power supply question

[John]

Hi, on the attached schematic (Heathkit cap checker) it appears to me that they're using the 1626 triode as a rectifying diode, since the plate is connected to the grid. Is that correct? And if so, could I sub a SS diode in it's place?


I'm sure I'm missing something obvious and possibly ticklish, which is why I didn't just try it and see. Thanks!

[sluckey]

Is that correct? And if so, could I sub a SS diode in it's place?

Yes. Voltage will increase though. Don't know if that would be critical to the rest of the circuit.

[John]

I'll be durned. I finally figured something out.


If voltage goes up too much, I'll try a smaller dropping resistor across the top filter cap. It's a low current output, so without doing the math maybe a 39K may end up being right.


Now off to my Basic Scope Training Class. 

[HotBluePlates]

The 1626 is kinda-like a 6L6 with a top-cap. (EDIT: Oops, was looking at the 1625 on the same data sheet; 1626 is a triode)  It's also a $12 tube that will probably last the life of the cap checker. If you already have this cap-checker in hand, is it worth replacing the tube?

[John]

I'm bidding on one from eBay for cheap. If that falls through, I figure I'll try out the diode for yucks. I haven't tested it yet. Just arrived yesterday with the 1626 in fragments because the PT wasn't bolted down, so guess what happened in shipping. I'm amazed the 1629 was unscathed. The seller gave me a refund, but it really makes me wonder how it powered up for the picture (with tube glowing) since the filter caps use the PT bolt as ground.


Before anyone asks, I got a  cap checker because I think they're cool, and I love old Heathkit stuff. It's a addiction hobby, that's all.

[eleventeen]

Hold off buying any more for a day or two, if you can. I think I may have a few NOS ones you can have. In storage.

[jjasilli]

My suggestion is to track down he Heathkit manual if you can find one.  I have a nagging feeling that the triode-wired-as-diode is done for a hidden, clever reason.  Perhaps the characteristics of the tube, as diode, particularly the grid-plate impedance, may enable the test circuit to function.    OTOH, probably the worst that may happen -- with a SS diode and fixed resistor in lieu of the tube -- is that the circuit becomes inaccurate as a measuring device.


EDIT: Then again, maybe that triode was used simply because it was cheap & plentiful in the late 40's & aerly 50's:


http://www.ohio.edu/people/postr/bapix/capchkr2.htm
http://www.ohio.edu/people/postr/bapix/OldHeath.htm
http://www.antiqueradios.com/forums/viewtopic.php?p=930689

[sluckey]

I have a nagging feeling that the triode-wired-as-diode is done for a hidden, clever reason.

Me too.

[HotBluePlates]

It's like half of a half-wave voltage doubler. A "quarter-wave doubler"? 

500vac in, but 150vdc + 410vdc = 560vdc total output. Probably winds up with that output due to the fact it's shut off half the time, and charges 2 series caps when the 1626 is passing current. The caps should probably be rated over 700vdc each, but I have no idea if they did that.

I have a nagging feeling that the triode-wired-as-diode is done for a hidden, clever reason.

Only one thing comes to mind; maybe PRR knows something extra.

The 1626 appears to be used as a diode. The grid could be tied to 0v, but the 0v gridline will act like a high-resistance diode (a high internal plate resistance).

The grid is tied to the plate, so when the "diode" is conducting the grid swings to the same high positive voltage on the plate. The tube should act like a diode with a much lower resistance; see any triode that has positive grid voltages shown on the characteristic curves.

I don't know why the choice to use a triode instead of a single diode. Maybe an intermediate value of resistance was needed: higher than a rectifier diode but lower then a triode.

[John]

11teen, thank you, much appreciated. I'll be happy to pay you though. I already get tons of free advice and info here. 


Jjassili, I downloaded the manual a few days ago. It describes it as  "half wave rectified " with no other comments. From visiting some antique radio forums, I gather that at the time, the 1626 tubes were almost free. Tons of military surplus, so maybe they were cheaper than diode tubes then. The plate current is only 25 mA normal operation, which maybe is a safety feature given the habit of guys like me to touch the wrong wire.


HBP, the 1626 doesn't have the top cap, maybe you're thinking of the '25? the '26 is called a transmitting triode, the '25 a transmitting beam power amplifier (whatever that means lol)


Anywho, before I do anything gotta check and make sure the tranny is good. Thanks all for your replies!

[PRR]

1626 is a 5 Watt triode; not 19W like a 6L6.

12V 0.25A heater is low current and also an unpopular voltage (at the time).

I suspect it was just CHEAP.

Many early radios used the most-common (cheapest) triodes for various diode/rectifier duties, including small power supplies. Yes, grid usually tied to plate.

Personally I think you could build (or heavily-mod) a better cap-check than these. But I guess if you like the old stuff, it is probably worth putting in a 1626 instead of a 1N4007.

[John]

PRR, thanks for weighing in. I did get the tube for $5 shipped, so I'll pop that in. Probably will put diode in series for protection anyway. I'll be replacing all the caps and any out of whack resistors. Any other useful mods you know of, I'd be mighty interested. Thank you!