Oddball Layout Needs shielded Heater Wire?

[David Root]

I decided to add a buffered loop to my BM-HRM in a Bassman chassis. The only place I can reasonably put a fourth pre tube is on the other side of the board, near the pots.

I am thinking it might be advisable to shield the heater supply wires to and from this extra tube because they will run under the board near the PI wiring.

I have some old mil-spec coax w/three 20 ga conductors, a plated copper weave shield and a transparent outer insulation. Marked on the insulation is 15851 M27027/48 JDDDX-3-600V-20AWG.

Is this necessary and is this the right coax?

See chassis pic below for layout. NOTE: The extra pre tube socket in line is just sitting there, that seems to be the only other option and I don't like it.

[David Root]

I thought it thru and I will put in a 12 VDC supply for the loop tube. Makes much more sense I think.

[ChrisM]

DC heaters does make more sense.

[ampdoc1]

Use the power supply from your relay board, and wire the heaters in parallel. The 1.5A rectifier in it will have no trouble with the additional current draw of 1 preamp tube (~.3A).

I just used that power board in my latest Express to provide DC to all the preamp heaters (see pic).

Since first designing that relay board, I've used it and the PS board for a number of "odd jobs". With a higher amperage FWB rectifier in there, you could provide DC to all the tubes and the relay.

ampdoc1

[David Root]

I believe there is a risk of noise using the same winding for powering both heaters and relays. That's why I didn't do it.

[David Root]

Double post.

[67plexi]

David, I would try to put a piece of alu. plate and space the 4, 9 pin sockets in a row, or run the heater wire through the chassis. the dumble way is under the preamp board two layer the under board is copper clad copper down and grounded for sheilding.

[Ken Moon]

I've spent zillions of hours playing with heater wire arrangements.

One thing that never seems to help is shielding the heater wires - I've used shielding tape, copper and aluminum tubing, aircraft shielding conduit, triax, shielded twisted pair, tried grounding the shields in every conceivable manner, and detected no differences.

What does seem to matter the most, is to make sure that any grid wires that go near the heater wires cross at a 90 degree angle, and it's better to space them at least 1/2" away from the heater wires if at all possible, usually by making the grid wires come "down" to the tube pin, while the heater wires are flat against the chassis (good for eyelet/turret board designs), OR, to have the heater wires coming down to the tube, while keeping the signal wires flat against the chassis (this works better for PTP)

[Structo]

Interesting, I wonder why shielding them doesn't help.
I guess because the actual heater wires are the source of the 60 Hz hum so shielding them or encapsulating them won't do any good.

On my D'lite amp I originally ran the heater wires done in the back corner ala Marshall because I had a solid state FX loop that would have been too close to the heater wires had they been overhead.

When I re-wired the amp to clean it up after many tweaks, I ran the wires overhead and I like that method very much more and I seem to have less hum.
Keeping them high and away from sensitive wires like grids just makes good sense.

[David Root]

I got a Hammond 166F12 filament transformer, built a 3/4 x 2 3/4 board with a BR34 bridge rectifier, 2x1000uF /25V caps and a Fairchild 12V/1.5A regulator, bob's your uncle! (As they used to sayyyyy!)

If I ever want/need to use 12VDC on all four preamp tubes, all I have to do is change the transformer.

[Alexo]

Interesting, I wonder why shielding them doesn't help.

..at 60hz, you're dealing more with a magnetic hum than an RF one, shielding can block out the hash and upper harmonics and noise, but it won't do much for the fundamental.

...twisting works though.