An amp I'm building has six tubes that can only use 6.3V (total load 1.55A), and one big guy that can use either 6.3V@1.6A or 12.6V@0.8A.
The original tubes on the PT's 6.3V secondary drew 2.7A. I believe that was marginal to begin with, because my 15% higher load is bogging it way down to around 4.5V.
I've got a 12.6VCT@2A filament trafo I can use to supplement or replace. I see three ways I can use this.
Option 1 seems like it would provide the cleanest power, running both secondaries well below spec. Am I missing something?
The original tubes on the PT's 6.3V secondary drew 2.7A. I believe that was marginal to begin with, because my 15% higher load is bogging it way down to around 4.5V.
I've got a 12.6VCT@2A filament trafo I can use to supplement or replace. I see three ways I can use this.
- Power the big guy at 12.6V and the rest from the PT. That puts the 12.6V trafo at 40% of spec, and the PT secondary at 58% of the original load.
- Power all the tubes from the add-in trafo, putting the 6.3V tubes across the center tap and one leg (I can divide the load 0.75A/0.8A), and the high-draw tube across the two legs. That puts the trafo at about 75% of spec.
- Power all the tubes from the add-in trafo, putting the 6.3V tubes in series pairs. Problem here is that I've got two good balances (0.15+0.15, 0.3+0.3), and one awful one (0.2+0.45, which would split the voltage 3.8+8.8). This is probably an exceedingly bad idea.
Option 1 seems like it would provide the cleanest power, running both secondaries well below spec. Am I missing something?
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Agree, this is GROSS:
Only 15% extra would not bog voltage down so much so it was already abused, option 1 seems safest.
Okay, so to my embarrassment, what we have here is a combination of dumb choices on my part, and unexpected Heisenberg effects (i.e., taking a measurement of something changing that very something).
First off, I was stupidly prototyping this with cheap 24 AWG test leads. That's where the biggest voltage drop came from - that wire just isn't cut out for that kind of current. Wire-nutting 20 AWG leads in their place was like magic - came right up to 6.289V, with all the tubes (even the big guy) running. And then I also remembered that I had been using my Fluke 45 in line as an ammeter, so I hooked that back in ... and the voltage dropped to 5.6V. Pull it out, back to 6.3V. Repeatably. Ugh.
So two lessons learned: Use the right gauge of test lead for the current, and beware the impact of measuring devices at low voltage/high current.
As for being over spec, when I ran with just the 6.3V tubes in place, it was only drawing about 1.25A, which is 300mA under the tube specs. So I'm closer than I thought.
Thanks for the advice!
First off, I was stupidly prototyping this with cheap 24 AWG test leads. That's where the biggest voltage drop came from - that wire just isn't cut out for that kind of current. Wire-nutting 20 AWG leads in their place was like magic - came right up to 6.289V, with all the tubes (even the big guy) running. And then I also remembered that I had been using my Fluke 45 in line as an ammeter, so I hooked that back in ... and the voltage dropped to 5.6V. Pull it out, back to 6.3V. Repeatably. Ugh.
So two lessons learned: Use the right gauge of test lead for the current, and beware the impact of measuring devices at low voltage/high current.
As for being over spec, when I ran with just the 6.3V tubes in place, it was only drawing about 1.25A, which is 300mA under the tube specs. So I'm closer than I thought.
Thanks for the advice!
did a cctv project many years ago and needed long wires for the cameras.
rather than just take any wire i bought in some small reels and did some resistance measurements.
some wires were much lower impedance than others even for the same wire thickness.
Clearly wire quality matters too.
rather than just take any wire i bought in some small reels and did some resistance measurements.
some wires were much lower impedance than others even for the same wire thickness.
Clearly wire quality matters too.
> prototyping this with cheap 24 AWG
Apropos of that thought: I just checked my four fire extinguishers and replaced two which had been recalled.(*)
Yes, I have burned #24 clip-leads. At night. In the car.
(*) Kidde seems to have several/many recalled models; First Alert has had no product recalls in the past decade.
Apropos of that thought: I just checked my four fire extinguishers and replaced two which had been recalled.(*)
Yes, I have burned #24 clip-leads. At night. In the car.
(*) Kidde seems to have several/many recalled models; First Alert has had no product recalls in the past decade.
Best regards!
I've got one literally right next to my workspace, in a place of honor. I subscribe to Alton Brown's theory that a fire extinguisher is the only unitasker allowed. (Then again, I'm not sure he ever used a Jokari wire stripper ...)
I should check for a recall. It's a Kidde.
Funny you should mention that, I brainfarted a pair of these into a 120VAC dead short just the other day. To their credit, there was no flame! But the smoke was impressive, as was their ability to melt the insulation on other nearby leads at the same time.
That actually brings up a quasi-related question. Because my line voltage varies from 121-127, I use a variac to feed 120 to the DUT. But if I put my series lightbulb current limiter in between, it acts like an open circuit. Zero volts, even when I short it.
It works fine plugged into the wall, and if I put it before the variac, but not after. Others don't seem to have this problem. What gives?
(Question is related because I'm pretty sure that would have kept those leads from smoking ...)