ax84 build question for Power Supply

[surfsup]

I am trying to figure out tube amp building and have a question (the ax84 forum is rarely visited) and figured I would try to ask it here:

http://ax84.com/hioctane/AX84_Hi-Octane_101004.pdf

For this schematic, the power supply is a EX269 from Hammond and would like to figure out how the values in red for the voltages of B1+ B2+ and B3+ were calculated. I've been following the valve wizard article on smoothing power supply:

http://www.freewebs.com/valvewizard1/smoothing.html

and got to the "THE PRE-AMP SUPPLY" part about 3/4 of the way down, and understood the sections prior on how the resevoir capacitor was determined, as well as the screen resistor and capacitor.

However, the Pre-Amp Supply section abruptly ends and doesn't really explain why the valve wizard article selects 1k, 3W resistors and 47uF caps, as well as how the BLUE VOLT values were calculated. Could someone help me understand how those blue volt values were determined?

I've ordered some books by merlin? and they haven't arrived yet. I'm all about figuring this stuff out THANKS in advance!

[mdroberts1243]

The last stage voltages are simple calculations based on the voltage you want to drop... "The value of resistor used in each stage depends on the voltage you want it to drop. The current flowing from the power supply through the filter resistors progressively decreases toward the input stage."

In the last stage in Merlin's example (the input stage) he shows 3mA going through a 1K resistor for a drop of 3V (E=I*R). The blue voltages correspond to this, dropping from 269 to 266V.

In circuits like the Dumble amps we're often shooting for a specific supply voltage on the preamp tubes to get in the right zone for tone and bias of the tube.

[ampgeek]

Welcome!
Each tube draws current from the PS rail. Since the tubes' high voltage demands are in series off of the rail, there is a progressively greater current draw through each dropping resistor along the rail from the first preamp tube back to the first supply point.

Use ohms law to calculate the voltage drop with an assumed current draw for each tube.

How much current each tube draws depends on how it is biased/set-up. My rule of thumb is that a 12AX7 draws 1.1 ma/section (2.2 ma/bottle). Not sure about the ef86 off of the top of my head. It has a plate and a screen current demand.

Cheers,
Dave O.

[xtian]

I'm not trying to disuade you from posting your questions here, but the AX84 forum is pretty active, and there are lots of smart folks there who can help with the projects.

[surfsup]

thanks. so the blue volt values would be used to draw the right point of the load line for the tube? So the load line starts at 282Va for the ECC81 for example?

The article also states: "Supposing this amp has two ECC81's, an EF86 and two EL34's, the average current drawn by each triode will be about 5mA and there are four triodes in all, making 20mA. The EF86 will draw about 3mA, making 23mA for the pre-amp."

Where do the 5mA and 3mA values come from? Is this given by the tube mfg? So if I want to "reverse engineer" the HO ax84 design schematic red volt values, I would need to know what each triode of the 12ax7 used in the HO design would draw (in mA). How does one know this?

xtian, I did post there, but since I'm trying to move forward in my understanding of this, the quickest answer is the best. I am blown away how quickly people responded here. If you guys don't mind, I would certainly like to ask more questions here since it is definitely more active. Again thanks for the help!

[ampgeek]

You can punch in a tube type here and pull up it's data sheet.

http://tdsl.duncanamps.com/tubesearch.php

Most will have curves that will allow you to estimate plate current draw at various grid and plate voltage conditions. Note that the grid voltage is referenced to the cathode voltage value. I always assume the grid is at -1.5 V for estimating purposes.

Not sure about the article that you reference but....EF-86s and EL-34s are definitely not triodes. They are pentodes. EL-34's, at least in a power amp position anyway, should be drawing a whole lot more than 5 ma. 40 to 50 depending on bias and personal tastes. So....use that "article" carefully!

Cheers,
Dave O.

[surfsup]

Not sure about the article that you reference but....EF-86s and EL-34s are definitely not triodes. They are pentodes. EL-34's, at least in a power amp position anyway, should be drawing a whole lot more than 5 ma. 40 to 50 depending on bias and personal tastes. So....use that "article" carefully!

I can see how the snippet I posted could misrepresent the author if the rest of the article is not read. The values for the triodes are for the ECCs only. He goes on to quote the EL34 as 22mA draw. Here is the full text, and I have an additional question about the EL34 value, actually looking at the bold, shouldn't the 22mA be added twice for ~210mA total since there are two EL34 power tubes?:

In the following example we have a 50W push-pull amp, and the HT after rectification is 350Vdc. We also need to know the average current the amp will draw. This will be the sum of the quiescent currents of all the valves.
Supposing this amp has two ECC81's, an EF86 and two EL34's, the average current drawn by each triode will be about 5mA and there are four triodes in all, making 20mA. The EF86 will draw about 3mA, making 23mA for the pre-amp.
We will assume that the power valves are biased to their maximum dissipation of 25W each (if the amp is Class-AB they won't be biased that hot, but we should assume worst case scenario). The average anode current they draw will be in the region of:
I = P / V
I = 50 / 350
= 143mA.

The screen-grids will also draw a quiescent current: The data sheet for the EL34 suggests a screen-to-anode current ratio of 6.5, so we can expect the screen currents to sum to:
143 / 6.5 = 22mA.
Added to the pre-amp current this makes 188mA in total for the whole amplifier. (Remember, this is the average current, and is not the same as the peak current that the amp will draw. In an amp like this, depending on the class of operation, the peak current might be 200mA for one EL34 (while the other EL34 goes into cut-off) plus the 23mA pre-amp current which won't vary much. That makes 223mA peak, and the power transformer would need to be rated for at least this much AC current, preferably 1.5x more if the amp is to be run at full power for long periods.

We decide to allow 10% ripple. The HT is 350V, so 10% of this is:
(350 / 100) * 10 = 35Vp-p.
The reservoir capacitor can be found using:
C = (t * I) / V
Where:
I = average load current drawn
V = ripple voltage peak-to-peak
t = duration between charging cycles and is equal to: 1/twice mains frequency.
In Europe the mains frequency is 50Hz, so: t = 1/100 = 0.01 seconds.

The reservoir capacitor required will be:
C = (0.01 * 0.188) / 35
= 54uF.

Here is the circuit design showing two EL34s in the power stage:

[img:763:351]http://www.freewebs.com/valvewizard1/smoothing5.jpg[/img]

[ampgeek]

Fair enough!

No. The 22 ma is for both EL-34s. The author is estimating screen grid current draw as a function of the plate current draw (6.5:1 in the cited example). The plate current draw used to calculate the screen draw is the combination of both tubes so the result is for both tubes.

Cheers,
Dave O.

[surfsup]

Okay, thanks so much! Another question.

http://www.audiomatica.com/tubes/el34.htm
http://tdsl.duncanamps.com/show.php?des=EL34

So here they say the "max plate dissipation" is 25W and a "filament voltage" of 6.3V. So:

Two tubes at 25W each = 50W power = I * V
350V out means I = P/V = 0.1428 current

This is the current flowing into the Power Transformer (PT) - I get that.

Then they use the value of 6.5 as screen to anode current ratio but I can't seem to find this on the datasheet at the above link. The closest number is the screen voltage of 6.3V, but I don't think that is what I should be using (more coincidence). Looking at the screen max current of 50mA and plate max current of 140mA, getting the ratio here is not 6.5

So I'm wondering where you find this from the data sheet?

[ampgeek]

My pleasure mate!

Trying to nail, exactly, the screen grid current draw will make your head hurt (unless you are using specialized software)! Too many interdependancies between plate and grid voltage (which will both change with varying current draws) too simply pull it off of a curve like this one:

http://www.triodeel.com/6ca7ap6.gif

So....the rule of thumb is to start with some type of factor that will get you close (I use 10% of the design plate current) and tweak the dropping resistors after the amp is built and operating. Even that excercise can be mind numbing if you are overly anal about obtaining exact voltages at each tube plate.

Cheers,
Dave O.

[surfsup]

so just to confirm, the max plate current is 140mA. So you would use 10% of this value = 14mA?

So for this example, if you wrote the article, you'd use a value of 28mA instead of the 22mA the author used at valve wizard (because there are two EL34s)? Yes? Do I finally understand the voodoo?

I certainly understand that mechanically, the tubes are not all exactly the same, don't behave the same way, the resistors, caps, etc are not all perfect, so there is margin for error. I just don't want to be way way off and have it not work properly.