Amp Computer

The problem here was banana cables.   At some point, the ‘Asshole Audiophile’ community got a hold of banana connections and now it’s damn near impossible to get banana cables at a reasonable rate.   I wanted to use banana cables instead of say, posts and alligator clips.    These guys :  Circuit Specialists solved that for me.   I paid a buck and a quarter per 20″ cable.  Woot!  Back on that soon, but the first two parts are in fab now!    I haven’t decided on the mounting system yet, Cheap boxes or panel/rackmount.   Tough call actually.

Real McTube III

This got some love yesterday.   The power circuit is done but not boxed yet.   I am going to box it separately from the foot pedal part because hey, it’s 140/12 power supply and it’s not like I’m gonna take it on the road.   I wired up the heater stuff, LED and footswitch yesterday and may take a crack at the signal today.   More on that heater circuit later, but these are the first two amp computer style components.

Real McTube Calculations

This is gonna take a while, I did the numbers and they worked out, but now I can’t find the book I used and describing this stuff is really painful.   I’m back into the Rider book but that is the most annoyingly pedagogical book I have ever read.   I’d love to find a good clean book on tubes, with our without the math but it must have full data sheet reading info

SongBird

I’ve started buying parts and will rip the old stuff out of the chassis soon.   There may be some original Gibson Transformers on EBay soon.  Maybe I will recreate the Crestline in the amp computer style.  Maybe the 102C84 will get up there.  That would be ridiculously hard to find anywhere else.

I’m not gonna do a full analysis either, but I want to cover some of the basics, and then measure once we are built.   I’ve done some back of the envelope already and I’m pretty sure I don’t know what I’m doing.  

12AX7

The tube is 12AX7, a very popular pre-amp tube, so knowing how it sounds and how it used in a circuit will be very handy.   This is a dual triode.   It houses two triodes in one glass bottle.

Power

Each triode is rated for 1 watt of power.    Power is    P = VI, so we can draw that curve on the standard datasheet just to keep ourselves safe.    The datasheet shows voltages from 0 to 500 and currents (in mA) from 0 to 4.0.    All we need to do here is pick a number, do the math and plot the point.

I’ll start with 250 V for no reason at all and find the current that creates 1 watt of power.  Please see the datasheet below

1 W = 250 V  * I  ->  I = 1/250 A = .004 A  = 4.0 mA.   Isn’t that handy?  Right at the top of the graph.  Wow, what luck!

Lets do some more

So just draw those points on the graph and connect them with lines to get a good idea of the safe ranges for this tube :

12ax7 data sheet with max power curve

Click on the sheet for a better picture, but there is no way we are gonna get close to the max power rating, it’s not even in charted territory.    Recall from the real McTube article that the max voltage is 140 V.    That would be over 7 mA of current and this is a stomp box meant to go in front of your amp.   I don’t think we need to worry.

Next up.. where on this chart are we?