Hi ThSpeakerDude88,
You really should read up a little on tubes. It'll help you and maybe give you some ideas. Also keep you out of trouble.
A tube will conduct a high amount of current without any negative bias voltage on the grid with respect to the cathode. The negative grid will reduce the cathode current. This is what gives you control of the electron stream and enables amplification. Without a reduced current, the maximum dissipation (heat) for the plate will be exceeded. This will shorten the life of the tube, in some cases to seconds. Not pretty.
In your circuit, the resistor causes a voltage drop. The cathode ends up at some positive voltage. The grid is referenced to ground and so is negative with respect to the cathode.
-Chris
You really should read up a little on tubes. It'll help you and maybe give you some ideas. Also keep you out of trouble.
A tube will conduct a high amount of current without any negative bias voltage on the grid with respect to the cathode. The negative grid will reduce the cathode current. This is what gives you control of the electron stream and enables amplification. Without a reduced current, the maximum dissipation (heat) for the plate will be exceeded. This will shorten the life of the tube, in some cases to seconds. Not pretty.
In your circuit, the resistor causes a voltage drop. The cathode ends up at some positive voltage. The grid is referenced to ground and so is negative with respect to the cathode.
-Chris
The short, quick answer to the negative bias questionis, it keeps your tube from melting. If you had a power supply that was capable of puting out jome serious juice, you would notice that your tube would get hotter as you reduced the resistor value. Your transformer feeding transformer power supply is probably limiting your ability to melt the tube, nevertheless you could be hurting it.
It is normal for the cathode (the rod shaped element that runs through the center of the tube) to glow red. If you see any red glow on the plate of the tube (the outer black or grey cylinder) you are pushing it too hard and it will die soon. Dial in some more resistance. A 50K pot is too big to set in the 100 ohm range, you are using about .2% of its resistance and it will be unstable, and possibly fry. Either use a regular resistor, or find a 500 ohm pot.
The 35C5 is a slightly different tube than a 50C5. It can handle less power. Of any of the tubes that could be found in an old radio, the 35C5 would be my last choice for a preamp, but It could be done. I think that distortion would be a problem.
It is normal for the cathode (the rod shaped element that runs through the center of the tube) to glow red. If you see any red glow on the plate of the tube (the outer black or grey cylinder) you are pushing it too hard and it will die soon. Dial in some more resistance. A 50K pot is too big to set in the 100 ohm range, you are using about .2% of its resistance and it will be unstable, and possibly fry. Either use a regular resistor, or find a 500 ohm pot.
The 35C5 is a slightly different tube than a 50C5. It can handle less power. Of any of the tubes that could be found in an old radio, the 35C5 would be my last choice for a preamp, but It could be done. I think that distortion would be a problem.
ah, ok thanks. This helps much! 
I think I understand much more now.
I have composed a circuit from the RCA schematic of the 42 to run my 42.
Tell me if I did this right....
What should I get the voltage to on that resistor, and what should the value of the cap be?
I think I understand much more now.
I have composed a circuit from the RCA schematic of the 42 to run my 42.
Tell me if I did this right....
An externally hosted image should be here but it was not working when we last tested it.
What should I get the voltage to on that resistor, and what should the value of the cap be?
tubelab, I'm sorry, as you were posting I was too! so I did not see your last post.
As I gathered from anatech's post I assumed my power supply is not capable of supplying enough current to melt the tube, ( I know because I "skin tested it")
I only have about 16 watts of power going into the circuit. I will see if for testing purposes I can find a larger pot of smaller value, it would be handy to have around at least. I believe that the reason my resistor there isn't heating up is because I am not supplying enough current to do so...that would explain it all. So as long as I keep the current low for now I can use lesser parts, but I'll end up finding proper ones for saftey purposes Thanks for the explination! wow I would be in trouble if I didn't know that and used too much current 
I also know that if the plates glow you have problems and no mine wasn't glowing, I don't have enough current.
As I gathered from anatech's post I assumed my power supply is not capable of supplying enough current to melt the tube, ( I know because I "skin tested it")
I only have about 16 watts of power going into the circuit. I will see if for testing purposes I can find a larger pot of smaller value, it would be handy to have around at least. I believe that the reason my resistor there isn't heating up is because I am not supplying enough current to do so...that would explain it all. So as long as I keep the current low for now I can use lesser parts, but I'll end up finding proper ones for saftey purposes
Thanks for the explination! wow I would be in trouble if I didn't know that and used too much current I also know that if the plates glow you have problems
and no mine wasn't glowing, I don't have enough current.
Hi ThSpeakerDude88,
I looked up the 42 and got 34 mA plate, 6.5 mA screen at 250V.
Anyway, you would use a 390R to 430R 2W resistor (actual 0.7W) for a -16.5 V bias on the grid. It's approximate to begin with. You could always look at a schematic for an old radio that uses that tube. What you have is close enough.
Oh, your supply. "It hasn't got enough power to hurt itself". That's a good thing for experimenting,
-Chris
I looked up the 42 and got 34 mA plate, 6.5 mA screen at 250V.
Anyway, you would use a 390R to 430R 2W resistor (actual 0.7W) for a -16.5 V bias on the grid. It's approximate to begin with. You could always look at a schematic for an old radio that uses that tube. What you have is close enough.
Oh, your supply. "It hasn't got enough power to hurt itself". That's a good thing for experimenting,
-Chris
I look at it this way, If you are careful not to get hurt, and you don't set your house on fire, and your parts are cheap or free, who cares if you melt a few parts. It is all a good learning experience. It is good to have a forum such as this to get advice, it would have saved me a lot of time years ago (before home computers were even thought of), but the ultimate teacher is experience.
In all tube amplifiers there must be a path from the control grid to ground (or the bias supply). This is the 500K (or 330K) resistor is for. It is missing in your 42 schematic. In your 50C5 schematic the volume control is that path. You must have the resistor, or the volume control, or severe distortion and possibly no sound will result.
A lot of my experiments venture into uncharted teritory, so I still blow up parts. When I am not sure of the outcome, I use "sacrificial" parts and tubes, so that I am not too upset when things fry. In another therad I "tested" some 6AV5 tubes to find out if they sounded good for audio, and what their true limits are. Here you will see the plates glowing.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=37403&highlight=
In my earlier days of tube experimenting, I obtained parts from old radios, and old tube TV sets. These used to be common in the neighborhood trash, but are not too common any more. It is still possible to find surplus tube type test equipment at flea markets and on Ebay. Often this is a good source of transformers and tubes. The transformers from this stuff can put out lethal amounts of electricity, you can't afford to "skin test it".
It would be a good idea to read the safety section of this forum, adt the safety related sections on my web site. There are some good ideas on how to set up your work area in a safe efficient manner. After all it is YOUR A** and you only have one!
http://www.tubelab.com/Safety.htm
In all tube amplifiers there must be a path from the control grid to ground (or the bias supply). This is the 500K (or 330K) resistor is for. It is missing in your 42 schematic. In your 50C5 schematic the volume control is that path. You must have the resistor, or the volume control, or severe distortion and possibly no sound will result.
A lot of my experiments venture into uncharted teritory, so I still blow up parts. When I am not sure of the outcome, I use "sacrificial" parts and tubes, so that I am not too upset when things fry. In another therad I "tested" some 6AV5 tubes to find out if they sounded good for audio, and what their true limits are. Here you will see the plates glowing.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=37403&highlight=
In my earlier days of tube experimenting, I obtained parts from old radios, and old tube TV sets. These used to be common in the neighborhood trash, but are not too common any more. It is still possible to find surplus tube type test equipment at flea markets and on Ebay. Often this is a good source of transformers and tubes. The transformers from this stuff can put out lethal amounts of electricity, you can't afford to "skin test it".
It would be a good idea to read the safety section of this forum, adt the safety related sections on my web site. There are some good ideas on how to set up your work area in a safe efficient manner. After all it is YOUR A** and you only have one!
http://www.tubelab.com/Safety.htm
Oh yeah, missing grid resistor. Missed it, I thought you were going the same way as before in the earlier schematic.
As tubelab posted, the best way to learn is by making the odd mistake. We were forced to do it that way before computers existed at home. There's nothing like rapidly burning resistors as things go south. You'll remember that for sure. Exploding caps remain etched on my mind.
-Chris
As tubelab posted, the best way to learn is by making the odd mistake. We were forced to do it that way before computers existed at home. There's nothing like rapidly burning resistors as things go south. You'll remember that for sure. Exploding caps remain etched on my mind.
-Chris
hahahaha! Hey someone told me once that the stuff in caps gives off a nice nerve gass when they blow up 
Anyways, thats about how I have learned what I know in electronics- trial and error, and making mistakes. I guess if we didn't break stuff, then we wouldn't learn what not to do, or why we don't do it, and we wouldn't be as careful or would never get anywhere.
I made sure I made a power supply with only enough current for the tube, so I didnt hurt something or someone. I do have a 13vac 5amp transformer that will light up a 60 watt light bulb very bright when stepped up, but I didn't want that much current incase something went wrong
Thanks for that bit of information on the resistor between in and ground, I read about what thats for but I forgot to put it in the schematic. Sorry about that. I got the current wrong on the tube, that was for running it at 317v I think.
Most of my parts come from old radios and stuff too, although I don't have a very big collection. I had a '67 RCA clock radio I tore apart cuz it was uglier than sin, and I got a useable transformer out of it, some caps, resistors, and of cource tubes.
I'm currently in the process of restoring a 1936 RCA 5T, I'm sure you have seen my post on it as well. Thats what the 42 came out of, I am going to end up replacing all the tubes when I am done.
Most of my other parts ( resistors, caps, pots, diodes, some regulators transistors and other chips, etc) came out of tons of old computer power supplys, amplifiers, radios, tv's, tape decks, etc.. So I have a good enough collection of that stuff. I still need to find some cheap tube stuff, I haven't been able to find anything I can really afford on ebay yet, and when I can , it always goes up
I'd really like to restore a pair of old 20 watt or so tube mono blocks from the 40's-50's, but can't really pay high prices for them. Meh- I need a job -_-
Gotta get my car on the road first, you know, gas tank, tires, brakes, exhaust... Then I can spend some money on that other stuff I want
Sorry, so off topic.
Anyways, I will hook it up this afternoon with the values we discussed. Do I need anything to drive this tube, such as a 12AT7 ?
Anyways, thats about how I have learned what I know in electronics- trial and error, and making mistakes. I guess if we didn't break stuff, then we wouldn't learn what not to do, or why we don't do it, and we wouldn't be as careful or would never get anywhere.
I made sure I made a power supply with only enough current for the tube, so I didnt hurt something or someone. I do have a 13vac 5amp transformer that will light up a 60 watt light bulb very bright when stepped up, but I didn't want that much current incase something went wrong
Thanks for that bit of information on the resistor between in and ground, I read about what thats for but I forgot to put it in the schematic. Sorry about that. I got the current wrong on the tube, that was for running it at 317v I think.
Most of my parts come from old radios and stuff too, although I don't have a very big collection. I had a '67 RCA clock radio I tore apart cuz it was uglier than sin, and I got a useable transformer out of it, some caps, resistors, and of cource tubes.
I'm currently in the process of restoring a 1936 RCA 5T, I'm sure you have seen my post on it as well. Thats what the 42 came out of, I am going to end up replacing all the tubes when I am done.
Most of my other parts ( resistors, caps, pots, diodes, some regulators transistors and other chips, etc) came out of tons of old computer power supplys, amplifiers, radios, tv's, tape decks, etc.. So I have a good enough collection of that stuff. I still need to find some cheap tube stuff, I haven't been able to find anything I can really afford on ebay yet, and when I can , it always goes up
I'd really like to restore a pair of old 20 watt or so tube mono blocks from the 40's-50's, but can't really pay high prices for them. Meh- I need a job -_-
Gotta get my car on the road first, you know, gas tank, tires, brakes, exhaust... Then I can spend some money on that other stuff I want
Sorry, so off topic.
Anyways, I will hook it up this afternoon with the values we discussed. Do I need anything to drive this tube, such as a 12AT7 ?
"What value and wattage should my bleeder resistors be on my filter caps?"
The usual rule is: 100 ohms per volt. That way, they'll pull 10mA. You can figure the power from either: W= I^2 X R or V^2 / R. You can make them larger, if necessary. Bleeders can help with voltage regulation, however, the main purpose is to allow the filter caps to discharge when turned off. Good for safety, yannow.
The usual rule is: 100 ohms per volt. That way, they'll pull 10mA. You can figure the power from either: W= I^2 X R or V^2 / R. You can make them larger, if necessary. Bleeders can help with voltage regulation, however, the main purpose is to allow the filter caps to discharge when turned off. Good for safety, yannow.
I see what you mean , I might be robbing current with such low currents to begin with.
I checked the radio schematic for the 42, its cathode is grounded, no capacitor and resistor. Is this because the regulated supply (80 rectifier) isn't enough to cause problems? Also, If I plan to use the 50c5 or 35c5 with the proper current regulation , can I run it grounded as well or will my power supply overheat?
I would also like to know if it is possible to rewind a wall-wart to get 150vac @ 200ma on the output. I have several wall warts opened up and the secondaries removed so they can be rewound for other things already, and was wondering if I could do this with them. I know their primary consumption (amperage or watts)
I checked the radio schematic for the 42, its cathode is grounded, no capacitor and resistor. Is this because the regulated supply (80 rectifier) isn't enough to cause problems? Also, If I plan to use the 50c5 or 35c5 with the proper current regulation , can I run it grounded as well or will my power supply overheat?
I would also like to know if it is possible to rewind a wall-wart to get 150vac @ 200ma on the output. I have several wall warts opened up and the secondaries removed so they can be rewound for other things already, and was wondering if I could do this with them. I know their primary consumption (amperage or watts)
If you check that schematic again, you will find that the 42 was fixed biased from a negative voltage source from somewhere. You can't run a 50C5 with zero bias, as it will go into hard saturation and probably burn out. Still, it won't amplify at all even if it doesn't. For such a simple project, stick with cathode bias: it's easy, and self correcting for component errors (and usually design errors as well
At least you won't lose the tube if you fouled up the design work).Sure, if the new secondary will fit in the core window.
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