Controlled zero-bias characteristics?

[tubby]

From time to time I stumble across tube data sheets where it says: "This tube is a direct replacement for tube x, but features additional controlled zero-bias characteristics."

Anybody could explain what that means? Why would you run a tube with zero-bias in the first place?
Is "zero-bias" meant as in no plate current is flowing?

[Helmholtz]

From time to time I stumble across tube data sheets where it says: "This tube is a direct replacement for tube x, but features additional controlled zero-bias characteristics."

I might refer to the Vgk = 0 plate characteristic/curve, which determines Raa, Vsat and max. output.

But please post a datasheet example showing this note.

[tubby]

I might refer to the Vgk = 0 plate characteristic/curve, which determines Raa, Vsat and max. output.

But please post a datasheet example showing this note.

Here is a datasheet for the 7408, which is a 6V6GT substitute with said addition in the data sheet.

[Helmholtz]

Thanks.

I think my explanation above is correct.

Zero-bias means Vgk = 0 and refers to the upper plate curve (= characteristic). Grid bias (Vgk) is the parameter for the plate curves.
Controlled means quality controlled.
So the 7408 might be a 6V6GT selected for low tolerance/variation regarding the plate characteristics.

[tubby]

Thanks.

I think my explanation above is correct.

Zero-bias means Vgk = 0 and refers to the upper plate curve (= characteristic). Grid bias (Vgk) is the parameter for the plate curves.
Controlled means quality controlled.
So the 7408 might be a 6V6GT selected for low tolerance/variation regarding the plate characteristics.

Ah yes, makes sense now. Thanks for explaining!

[pdf64]

Thanks.
…Controlled means quality controlled.
So the 7408 might be a 6V6GT selected for low tolerance/variation regarding the plate characteristics.

That’s reminded me of a query I’ve not yet found an answer to, despite a fair bit of trawling -
What is the normal level of acceptable variance of characteristics from the type nominal?

[Helmholtz]

That’s reminded me of a query I’ve not yet found an answer to, despite a fair bit of trawling -
What is the normal level of acceptable variance of characteristics from the type nominal?

Haha, good question.
Tube manufacturers don't seem to like publishing that kind of info.

As long as I don't know better I assume +/- 10% for quality tubes.

[pdf64]

So ‘controlled’ types would be tighter than that?

[Helmholtz]

So ‘controlled’ types would be tighter than that?

I would hope so .

[g1]

So ‘controlled’ types would be tighter than that?

Do you mean like milspec, JAN, etc.? I wonder if some of those old requirements are published.
Sometimes it means every single one is tested to meet spec., rather than say, 1 in 10, or 1 per hundred.

[The Dude]

I've often wondered how many tubes are rejected by first tier dealers that actually test tubes before reselling. In other words, how many tubes don't meet the cut directly from the manufacturer. Depending on where you buy tubes even as a tech or end user/consumer, you can have a pretty high failure rate. Just imagine how many are kicked back before they even get to us.

[g1]

I've often wondered how many tubes are rejected by first tier dealers that actually test tubes before reselling.

There used to be speculation that JJ from eurotubes were more reliable than from other sellers due to 'special' relationship with the factory.

[Enzo]

I used to deal with Bob at Eurotubes from time to time, and the occasional correspondence. If my aging memory serves, seems to me you had options. You could buy bulk tubes still in the crate from the factory, or you could pay a premium for tubes that had been tested. Either you pay less for tubes, expecting higher dud rate, or you pay more for tubes, but expect them all to be good. Take your choice.

[The Dude]

There was a time when it was worth it to buy bulk untested tubes, but as tube QC devolved it became a losing proposition. The last time I bought bulk untested tubes, about 60% were defective. To make matters worse, there was no refund for defective tubes. Another negative was that they were so bad you couldn't count on a repair to stay fixed. I started buying tested tubes and never looked back. It's a no brainer.

[Enzo]

I used to buy a lot of my tubes from Tom at Magic Parts (The Ruby Tube folks). They cost a little more but they were all tested and came with a warranty. Worth it to me.