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I am building a PPT stereo amp that uses 2xEL84 for power, a 12AX7 inverter and a 12AU7 input per channel. Only half of each 12AU7 will be used. My PT is a Hammond 372JX that supplies a single CT 6.3V winding (8A rated) for filaments.I will connect the filament windings to 2 posts as a take-off point, then wire each channel separately.
Here's the part in question: For partially educated reasons, I (tentatively) plan to run a single connection to each channel that would connect the heaters in the following sequence: EL84, EL84, 12AX7, 12AU7. Please advise me if there is a better way to do this.
Current draw for each tube is:
12AU7 Ih = 300mA @ 6.3V
12AX7 Ih = 300mA @ 6.3V
EL84 Ih = 760mA
Total current per channel ≈ 2.12A (winding is rated at 8A, nearly double what is required)Also - I plan to use stranded 18 ga. shielded, twisted pair for the heater connections. The shielding would be connected to the main system ground lug only and would follow the heater wires from socket to socket and remain insulated the entire way. Please also advise if there is folly in this method.
Follow Ups:
Twisting the wires nulls hum pickup, using shielded wire may cancel that effect.
Maybe I misunderstand you, but semantics are important.Twisting the wires does not null hum pickup.
Twisting the wires keeps them closely spaced together, which will permit the magnetic field from each (the two currents are 180 degrees apart) to effectively cancel, or at least be at a minimum. You will obtain very similar results with speaker wire. No doubt some will claim twisting is superior, but the foundation is the same, to minimize magnetic fields by cancelation.
The result of this is that nearby circuits will not have currents induced into them by this magnetic field. So the proper description is "twisting wires minimizes the radiated magnetic fields, reducing inductive pickup in other circuits"
With regards to shielding, you are dealing with a totally different animal. Traditional foil or mesh shielding creates a static shield around the conductors, forcing the electric field intensity to zero at the shield (relative to the inner conductors). So any electric fields generated by the conductors cannot radiate into the surroundings, as the shield makes the wire "look" like it's all at ground potential.
Totally different from the magnetic issue above. Magnetic fields will pass right through a copper or foil shield; electric fields will not.
So with the statement, "shielded wire will cancel that effect", I disagree. It is rather unrelated, and will only benefit. The twisting deals with the magnetic fields, which is generated as a result of the current magnitude. The shielding deals with the electric fields, which is generated as a result of the voltage magnitude. They are separate, and I encourage the use of both shielded and twisted.
We won't address the fact that magnetic and electric fields propogate each other, as this just adds confusion. As usual, the statements made above are simplifications that at least permit us to understand the basics of what is going on.
Last point is that the greater culprit in tube amps is the electric field strength; with the exception of heater circuits, the currents we are dealing with are quite low. The voltages we deal with, however are significant. It is for this reason that shielding is very effective in keeping an amp quiet.
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One could write an entire chapter on heater wiring (see Morgan Jones' books), but here's a few tips.If all your heaters are wired in parallel, there will be very little effect the actual sequence will have on operation. Sure, there will be a little voltage drop across the 18 gauge wires, but I don't think 0.1 or 0.2 volts will kill you. Initially I would think you would wire the smallest current draw loads first, with the big loads last in the train. This would make the smallest loads have the least affect from the big ones. But then again, the real goal I would pursue is to use the least amount of wiring. This would probably not result in the desired sequence.
If you are that concerned about voltage drop, consider separate runs to each tube or set of tubes. Also, note that a 6.3V 8A winding is typically rated to output 6.3V at rated load. Being that you are only pulling 25% of this, your actual applied voltage will probably be greater, maybe 6.6V.
If you find this to be the case, consider a series resistor(s) to bring the voltage into spec. Some would insist on a resistor in each leg; if you have a grounded center tap, this might not be a bad idea.
Finally, if you have any elevated cathodes, an elevated heater voltage might be worthwhile. This would require another transformer winding.
That all makes complete sense and gives me a few ideas. I have both of Mr. Jones' books but didn't encounter much info on this particular subject. Perhaps I must take another look.I have a pretty hearty stock of resistors if I need them. Are there any types of composition that you would recommend NOT using for this application?
For starters, determine the size and power rating you need. I like to run my resistors at no more that 20% of their wattage rating, to keep temperatures under control and limit the positive tempco most resistors exhibit. In critical places, I go for 10%. Heater supplies are not critical; you will be more subject to line voltage variations.I would likely end up using a standard cheap wirewound; they are very sturdy and come readily in the sizes you are likely to require (somewhere in the <1 ohm range). Non-inductive is unnecessary; your source has lots of L anyway.
I would remind you of the importance and sonic benefit of elevating heaters where necessary. Don't underestimate this consideration. Jones spends a lot of time discussing heaters in the chapters where he actually steps you through building an amp, though I think there are some places prior where he discusses some other issues. Personally, I don't buy into his THINGY design; unneeded, IMO.
I love this hobby. Thanks for the replies.Very interesting stuff, Kurt. I need to move beyond my "Principles of Electronic Circuits" text book and learn some more intensive electrical theory.
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