advice needed on wiring tube ac heaters . . .

[NewYorkDave]

Point taken, Brad. But you'd only need to go a little ways positive of the cathode to reverse-bias this unwanted diode, right? I was mainly trying to warn against sitting the heaters up on +50V or higher for no good reason other than "I heard it was a good idea."

 

[bcarso]

[quote author="NewYorkDave"]Point taken, Brad. But you'd only need to go a little ways positive of the cathode to reverse-bias this unwanted diode, right? I was mainly trying to warn against sitting the heaters up on +50V or higher for no good reason other than "I heard it was a good idea."[/quote]

Yep---Cherry and Hooper say about 10V (pp. 431-432, Amplifying Devices and Low-Pass Amplifier Design, Wiley, 1968).

I encountered this effect empirically with a 6SN7-based fairly-high-gain circuit with unbypassed cathode resistors. I had cascaded two stages of voltage gain with the plate and cathode loads adjusted so that (according to PSpice :twisted: ) the second-harmonic distortion would cancel. Although I was within heater-cathode ratings the first stage had a bunch of really squirrely noise which was present even with d.c. heater supplies.

The Sovtek SN7's may not have been that good either :sad:

 

[mikka]

Must be showing my age. I like CJ's dynamite for DC. If you use the two 100 ohm resisters, I've found they work best if placed at the first preamp stage.

 

[Svart]

nothing beats new sand... :green:

 

[CJ]

Don't write off the gel cell and charger circuit.

 

[mr coffee]

I'm with bcarso,

as I understand it the bias can sometimes be helpful when the cathode-heater system acts like a little vacuum diode of its own, particularly when there is contamination, some cathode coating that has found its way to the heater during fabrication. Then, if the cathode is not at a particularly low impedance, the zero bias (or more properly periodically shifting bias for an a.c. filament) leads to a lower impedance between filament and cathode and hence greater induced hum/noise. The positive bias of the filament relative to the cathode repels the electrons and reduces the coupling.

I've heard it described differently, that by saturating the (unintended diode) between the heater and cathode, it reduces hum conducted into the cathode circuit and therefore into the signal circuit. In any case, it is a good "knee-jerk" approach if done, as was pointed out, in moderation (10 volts or so) in low-level stages. FWIW.

 

[Emperor-TK]

On a related topic: Is there any reason not to wire up heaters with solid core wire? I have some nice 20ga wire that I would like to use. The advantage I see to the solid core is that it will hold a twist nicely and stay tight against the chassis. The amp in question is 100W w EL34's, so I guess the heaters are drawing about 7 amps at 6.3V.

Thanks,
Chris

 

[mr coffee]

Chris,
I would say solid wire is fine if you don't move the equipment around. Most houses are wired with solid wire. AFAIK, the only advantage of stranded wire in a 50-60 hz. environment is that it is less prone to break when subject to vibration.

Mil specs used to (probably still do, I dunno) require stranded wire, and even required you to use anti-wicking tweezers (to keep solder from joining the strands up under the end of the insulation). And a maximum of 3 nicked strands out of 19, none missing. Picky, huh?

Back when I was a wirer-assembler making aircraft navigation systems I asked my supervisor why we didn't use solid wire because it was so much easier to use, and he swore if we didn't use stranded wire and "put the equipment on a truck and shipped it to California that there would be 5-10 broken joints by the time it arrived." I don't know if it's really that bad or not, just what I was told. We sure didn't wire stuff that went in airplanes any way but mil spec. Of course, somebody's life might depend on that. I rarely see solid wire in commercial audio equipment, though. FWIW.

Ty-raps will do a good job of keeping heater wires up and away from the audio circuitry, and stabilize them so they don't vibrate so much when equipment is on the road. I like the old Fender approach of wiring filament power twisted together and routed up away from the top of the chassis where the tube sockets are mounted and against the bottom plate of the chassis (don't let me confuse you - Fender chassis are upside down in the amp). A Ty-rap between the coming and going twisted pair at each socket firms the whole heater supply up. Then you can route all the audio signals up against the chassis and don't need so much shielded wire. YMMV

A little dab of RTV goes a long way in stabilizing the end of long runs of wire, which is where wires tend to break in my experience.

 

[CJ]

Solid is ok if you don't knick it when you strip it.
Didn't Fender use solid core for filimaent hookups? Maybe I'm on crack.
Solid is good on heaters becuase usually you have two wires per socket terminal, since you tend to daisy chain.

We use 104 strand wire here for our test leads. Super expensive but it will not break.

 

[bcarso]

[quote author="mr coffee"]
Mil specs used to (probably still do, I dunno) require stranded wire, and even required you to use anti-wicking tweezers (to keep solder from joining the strands up under the end of the insulation). And a maximum of 3 nicked strands out of 19, none missing. Picky, huh?

[/quote]

Silver-coated stranded-copper teflon-insulated wire is especially prone to embrittlement and fatigue breaking---there's a piece of Pease's about his youthful experiences at Philbrick with a design that used the wire and had horrible field failures. It's also a biatch to strip wihout nicking, but otherwise it is a great high insulation resistance and high temperature wire.

 

[mr coffee]

Hi bcarso,

You wrote

Silver-coated stranded-copper teflon-insulated wire is especially prone to embrittlement and fatigue breaking---there's a piece of Pease's about his youthful experiences at Philbrick with a design that used the wire and had horrible field failures.

I missed Bob's piece on that. Can you give me the gist of why it's prone to embrittlement and fatigue-breaking, or do you know where I can find the article myself? I still use teflon for some in custom work for customers that want it.

It's also a biatch to strip wihout nicking, but otherwise it is a great high insulation resistance and high temperature wire.

It sure as h*ll is <lol> We used to strip it with a thermal stripper, which was a pain, but even the high-dollar, exact wire-size, ground-blade mechanical strippers nicked strands once or twice out of every 10 strips (my own informal study, funded by the Frustration, Perfection, and Boredom Fund for the Arts) :roll:

Teflon is nice if you have to solder in really tight spaces, though. You can just lay the soldering iron on it by accident, and no big deal, no rewiring 3 other connections. IIRC it is supposed to give off some baddie gas when overheated that ain't so good for you to breathe, though (maybe Florine?).

 

[bcarso]

[quote author="mr coffee"]Hi bcarso,

You wrote

Silver-coated stranded-copper teflon-insulated wire is especially prone to embrittlement and fatigue breaking---there's a piece of Pease's about his youthful experiences at Philbrick with a design that used the wire and had horrible field failures.

I missed Bob's piece on that. Can you give me the gist of why it's prone to embrittlement and fatigue-breaking, or do you know where I can find the article myself? I still use teflon for some in custom work for customers that want it.

It's also a biatch to strip wihout nicking, but otherwise it is a great high insulation resistance and high temperature wire.

It sure as h*ll is <lol> We used to strip it with a thermal stripper, which was a pain, but even the high-dollar, exact wire-size, ground-blade mechanical strippers nicked strands once or twice out of every 10 strips (my own informal study, funded by the Frustration, Perfection, and Boredom Fund for the Arts) :roll:

Teflon is nice if you have to solder in really tight spaces, though. You can just lay the soldering iron on it by accident, and no big deal, no rewiring 3 other connections. IIRC it is supposed to give off some baddie gas when overheated that ain't so good for you to breathe, though (maybe Florine?).[/quote]

It seems to me that there was a more elaborate war story somewhere, but there's a short section in Pease's Troubleshooting Analog Circuits, pg. 62, "Weird Wired World."

As far as the breakdown of teflon, at least it doesn't produce elemental fluorine gas (the toxicity of which is so high that the concentration to detect by smell is 1/5 the lethal con.). However there are a plethora of other byproducts---see for example this site:

http://tuberose.com/Teflon.html

Clearly one should not push the no-meltback-carbonization issue when soldering!

 

[mr coffee]

Thanks for info.

 

[Emperor-TK]

Fluorochemistry is some scary stuff. The flourine ion is enormously reactive. I was once using some concentrated HF to etch some silicon wafers for a research project when the syringe I was using to neutralize the acid popped and a drop of HF of unknown concentration flew into my eye (right under my safety glasses). 20 minutes later, while under the eyewash station, I was happy to relize I didn't crap my pants in my panic. I have since refused to use HF on at least one other occasion. The fun thing about HF is how it can attack your body. HF can diffuse right through your flesh and into your bones, where the flourine ion reacts with the calcium in your bones or blood, which plays havoc with the calcium/potasium transport mechanism in cellular respiration. Killing you from the inside out over an extended period of time. Fun, huh? :shock: Concentrated HF on only 3% of you body surface is lethal IIRC.