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Are 49 tubes useful for anything?

currituckco

currituckco

Super Member
I got a couple NOS 49 tubes from a yard sale. One is Sylvania and the other is National. Are these useful for anything? Sadly the dynaco branded mullard EL34, tung sol 6SN7, beige base raytheon 6sn7 and two grey glass RCA 6SN7s all were bad! :tears: Luckily they were only a buck each. The GE 6L6 works, and an RCA 12SN7 tests good, as well as a military 12AU7. Haven't tested the pretty silvertone globe 280. Super bummed on the 6SN7s and the EL34 though. But I digress.

According to NJIP the 49 is a dual grid power tube, single ended it only yields .17 watts. Push pull it says they can put out 3.5 watts...but I only have two. Could I build a PP guitar amp with these? A headphone amp? What would be something useful I could do with them?

http://www.nj7p.org/Tube4.php?tube=49

Thanks all!
 
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I was thinking that - but don't really know enough to design it without help. I didn't know if these guys would have the right characteristics. Would they work?
 
It could be extremely simple. If I were doing it, I would use the parameters shown on the datasheet, with 155 volts on the plates and a 3333 ohm (or approximately equal) cathode resistor to create a 20 volt cathode voltage (aka cathode bias the thing).
What I would look into before doing anything with these is why the datasheet shows the class B PP amplifiers having the grid at 0 volts, and the current at 4 ma, whereas the class A SE type shown on the datasheet shows the grid at -20, current at 6 ma... it's like one expects to be cathode biased and the other expects to have the cathode at 0 volts. Or maybe they expect you to look on the loadlines and find the proper grid/cathode voltage for the specified current, and the -20 grid volts shown on the class A amp is just... there for no reason, I guess?
I would certainly find grid curves, and better datasheets with stuff like maximum dissipation on them.

Things to keep in mind-- they're triodes, so they won't mind higher loads than expected, within reason. You could be able to use different taps for different ranges of headphones (8, 60, 120, 300, 600 ohms?) which you might need a specialized xformer for...
Then, you would have to either just not have it foolproof and assume the user is smart enough to plug headphones in before switching it on, OR some system in which you have switching jacks with their grounds all in a row and a resistor between the top one of them and the tap that properly utilizes that resistor, so that if you plug headphones into any one of the taps, you break that resistor's connection to ground and replace it with the headphone's load. I'll try to draw up a schematic later, or just a drawing, the idea is quite simple but I'm doing a terrible job of explaining it.

One idea I've had to utilize a higher powered amp as a headphone amp is basically this:
8 ohm load resistor in parallel with a 16 ohm resistor, in series with headphones.
Worst case, no headphones connected, amp sees 8 ohms, OR short circuit on the headphone jack, amp sees 8 in parallel with 16 ohms, which is darn close to what it needs, certainly close enough to make it not blow up. This also divides the power of a large amp for headphones, but since you have a tiny amplifier here you need to match the impedance of headphones, more or less a nightmare with all the varieties of them out there.
 
Lines of constant current have nice equal grid voltage spacings wired in triode, and gain is low (about 4 to 5 V/V), so you could make a nice CCS loaded linestage (like the 12B4 threads). Take mu out for low impedance to drive long cables or possibly headphones...I didn't see any plate dissipation specs so it might not be good to run them too hot. Hopefully they aren't microphonic as well...

Some plate curves: http://www.mif.pg.gda.pl/homepages/frank/sheets/049/4/49.pdf
 
One other project that could be built with the 49 is the "hiker's radio"... the circuit was published in the '30s for a radio that needed only 6V B+ supply, by wiring the '49 tube in "space charge" mode - 1st grid biased positive. There have been one and two tube versions published - here's a recently built example: http://www.sparkbench.com/hikers/hikers2.html
 
.12 amps@2 volts will run for hours off of modern rechargeable AA batteries. It could be interesting to see a battery powered radio with the things...
But I would still love to see a headphone amp with those, just because it apparently hasn't been done before-- and headphone amps are cool.
 
jrsh92 said:
.12 amps@2 volts will run for hours off of modern rechargeable AA batteries. It could be interesting to see a battery powered radio with the things...
But I would still love to see a headphone amp with those, just because it apparently hasn't been done before-- and headphone amps are cool.
Click to expand...

I would love to get a headphone amp going with these things...if someone could help me draw up a schematic I would gladly be the guinea pig.

I'd be thinking about possibly getting some grado sr80s or something, which are 32ohm impedance....
 
I'd highly recommend the Sennheiser HD-555's, wonderful headphones, and extremely comfortable too. As long as you don't plan on going running with them (the 10 foot cord will get tangled in everything and they'll fall off your head) I think you'll like them, I love my set. I haven't compared the sound to the Grados, or pretty much any other high-end headphone for that matter, but compared to cheap earbuds and almost any mid-range speakers (we're talking about real stereos here, not cheap boom boxes, though I haven't compared them to Klipschorns or anything) they kick ass. I'd try them out if you can.

I'd like to try to draw up a schematic, just for the hell of it. I might get stuff wrong, I'd have a qualified circuitdesignerperson look over it (there are plenty of them here) but I'd like to try my hand at actually making functional designs.
What would you be using for preamp tubes? Are the grids of those sensitive to drive from a line level source?
I probably need current graphs to really have an idea of this stuff, but I'll take a stab at making something functional.
 
No idea about the preamp tubes. So are most headphone amps run straight from a source? I guess I'd pictured it coming after the preamp, or maybe run off the tape outs. I'll certainly look into the sennhieser. They have a very different nominal impedence (120 ohm) than the grados so i'd have to decide before i build the amp!

But that would be awesome if you wanted to take a stab at putting together a design. It'll be a little while before I could start building, but that would give the peanut gallery time to chime in with additions, subtractions, suggestions and so forth.

So are most headphone amps output transformerless? Most tubed ones seem to use preamp tubes rather than power tubes, right? but these have such a low power output maybe they'll be good. who knows? not me! but i'll be stoked to learn!

also jrsh92, as much description of your thought processes when designing would be really helpful to me. thanks so much!
 
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