PT Current Draw Page Gone

[jazbo8]

(1) When it is filled, I am using power output estimation formula from Merlins page (towards the bottom) http://www.valvewizard.co.uk/pp.html . Admittedly for a push pull stage. Then P is divided by DC for current draw calculation. <snip>

(2)When it is not filled - the Rload is estimated as per SE output stage
<snip>

Niki

Great job on the calculator - it's new and improved!

I think Merlin's PP formula should be modified, since the actual Pdiss is usually 0.6~0.7 x Pmax. Perhaps you can add a switch for SE or PP, that way the correct Pmax thus RL (if left blank) could be applied.

[Synchu]

Good points!
Will think on how to add the switching option, as the sequence, once you say select Merlin's formula will be, if there's no Rload entered:
- Rload will need to be calculated - R ~ (DC^2)/P . And P here will be the selected tube max plate dissipation.
- Then, if we use the calculated load to calculate P via Merlin's formula ((V(dc) - 50)^2)/Rload, there obviously will be inconsistency
It is possible, but I am still contemplating on how plausible it is.

Scratching head here ....

Niki

(by the way - 0.6-0.7 * max. dissipation will be your bias point current, i.e. the actual current will vary around, and that is why most PP amps are biased at most 70% of max dissipation)

[jazbo8]

Good points!
Will think on how to add the switching option, as the sequence, once you say select Merlin's formula will be, if there's no Rload entered:

Actually I just realized that you have a switch for SE already, by selecting "1" for the number of output tubes to - that's your SE, in which case use Merlin's SE formula, and for all other cases, use the PP formula. The inconsistency in Po/RL was already noted by Merlin, so as they say it is "close enough for rock & roll"...

[Synchu]

To some extent - yes, but still - you can have SE with more than 1 tube (as you may have Class A, Class AB operation and a number of other factors that have been only approximated in the current calculation).

Anyway, I did few updates in the hope to make it a bit more understandable. But I will be ultimately looking to collect and enter the reference data from the datasheets and display it along the calculations so it can be compared.

Niki

[Synchu]

Hey all,

Fellow forum member dorrisant posted me a few suggestions that I've implemented on this (already old ) calculator. Thanks Tony!
- custom transformer voltage (center-tapped transformer is assumed)
- custom power tube (with the ability to enter the output power and the filament current draw)

Both values are used in any further calculations exactly as the pre-defined ones.

Let me know how these work for you, prior to me replacing the old one in the next few days or so.

EDITED: Please, use the original link
The updates can be found here:
https://thesubjectmatter.com/calcptcurrent.html

Niki

[thetragichero]

I've been using the old one quite a bit so thanks for the update!

[dorrisant]

Yes, thanks again Niki!!

Now we can calculate for odd tubes and whatever voltage your off the shelf PT will put out. This will be a closer approximation of what you will get with whatever combination of tubes an TX you might use.

A great tool made better!!

[roberto]

Thanks guys!

[Synchu]

Thanks!

This has been moved to:
https://thesubjectmatter.com/calcptcurrent.html

The "test" link remains for the time being as well at:
https://thesubjectmatter.com/calcptcurrentv2.html

Niki

[martin manning]

Seems like there is some error, around a factor of two in the results. When I input parameters for a 4x 6L6GC 2k Zpri with three preamp tubes (Dumble/Fender Twin), I see a PT current requirement of nearly 800mA.

[Synchu]

There isn't - at least from calculation point of view.
It uses Merlin's "approximation" formula, shown and referenced when you hit "Show formulas" button.

Gladly accepting better ideas

Niki

[martin manning]

You have this: P = (NoPwrTubes * (DC - 50)^2)/Rload [W]

Blencowe’s equation is 2*(DC-50)^2 / Zaa. http://www.valvewizard.co.uk/pp.html

The number of tubes doesn't matter, the power and current are determined by voltage and load.

Idc = P / Vdc, Where Vdc should be the same voltage as was used to get the power, DC-50.

For the case I looked at above, with 4x 6L6GC, 2k Zaa, and 445Vdc, P = 156W, and Idc = 0.395A (power stage alone). For a 2x 6L6GC amp with the same voltage and 4k Zaa, the results are reduced by half as expected to 78W and 0.197A. I would consider increasing the results by 5-10% to account for screen current.

[Synchu]

Thanks Martin,

I will look at it and get back.

Niki

[Synchu]

Martin,

Got you!

You're totally correct - and it was fixed.

Niki

[martin manning]

Niki I still get the high current using the links above.

Another question, the current for preamp tubes based on max dissipation seems high. 12AX7's usually only draw 1-1.5mA. 12AT7's are common for PI and reverb drivers, and they will draw more, but I don't know if the extra complication of multiple preamp types is worth the trouble.