Determine proper HT current reserve for PP from load line

[tschwarz]

Hi builders,

Given the following load line for a set of 2 ECL85s, how do you determine the HT power rating for the PT? Let's ignore the pre-amp for the moment? Surely it's not just the bias current x 2 (22mA per pentode). It also is probably not Imax at Ug1=0 (140mA). How does the reservoir capacitor factor into this calculation / estimation ?

Cheers

[PRR]

...It also is probably not Imax at Ug1=0 (140mA). How does the reservoir capacitor factor into this calculation / estimation ?...

If the peak of a sine is 140mA, what is the RMS?

[tubeswell]

Here's some examples of how to calculate it on different 6V6 load-lines (Courtesy HBP)

[tschwarz]

Well... 100mA. But the peak current drawn must be provided from somewhere, no? Do you just multiply by some factor when choosing the PT? Like "twice the RMS" etc. ?

[Edit]: Ooops, ignore that. RMS power is ((300V - 9V) * 137mA) / 2 = 20W...

[tschwarz]

Here's some examples of how to calculate it on different 6V6 load-lines (Courtesy HBP)

I get that. That's how I calculate the output power: (dV * Imax) / 2. In the example case that's ~20W. What I'm looking for is the PT power / reservoir capacity rating required to reliably deliver the 20W output with enought reserves for the peak current...

Thank you for your input!

[shooter]

reservoir capacity rating required

Ecaps care way more about Volts than current, so max operating VDC, round up to nearest vdc rating.


PT cares most about heat over time.  guitar players care about sag, brown sound.  Stiff, clean.
so the type of rectification matters there.


a tube Rec, "lightly" filtered, with a PT close to the VDC and I of the PA section
or SS rect. heavily filtered, with a PT above the VDC, say +20% and a current rating +40% of the PA section.


but the BEST way, find the commercial amp closest to the one you're building and Steal the values from it 

[pdf64]

Yes, to go deeper than borrowing and tweaking pre existing power supplies, things get rather complex, eg relating the power amp’s HT DC current requirement to the AC current rating of the HT winding in a typical 2 phase rectifier.
Maybe try downloading Duncan’s PSUD

[tschwarz]

reservoir capacity rating required

Ecaps care way more about Volts than current, so max operating VDC, round up to nearest vdc rating.

Electrolytics voltage rating is clear. What I would like to know is how to at least estimate eg. a 100uF reservoir's contribution to the impulse current drawn when Ug1 approaches 0V. That current, resp. power quantum, would then not have to be delivered by the PT and rectifier. Assuming I'm not feeding my amp with a sine wave input at full blast...

Every builder here must be constantly facing the same question with each new (non cloned) project - "how big must the PT be for this amp, based on HT and the power section's load line". Right now I'm guessing it's some magic factor, like "twice the quiescent current"...

[shooter]

Every builder here must be constantly facing the same question

nope, in 18 different design/builds the same Ecap went into the 100w amp n the 5W amp


450vdc, 100uF F&T  Sometimes I did cheat when vdc was over 500vdc at power on, I'd use 2 in series and balance with a couple R's

[tschwarz]

Every builder here must be constantly facing the same question

nope, in 18 different design/builds the same Ecap went into the 100w amp n the 5W amp


450vdc, 100uF F&T  Sometimes I did cheat when vdc was over 500vdc at power on, I'd use 2 in series and balance with a couple R's

...the electrolytics aren't in question here. Their dimensioning is trivial - "the bigger the better within the rectifier's limits"...

The power transformer max HT power rating is what interests me. More PT iron costs more and uses up more chassis real estate. So how small can it be made for a given power amp's parameters (HT, load line, bias, ...)?

So - in those 18 different designs, did you always use the same PT?

Thanks again for chiming in!

Cheers, Tom

[PRR]

Well... 100mA. But the peak current drawn must be provided from somewhere, no?....

The B+ cap.

Current swings from (say)22mA to 140mA and back again, the cap carries the peak, the transformer supplies the average.

[shooter]

nope, but close.
This isn't cork sniffing HiFi, this is guitarist using the amp as part of their system.  some want that brown sag you get from an under-sized PT.
some want that crisp clean, ring a note all day long you get from a PT that is twice what you need.


unless you want a deep dive in thermo-dynamics, for your own satisfaction, steal the PT specs from the already designed n engineered amps that have been around since WWII.

[tubeswell]

Here's some examples of how to calculate it on different 6V6 load-lines (Courtesy HBP)

I get that. That's how I calculate the output power: (dV * Imax) / 2. In the example case that's ~20W. What I'm looking for is the PT power / reservoir capacity rating required to reliably deliver the 20W output with enought reserves for the peak current...

Thank you for your input!

The current is in there. (And unless your amp has bazillions of preamp tubes, you can assume most of the current will be drawn by the output tubes. 2 output tubes draw 2x as much as one tube in Class A etc, and in Class B only one side of the power amp is on, so the total current load on the PT B+ barely goes above the Class A peak.) The loadline tells you the power efficiency (including the current demanded) for a given load. The screen current component of the load on the B+ can be estimated from the Ip:Ig2 ratio on the tube datasheet, but rarely will be above about 5% of plate current (per tube)

[HotBluePlates]

... how do you determine the HT power rating for the PT? ...

If the peak of a sine is 140mA, what is the RMS?

Current swings from (say)22mA to 140mA and back again, the cap carries the peak, the transformer supplies the average.

As PRR keeps saying, figure out the RMS current implied by the peak current for your tube/voltage/load at full power output.

A clue is that Hammond Transformers (and maybe some others) advised that the "D.C. Current" rating on their transformers was equal to the RMS Load Current the transformer could supply.

If you're running Class A, RMS current is very-close to idle current.  If you're running Class B, RMS current is very much higher than idle current.