode with plate-grid NFB into a suitable OPT, and not need global NFB around the OPT to drive a speaker with "triode-friendly" impedance curve. In other words, a better "SET" amp.

But if the 6L6 with plate-grid NFB still has higher output resistance like a pentode with no NFB, then global NFB would still be *required* to drive just about any normal speaker with a crossover.

Am I way off?
--

 

[MarekH]

Does that mean a 4p1L is ambidextrous? It seems to have as many grid lines above 0v as it does below 0v

kind regards
Marek

 

[Wavebourn]

The reason I was asking was because I'm still trying to figure out how a 6L6 with plate-grid negative feedback yields curves like a triode, and with low output resistance. If that's actually true, then it should be possible to use a 6L6 in pentode with plate-grid NFB into a suitable OPT, and not need global NFB around the OPT to drive a speaker with "triode-friendly" impedance curve. In other words, a better "SET" amp.

But if the 6L6 with plate-grid NFB still has higher output resistance like a pentode with no NFB, then global NFB would still be *required* to drive just about any normal speaker with a crossover.

Am I way off?
--

You can even get much lower output resistance in pentode mode than in triode mode, like McIntosh did. All depends on feedback ratio.

Does that mean a 4p1L is ambidextrous? It seems to have as many grid lines above 0v as it does below 0v

Any tube has as many grid lines above 0v as it does below 0V. The question is, for how long. Grid current heats it up and can damage.

 

[ricardogerassi]

Thanks for the replies folks. I am learning a lot in here.