thermistor on heater lines.. Question

[iccaros]

So with my tube pedals, and my amps, I have been reading Merlin's page. He suggest a Termistor on the heater lines to extent tube life by reducing in rush current. But what value would be a good value , I have used thermistors to control when fans come on, but not to limit current so have no ideal, tried Google and search here, and ax84 but no real answers on which size, as it is a resistor I do not want to divide my 6.3 heater lines by more than .6 volts..

Thanks

[R.G.]

So with my tube pedals, and my amps, I have been reading Merlin's page. He suggest a Termistor on the heater lines to extent tube life by reducing in rush current. But what value would be a good value , I have used thermistors to control when fans come on, but not to limit current so have no ideal, tried Google and search here, and ax84 but no real answers on which size, as it is a resistor I do not want to divide my 6.3 heater lines by more than .6 volts..

I put that into the production Workhorse amplifiers. It works. But it's tricky to design.

The way to determine the thermistor is to (1) measure or calculate the normal operating current of your heaters. Let's say it's 5A for example. (2) decide how much voltage drop you can stand. You've already picked 0.6V. (3) calculate the hot resistance of the thermistor; in the example it's 0.6V/5A = 0.12 ohms.
(4) go find a thermistor that does that - has a hot resistance of 0.12 ohms and is rated for 5A or more.

The cold resistance is less critical. You pick this to be the value that limits your inrush. The heaters will be about 1/10 their normal resistance when cold, so they'll be 6.3V/5A = 1.26 ohms hot and about 0.13 ohms cold. You can then pick an cold resistance that's between 1 ohms and maybe 5 ohms to limit starting current to 1X to 0.2X the normal operating current. Notice that the bigger the cold resistance, the longer the amp will take to come on. The higher the cold resistance the longer the amp will take to heat the heaters. This can be a couple of minutes if you make a bad guess...  Ask me know I know this. 

It may be tough finding a thermistor with such a small range of cold to hot resistances.

Finally, you have to worry about static DC power losses. The thermistor works by self heating. It has to stay hot to work in normal operation. So it has to heat up to 150-250C. That's HOT. You have to pick the size of the thermistor's static dissipation so it stays hot enough to work, but cold enough not to melt the solder off its leads and/or char the PCB or other parts around it. The thermistor data sheet will list the maximum DC power dissipation. In this case, the maximum dissipation is your 0.6V number times the current, 5A in the example, so the actual dissipation is 5*0.6 = 3W. You have to pick a thermistor that dissipates at least 3W, and probably with a specified max dissipation of 4-5W to keep it from overheating in application.

[iccaros]

I got it, I think  , Now to the bench to test..

Thanks

[PRR]

> extent tube life by reducing in rush current.

How many failed tube heaters have you seen?

Not counting tube TVs (special case), I only recall one, and that one had been thrown 8 feet into a dumpster.

[iccaros]

> extent tube life by reducing in rush current.

How many failed tube heaters have you seen?

Not counting tube TVs (special case), I only recall one, and that one had been thrown 8 feet into a dumpster.

none, but
Funny you should say TV, as the  tube I am using right now is from a TV, a 6AF11 that I am using in this new amp build.  a Champ style with a kick

[R.G.]

How many failed tube heaters have you seen?
Not counting tube TVs (special case), I only recall one, and that one had been thrown 8 feet into a dumpster.

Actually, I've seen a reasonable number. They do wear out, although "modern" usage of preamp tubes does cause a lot of mortality from other causes first. Admittedly, this comes more vicariously from my amp-tech friend than personal experience.  A preamp tube if treated properly will last about 40,000 hours according to one analysis I remember. As much as anything else, they don't burn out, they snap from thermal cycling, and the newer the tube the less care that was taken with the mountings and the more susceptible they are to thermal cycling.

$1 spent on a thermistor is cheap insurance if it saves even one tube.

I also like thermistors on AC power inputs. The less brutal the inrush, the less stress in everything in the power path. Again, not a huge thing, but the insurance is very cheap.

And I've never seen a tube heater fail in an amp with a heater thermistor.   

In generalizing like I'm prone to do, here are some things I posted in a more amp-oriented forum:

1. One fuse per winding on the power transformer. The AC power inlet fuse is there to prevent fires. It's a happy accident if it protects the power transformer. Put a fuse in the heater winding(s) and one in each side of the centertapped high voltage if so provided.
2. Install silicon safety diodes in series with a tube rectifer, one in series with each anode. Then if the [rectifier] tube shorts, the silicon picks up the slack and does not let your power transformer and filter caps die.
3. Put bleeder resistors across the first filter cap. This ensures that it will bleed down in some small time to save YOU from shocks when you service it. Pick how much current you can allocate to them, or how much power you can waste, then figure the resistors. For instance, if you have a 400V supply that goes to 500V if there are no tubes in, and you don't want to waste more than half a watt, calculate P = Vsquared/R = 0.5 and get R = 500K, and rated at 1W. A 470K 1W would do, as would two 220K half-watts in series, or two 1M half watts in parallel.
4. Build it with a 3-wire AC cord and safety grounded chassis. There is no two-wire magic.

That got the following good ideas in response:

Another thing I would say is a must is that the IEC pin terminals should be shrink wrapped, this is the only place I have ever been shocked. Amp turned off forgetting that its plugged into the wall.

That was a comment that assumed that an amp builder would use an IEC inlet receptacle, which is a Good Idea, especially the ones with the fuse in the little tray in the receptacle.
Here's another that could save some pain if not the life of a DIY amp builder:

As an embellishment, how about soldering on one of the neon power indicators to the back of the IEC so it's always lit when there's power coming in? I put LEDs in series with the bleeder resistors on the Workhorse line as an indicator that there was voltage on the filter caps and as a debugging aid. No light, voltage stopped before here.

One more thing to add. The light-bulb limiter. Build one and always do your initial power up with it. It will save you a lot of agony later.

What about a varistor (or 3) across the PT primary? - I seem to recall that in one of your articles from somewhere.

How about a 220R resistor ... across the secondary leads. Shouldn't effect impedance notably but probably enough load to save the OT from an open load spike.

I've been told by safety certification test engineers that bolting safety ground to the chassis by a transformer bolt was once considered passing, and no longer is; the reasoning was precisely that the mechanical loading could loosen the nut, and that replacement processes invited leaving it out or making the connection incorrectly during service. There was another issue, that being that all grounding with a transformer mounting bolt uniformly connected the safety tag above the transformer leg, not directly on the chassis. The thinking was that the whole point of safety ground was connecting to chassis, and the transformer leg may or may not make good connection to chassis. But I suspect that each safety lab may interpret it themselves. The practice of using a dedicated bolt, used for nothing else, with toothed washers between the ground terminal and chassis is open to less question about integrity. The guy I talked to said he'd have failed my unit if I'd put it on the transformer mounting bolt.

Good one, Jon. This is one that could save someone's life.

I think that the original 2-wire setup had the switch in one side and the fuse in the other, because that makes wiring the power line in easy. There is a terminal on the switch and one on the fuse where one of each line cord wires go. Converting this to 3-wire requires finding somewhere else to put the neutral wire, which now can't have the fuse in line.

A good solution to this impasse is to change out the power switch for a double-pole switch instead of a single pole. The double pole switch gives you a terminal for each side of the AC line, and you now have a terminal to run all the wires as well as breaking both sides of the AC line. This is a good companion to wiring a neon indicator on the inside of the chassis so you always know there is power inside the box when there is.

It occurs to me that I should make it really clear: any suggested circuit modifications I present are presented solely for the purpose of technical performance issues; I make no representations that they will make an amp safer nor compliant to any standard whatsoever, nor that they are free of inaccuracy or omission.

Presented as an example of a BAD idea:

Another one I've seen are people who are cloning a vintage Marshall as per the schematic and they actually install the Polarity switch with the death cap...all because it is on the original schematic...and a 3 prong power cord. Not a common thing but I have in fact seen that.

An amp will work with none of these; but a DIYer can produce a much safer amp for them to tinker inside later and to use by considering those ideas.

[iccaros]

Good reminder R.G.
I have worked thought most of those suggestions, I like the bleeder on the on the caps and have added a LED to let me know if there is voltage there or not. The power switch I bought has two neon bulds, one red when power is plugged in but the switch is off, the other green when the switch is on, nice way to remind you that the amp is plugged in.

I am looking at the size fuse needed on my heaters, one tube pulls 1.2 amps the other 150 ma