I withdraw the 'fatally flawed' comment and request that it be read as 'somewhat flawed'
Perhaps even as far as 'significantly flawed', in view of the impression of technical validity and accuracy the user might take from the calculators.
Ideally I would have reviewed and amended that before it got picked up on.
I have no reason to believe that such calculators do not work as expected from the data fed into them. The flaw I refer to is that different valve types / manufacturers / regions / time periods used different rating systems, so the published info should not be assumed to be directly comparable, from one valve type / manufacture / date to the next, and so should not be taken at face value, but rather should be interpreted from their particular context.
pompeiisneaks wrote: ↑Tue Mar 16, 2021 8:05 pm
...The document defines there are three types of ratings systems. Rob Robinettes calculator clearly denotes his is for max design dissipation, not the other two. also the design maximum (max design? Same thing?
I don't think so, ie the 'max design' dissipation noted on the calculator does not seem to correspond to a specific rating system.
For convenience and so we're all on the same page, here's Rob's calculator
https://robrobinette.com/Tube_Bias_Calculator.htm
There's a 'valve type' selection menu that provides anode dissipation ratings taken from manufacturer's info. For this calculator, that's where the user might be expected to get that data, though helpfully all integer values over the relevant range are also provided as an option.
I think the menu has examples of valve anode ratings from all 3 systems, eg 6L6WGB 26W - abs max, 6V6GTA 14W - design max, 6V6 12W - design centre.
It might be a useful improvement if the rating system used by the menu was made clear, as it tends to be noted on the published info, particularly for design max.
https://tubedata.altanatubes.com.br/she ... 6L6WGB.pdf
https://tubedata.altanatubes.com.br/she ... 6V6GTA.pdf
https://tubedata.altanatubes.com.br/she ... /6/6V6.pdf
...Is your implication that maybe this latest design maximum methodology is flawed and that the design centered system is more optimal?
All the systems are fine in and of themselves, what's non optimal is it isn't a level playing field for valve anode dissipation ratings, so in my view that acts to further erode the conceptual validity of idling all power valves in fixed bias to a specific, generic, target anode dissipation.
That being something that some people seem to fixate on.
My view is that for general use, as with such calculators, it would be best if design centre ratings, either published or estimated, were used, because they include an additional safety factor for component and supply variations. That being something non EEs may not consider when using ratings that were derived under the design max or abs max systems.
From the few published examples available, design centre ratings tend to be about 15 - 20% lower than design max.
...is the JETEC finding that design max was the most valid one a bad decision then?
Very clever engineers put forward a compelling technical rationale for it. But then there may be a more cynical commercial motivation, as valve types rated under the design max system get an apparent 20% boost in their anode ratings, compared to equivalents rated under the design centre system. Which gave the new 6L6GC a marketing advantage over the (European) EL34 it wouldn’t otherwise have had. However the Philips organisation seemed content and did fine staying with the design centre system. So on balance, given the confusion it causes, my view is that it would have been better for the whole global industry to do so too.
It’s unfortunate that the ‘big guns’ such as Merlin and Aiken, whilst providing guidance on max idle anode dissipation % in fixed bias AB, don’t qualify it by providing any indication about how the rating system used to derive the anode dissipation limit might affect that (if at all?). However the context to both examples seems to be EL34, a design centre type. We might take from that design max rated types should be idled a bit cooler than the 85 and 70% points they suggest.
The semiconductor industry uses only abs max ratings, perhaps they learned as lesson from this mess.
Stevem wrote: ↑Wed Mar 17, 2021 10:53 am...theGenlex ones rated at 45 watts where killer tough!
See
https://tubedata.altanatubes.com.br/she ... k/KT88.pdf
Design max - 35W, abs max - 42W.
From the typical max/centre ratios available for other valve types, I think its design centre rating would be about 30W.
It should be noted that abs max ratings do not include the allowance for valve variation, whereas the design max rating does. So from that, we may take it that many, perhaps up to half, KT88 would be excessively stressed as their average anode dissipation was increased above 35W, up to 42W.
The inclusion of the normally withheld abs max rating for the KT88 in that info may have been a desperate, cynical ploy by the GEC marketing dept to make it look competitive with GE’s 6550A. The 6550A had a 42W design max rating.
It would be interesting to know what rating systems are used by modern manufacturers.
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