AC30 build help - a few questions

[markmalin]

Hey guys,


I'm building an AC30 in the style of the Hoffman layout, but also looking at TUT3 for Kevin O'Conner's version.  I have a few questions and hope it's OK to post them all here in one thread. I can break them up if necessary.  These are probably in the vane of stupid questions, but it's been a tough week and I want to be sure I understand this before moving forward.  (read, "brain not functioning well")


1.) O'Conner has an AC layout in his book.  He talks about heater wiring in the diagram I've attached.  Normally I wire 12AX7 tube heaters such that pin 4/5 goes to 4/5 on the next one, and 9 to 9, etc., but for the power tubes I've always thought it's best to alternate the heater wires (so for an EL84 assuming "H1" is one heater wire and "H2" the other, tie H1 to Pin 4 of the first power tube, then to pin 5 of the next and so on so they alternate).  I've heard this helps with noise.  I see in TUT's diagram O'Conner has all the pin 5's to H1 and all the pin 4's to H2.  Is this better?   I need to know before I move further because it's going to be easier to get at the wires now than later.


2.) The Mercury PT I'm using has on the Primary side 0V, 100V, 120V.  Why is this?  I assume I use the 0V and 120V and tie the other lead off (i.e. cut it, put heatshrink over it and tiewrap it somewhere where it won't get into trouble.  Correct?


3.) Pretty much all of the amps I've ever built have been Class AB with fixed bias (like a Fender style adjustable fixed bias) where you take the 60V secondary tap and connect it to the bias rectifier circuit.  But the AC 30 is cathode bias.  So the cathode biased EL84's use a voltage from the power supply dropping resistor string...on my PT then, I just tie off the extra 60V taps (i.e. cover then and secure them somewhere where they won't touch), correct?


4.) This is embarrassing, but I've reversed (mirrored) the layout that Doug provided because I want the chassis "tubes up", but the input on the left side.  So I took Doug's diagram, reversed it in Paint.NET and am going off of that.  Thing is, the tube pinouts are now backwards with respect to reality.  This is going to sound really stupid, but it's been the week from hell so bear with me.  On my pre-amp tubes I'm using the opposite sections of the tubes from Doug's drawing, so pins 6,7,8 are the first gain stage, pins 1,2,3 the second and so-on.  The two halves of a 12AX7 are identical, yes?  I just want to be sure I'm not missing any nuances.


5.) Finally, check out the pictures I'm posting of the chassis and if you see something idiotic, please don't hesitate to call it out!


Thanks guys.  Again, probably all pretty stupid questions, but just having you guys there to sort of "talk out loud" helps :)


Mark

[sluckey]

1. I've always wired my EL84s pin 4 to pin 4 and pin 5 to pin 5. I don't see any benefit in crossing them.

2. Maybe the 100V primary tap is for use in Japan? Just insulate and tie it off.

3. Just insulate and tie off the bias wires. BTW, the cathode biased amps get there bias voltage from the current flowing through the cathode resistor, not from some power supply dropping resistor. Cathode bias is also called self-bias. It works just like a typical cathode biased 12AX7 in a preamp.

4. No problem swapping the halves of a 12AX7 around to accommodate your layout. I do it too.

5. Chassis looks good (when I hold a big mirror next to my monitor)! 

Take your time with the mirrored layout. Little gremlins can creep in and hide very well in a mirror. Please continue to post about your project. I'm very interested.

[markmalin]

...BTW, the cathode biased amps get there bias voltage from the current flowing through the cathode resistor, not from some power supply dropping resistor. Cathode bias is also called self-bias. It works just like a typical cathode biased 12AX7 in a preamp.

D'oh!  Right. I knew that, I'm not sure what I was thinking when i wrote that.


Thanks for the input, Slukey.  I'll post more as I go.  I'm going to get the PT all wired up and move on.  It's kind of a challenge doing this in reverse (mirrored).


Mark.