Heater Ordering....does it matter?

[Mook]

This is just for argument sake, but does ordering matter in the Heaters?    For instance, can I wire up EL34, 12ax7, EL34, 12ax7, lamp?    I'd say this usual parallel wiring with Center Tap 6.3 VAC from PT.


Then.....the same question for DC heaters.....



Thanks!

Mook

[shooter]

for the AC || I don't think it much matters.  Pretty sure the "phasing" issue has been argued to not really matter.  The biggest issue would be wire routing, keep them away from low level signals, B+ wires, anything that could get cross-coupled into the signal path.

I've done 2 DC filament builds and both have been problematic for me, the last I'm pretty sure is filtering because I used a wal-wart 12VDC 1.5A for my pre-amp there was still to much hum.  when I used a DC battery to troubleshoot, all the hum was gone.  the other used a 12VAC SS rectified/filtered PS which induced hum, I think, from the transformer into the electronics - somewhere.  I think most here that have used DC didn't have any issues.  Go to the top of the main page, select entire forum and put in DC filaments then search, should be some good reading.

[HotBluePlates]

... does ordering matter in the Heaters?    For instance, can I wire up EL34, 12ax7, EL34, 12ax7, lamp?    I'd say this usual parallel wiring with Center Tap 6.3 VAC from PT.

For A.C., what Shooter said.

My opinion is that heater phasing is pure b.s.

...the same question for DC heaters...

If all heaters are in parallel, then no. If you arranged the heaters in series, then maybe.

Say you have oriented your use of the heater pins to power 6x 12AX7's with each drawing 150mA at 12.6vdc (series arrangement of the internal heater sections). And you further decided to place each 12AX7 in series, powered by a d.c. supply of 12.6v * 6 = 75.6vdc @ 150mA. You probably did that because you were sharp and knew pulling low current through a voltage regulator eases design requirements and yields better regulator performance, and that there are available sources of power in the amp which provided well over 75v peak a.c. which can be regulated down to the desired heater voltage.

Now if you had the above arrangement, one end of the heater chain is connected to the regulator output and the other end of the chain is grounded. You'd probably want to have the phase inverter at the higher-voltage end, because a 12AX7 is rated for a max of 100v from heater to cathode, with the heater positive, and the cathode of the phase inverter is very likely at an elevated d.c. voltage already (for most phase inverter circuits). So the input stage would be the one that gets grounded.

Transformerless amp with series-string a.c. heaters do the same thing, with the rectifier connected directly to the line, power tubes next, all the way back to the input tube with has one heater pin grounded. They did this because the smaller tube was likely to have the lowest rated voltage allowance between heater and cathode, and because having the smallest a.c.in the tube was least likely to induce hum in the input stage.

But you won't be copying any of those amps, because as a sharp guy you're willing to pay a few bucks to acquire the isolation from the power line that a power transformer offers, and because saving a few bucks isn't worth risking electrocution.

[trobbins]

If your heaters are connected in a 'line', then make the first heater closest to the PT an output valve heater, and the heater at the other end of the line the input stage valve.  The AC current flowing along the line then reduces in level as you go away from the transformer, and go more towards the input stage.  The current in the heater winding can noticeably couple in to the signal, especially when the signal is a low level.

[2deaf]

If your heaters are connected in a 'line', then make the first heater closest to the PT an output valve heater, and the heater at the other end of the line the input stage valve.  The AC current flowing along the line then reduces in level as you go away from the transformer, and go more towards the input stage.

If "in line" means in series, the current is the same throughout the circuit.

[sluckey]

If "in line" means in series, the current is the same throughout the circuit.

I really doubt that is what he's referring to. He's describing the typical Fender tube layout.

[Ken Moon]

This pic illustrates trobbin's post (from Merlin's website).

Power valves closest to PT, then PI and preamp valves, with the input stage valve at the end of the chain.

[jjasilli]

More from Merlin, posted on another Forum, seconding trobbins point:  http://ax84.com/bbs/dm.php?thread=283738


The last heater in the chain (1st preamp stage) has only 0.3 A flowing.  Less likely to induce hum & noise into signal.  If the heater wiring were reversed, 3.9 A would flow there.  This might be noisier.

[trobbins]

Yes, to be more clear, the current in the wiring going from the second last tube to the last tube in the 'line'  only has eg. 0.3A flowing in it.  The last section of heater wiring is in close proximity to the low level signal wiring of the input stage circuitry, and the possibility of the induction of AC hum from that heater current is less if only 0.3A is flowing than if say 3A is flowing.

The other techniques commonly used to reduce he possibility of hum induction is to make the wiring between tubes in a twisted pair configuration, and to not splay the wiring as it gets to the valve base, but to keep the 'loop area' as small as possible.  But this is all low level noise stuff, so may be of no significance in many circuits.

[2deaf]

open air coil induction

You talking Simmons Beautyrest here or one of those cheap Chinese knock-off's?

[jjasilli]

in theory, it shouldn't matter, because the heaters would only be a resistance.   but in the real world, The heater sequence should follow the B rail voltage.  My reasoning, is the way the heaters are wound, they have will have open air coil induction characteristics. 

If someone wants to argue, and calculate the coil inductions and prove me wrong, then let it happen.

The issue, per Merlin, is not the heaters themselves but rather the heater lead wires inducing hum into the signal path.