Rail Question

[surfsup]

Perhaps it is the 12 hr work days, perhaps it is too much amp study but I am getting confused on something that should be simple.

I want to order the correct iron for the X10 Xits build and created a quick Ohm's Law spreadsheet to determine voltages at each node. My assumption:

1) 120mA total draw (2-EL84 = 100mA, 2-EL84 screens 10mA, 1-PI=5mA, 2-12ax7= 5mA)
2) The EL84s should probably see around 330V @ plates like Vox NT

My questions:

1) The first resistor (1k 5W) should I use that to calculate the V before the rectification using half the draw (60mA) or full 120mA current draw (i.e. V=IR=1000*60mA=60V drop). I assume half because I am using the half voltage to calculate rectified voltage 425VDC after the diode.

2) What does a typical PI draw? I have it at 5mA.

3) If I use a choke, I am substituting that for the 1k 10W? or do I use it in addition? The choke would have 400-ish R resistance so I would want to use a 500R resistor?

4) Should the choke go between the first two filter caps? Merlin says to use it before the OT but in this design the OT pulls from B+1

I put the spreadsheet online to show what I have right now. Any comments would be appreciated. (spreadsheet updated in latest post)

[NEjoe]

Since nobody else is taking a shot at this I will.

I think you're making this harder than it needs to be.

You want to know what power transformer for a pair of EL84s and associated preamp tubes, with max plate voltage of about 330V.

I think you only need about a 250-0-250, but that's just a guesstimate, why don't you drop this into the power supply designer at duncan amps.

A quick look over at the offering from hammond seems they don't have any 250V types in the amperage you need, but if you go up to 275V range you can get a 270EX with 144mA capability, that might put the first B+ around 360V but you can cut that down with a few resistors or a VVR. If you're not doing a tube rectifier you could go use an Antek toroid (no 5V line), they have the AS-1T250, but not everyone likes the look of toroids. Put in a PI filter with about 47uF/220R/68uF and you're there.

If you're going to put in a choke then you should make sure to draw your voltage after it. No sense in putting in a choke to smooth out the voltage and then not use it.

I'm not so sure about the design of the circuit you posted though, I've never seen anybody (Fender, Marshall, etc) put resistors before rectification, it just kind of seems they wouldn't do much there. It also seems like the cap values are a bit on the small side, and a single 33uF cap is going to probably cost less than two 16uF in parallel. (Maybe the company doing this design gets a pricing break on 16uF caps, but I know I won't). I'd look for a different design. Just my opinion.

[Structo]

See if this will help you.


http://www.dreamtone.org/Calc-Current-Draw.php

[XgamerGt03]

I'm going to guess that you don't really need those resistors in front of the rectifier if you are going to pick a transformer specifically for this. I've seen those put in front of a rectifier anytime that a company uses the same power transformer for multiple amps, especially if one uses a tube rectifier.

That simplifies your conundrum a little bit.

I typically say about 2-3mA for a phase inverter.

You would substitute the choke in for the 1k 10W resistor. You don't have to put another resistor after it, but if you don't your voltages will rise a bit. This may or may not be a bad thing.

I like to put the choke between the plate and screen supply caps. The output stage will cancel out any ripple in the voltage that feeds the power supply. Since your using a solid state rectifier you could put a choke up front, but i don't see a point in doing that.

[Colossal]

Surfsup,

To put some clarity on that 1k 10W resistor, we do not see it in the X10 version 2 gut shot so it is assumed that XITS switched over to a choke. The 1k value was used as a placeholder on the drawing as there *was* a large power resistor on the version 1 board although we never did confirm what its value was. Since the original X10 seemed to share a lot in common with the TW Liverpool, it was assumed that the power-resistor-in-question was likely a 1k 10W similar to the 1k 25W power resistor that is used in the Liverpool and Express in the same location.

As for the added power resistors on each leg of the AC secondary, I agree with the other posters that it may be a case of Xits being able to use one PT for multiple amps and they wished to just drop the voltage down slightly for the X10. However, I think in this case I would say that they are likely being used inentionally as sag resistors, not just repurposing a PT from another build. I concede that's just speculation though.

For your amp, I would just use a choke in that position.

[surfsup]

NEJoe, it is a collaboration on the X10 Xits amp seen in the thread in the trainwreck forum, FYI.

Xgamer, okay I had the PI pulling 2mA. I figured it would be the same as a 12ax7 using both triodes. The choke is in the right spot then.

Colossal, the vox night train is putting 330V on the plates, that's why I was thinking the higher PT V rating because everyone was saying its "voxy". So I was thinking of putting up around 315 to 330 on the plates as well. What were you thinking? I think azat said he was over 300 on the plates too in his prototype?

My main question which still isn't answered is the "sag" resistors on the PT secondary, did I calculate their drops correctly using 115 mA across them or should I be using 60mA since there are two sides to the PT secondary?

[XgamerGt03]

If I remember correctly current only flows through one side of the secondary at a time. But at that point its AC current and not DC so you have to take that into account. So you should be subtracting from the action voltage before multiplying it by 1.414.

[Colossal]

Colossal, the vox night train is putting 330V on the plates, that's why I was thinking the higher PT V rating because everyone was saying its "voxy". So I was thinking of putting up around 315 to 330 on the plates as well. What were you thinking? I think azat said he was over 300 on the plates too in his prototype?

340VDC is typical for 18W iron, if that's what is being used for that amp. My guess is 290-0-290VAC for about 340V tube rectified or "sag" rectified in this case. It very well could be lower PT secondary voltage. I think we were thinking in the 330-350V range because of the 125R cathode resistor and this is on par with Marshall 18W amps which operate with 290-0-290VAC secondaries tube rectified (EZ81).

[surfsup]

okay so I re-ordered the math and get these values. I am wondering if you guys are right in that this PT is probably oversized but I can't imagine getting one oversized expensive PT is cheaper than just ordering two PT values from the same source. Xits is definitely selling amps per the waiting list. You can see I am using 115mA and the drop is in the high 200s, not even in the 300s using a 310-0-310.

Something is wrong because the B+ values for the preamp are under 100 and this doesn't even consider the plate resistor...

[img:765:434]http://chicagocadcam.com/ChrisHahn/schem/X10_PS.jpg[/img]

[XgamerGt03]

I think I may have been incorrect in my thinking then. 310-0-310 with a 60V drop across the resistor would give 250 which would be 320ish after rectification with diodes.

[ampgeek]

Jump here for a really quick check that I have found to be pretty accurate for this type of work.

http://www.dreamtone.org/Calculate_Current_Form.htm

I have set up similar spreadsheets for various designs and, after all is said and done, the Dreamtone output, my spreadsheet and actual readings all seem to fall within a ~5% error band.

So...I now just default to the Dreamtone values in the interest of time.

You may have to interpolate for different secondary values so your spreadsheet will still be your friend!!

Cheers,
Dave O.

[ampgeek]

Oops..just noticed Structo suggested the same.
Sorry for the repeat.
Dave O.

[NEjoe]

Hmm....when I clicked on the link Structo provided it told me "You can't see this page without submitting the form"; the link you posted let me in to use their calculator, so it really wasn't a repeat.

[surfsup]

that link says two el84s and two preamp tubes is 72mA total draw. I have 115.

48mA for each plate = 95mA plates
5mA for each screen = 10mA screens
5mA for PI
5mA for two preamp tubes

Also the drop I calculate is 1000*0.115=115V
Should I be using half the current value on those resistors before the rectifier?

[Structo]

Sorry about that, I should have double checked the link.