First Try at reverse-engineering a simple amp ...

[surfsup]

Here is the schematic: I am trying to reverse engineer this to figure out what's going on and why.

http://ax84.com/p1x/AX84_P1x_101004.pdf

This is the AX84 p1 Xtreme schematic. The Transformer is kicking out 388V (i'm using the red value on the schematic here). However I can calculate this from:

http://www.hammondmfg.com/pdf/5c0026-27.pdf
secAC = 550 *0.7 = 385V so the 388 is good to go...

I want to figure out the rest of the red values using the resistances and some assumptions about the single 12ax7 and EL34 tubes.

B1:
The EL34 tube is rated at 25W max dissipation (Pa) from here;
[link]

Total is 388 so 25/388 = 64.4mA draw into B1

B2:
I read on another forum that the EL34, or any power tube, will pull about 10% of peak current which is 140mA for the EL34. So I'm using 14mA for B2

B3: The 12ax7 will pull about 1mA but I'll double that value, then double it again for the fact there are two in the tube = ~5mA for B3

That's 84mA total being drawn from the transformer.

So Voltage = I * R

Voltage drop across the 100ohm resistor is 6V per the difference in red volt values listed on schematic (388-382=6volts)
Per my calculations... V=84*100 = 8.4V (red is 6v)

I have 19mA across the next R=1k
V=IR=0.019*1000=19v (red is 9V)

I have 5mA across last R=47k
V=IR=47000*0.005 = 235V (red is 99V)

If I use the R and V values from the schematic:

B1=Vdiff/R=(388-382)/100= 60mA (I had 64.4mA)
B2=Vd/R=(382-373)/1000= 9mA (I figured 14mA)
B3=Vd=(373-274)/47000=99/47k= 2.1mA (I figured 5mA)

So I was kinda close on the mA side (I should state I *do* know a small change in current leads to a larger change in V). What I'm wondering is if I was to design this amp, I would be *creating* the schematic and I would have to have the mA numbers to calculate the V first, right? So the way I did it, would I be way off base? Or do you pick the V for each tube and design the resistors around those values?

If I am assuming the mA values first, calculating the voltages for each stage/tube, I would then try to draw the load lines for each of the tubes, 12ax7 and EL34, and attempt to get the bias point close, then adjust the numbers again on actual values?

I guess what I'm asking is to design an amp, the first thing one needs to do is:

1) select the tubes you want to use (how many and where)
2) guestimate the current draw (as I did with values 64.4, 14 and 5mA)
3) find a Transformer that would handle the load and get the Vdc (388v here)
4) calculate the V readings at each T/PT/PreT 5) start your load line and bias selection
6) design around the main parts of the circuit (tubes/stages) to add stuff that might force you to recalculate some stuff, but "close is close enough"?

--- OR ---

Do I,
1) select the tubes I want to use
2) select some Voltages for each tube
3) select the transformer, design the R values
4) calculate the current draw
5) pick biases to closely resemble the values
6) adjust for extra resistors/capacitors in the circuit

[surfsup]

I know I know its a long post but please, anyone? Please? bumpity bump?

[FunkyE9th]

You have to learn how to do loadlines... some good info info here http://www.audioxpress.com/resource/aud ... /index.htm

Also if you search the forum I believe there is a thread discussing a lot of reference material you can read.

[Phil_S]

To be brutally candid (no ill will intended), this is a long post that isn't that interesting to wade through. You've asked us to check your math in a number of places, or so it seems.

I believe you are over thinking the whole thing. As FunkyE9th says, search this forum and others -- particularly the AX84 forum where this design originates. There is plenty of material to cover your questions. That aside, find some tubes, find some iron, and go to it. Tubes are pretty forgiving within a reasonable range.

You've already figured out you need a PT that's 275-275. You will be good +/-10% on that and there is a very good BOM with the schematic. The rest falls into place if you build the schematic as draw. Go ahead. Give it a try and try not to think about which came first, the tube or the transformer.