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ThSpeakerDude88 17th November 2005 12:55 AM

quick 50c5 to play around with
I found this schematic online here, and I have a 50c5 and a few other tubes laying around.

does this look right? I'm not sure this is the whole circuit.... Anyways Id like to throw one together on my protoboard tonight or tomarrow. I'm gonna use a 12vac wall wart and run it through a 12 volt transformer to step up the voltage again. Tranny doesnt have to be too big as this tube only requiers a few ma of current I assume.

I also came across this one

I have all these tubes laying around, except for the 35w4, I am going to be using diodes instead.

anatech 17th November 2005 01:05 AM

Hi ThSpeakerDude88,
You have to safeguard against a live chassis. Use a 1:1 isolation transformer for circuit #2 for sure. Otherwise you may electrocute yourself or others. At the very least you may destroy your signal source.

You must avoid any direct connection to the AC mains at all costs.


ThSpeakerDude88 17th November 2005 01:13 AM

if you will see above:

"I'm gonna use a 12vac wall wart and run it through a 12 volt transformer to step up the voltage again."


that poses another questrion. Even though I will be isolated from wall voltage, and the current will be much less, I still want to protect the input so I don't blow up a cd player or guitar preamp or something. How do I isolate the input from any high voltage properly?

anatech 17th November 2005 01:46 AM

Hi ThSpeakerDude88,
You have about 30W in heater power alone, add 5W or more for the plate of the 50C5 and maybe a watt for the 14GT8. So you are over 35W requiring a 3 A wall supply. This is with or without the 35W4. A 1:1 transformer would work better. Look for a surplus one and save some money. You can test it with a bigger transformer.


Miles Prower 17th November 2005 02:08 AM

The first circuit looks OK. For some really dirt cheap record players made back in the "day", the amp used was just like that. However, don't expect any performance that you can write home about. The output iron was highly inadequate, and running 50C5s with those voltages resulted in some nasty nonlinearity.

As for the second :bigeyes: there's a helluva lot wrong with it. First off, that PS is especially bad. Connect that thing to make the "ground" 120V hot will likely ruin whatever you connect to it, and/or could electrocute you. They did that in the bad ol' days to keep manufacturing costs down. (And quite a few guitar players had some shocking experiences :hot: with those cheap "practice" guitar amps.) They might have gotten away with it for radios and TVs where the only thing the end user would be expected to connect is an antenna. You need to redesign that with a proper PS xfmr.

Secondly, three watts out of a 50C5 is dreamin'. That won't happen unless you bust the specs and then they certainly won't last too long.

Finally, since you need to redesign that PS, there are some improvements that can be made. Get a 1:1 isolation xfmr and use a full wave doubler circuit. This will give right around 300V. Add enough filter resistance to get that down to around 150Vdc. Next, add a voltage divider that can produce 90Vdc. Running the 50C5 with a higher plate voltage, and a lower screen voltage, gives a good, linear, transfer characteristic. Much better than what you'd get with the usual manner of using a 50C5. Get some decent output iron, and add some global NFB, and you could have a pretty sweet, low power, amp.

ThSpeakerDude88 17th November 2005 02:34 AM

I just went over the second schematic again too. I noticed that the audio input goes through a pot, and the ground side of the pot goes to all the heaters. I'm not going to run series heaters, I think I will step the voltages down from the transformer. How much voltage does the 14gt8 requier on its heater, 14 volts I am asuming? Power rating: Yeah I know I saw the 5 watt max thing and laughed. 1.9 watts is what the datasheet says, and thats at high distortion. I heard elsewhere that the 50c5 runs much better on a higher plate voltage and I was planing on doing that as well :)

As for the power supply... hmmm.

Well I was just going to use a full wave br setup with silicone diodes (yes laugh at me please.) I know its a little harder on the tubes than a soft start with a tube rec, but hey these tubes are cheap and I don't need anything fancy. Just thought I'd throw this together with junk parts. I actually have a heafty 3 amp 12 volt trasformer lying around ( I just recently took apart an old stereo...peice of...crud..)


Ok I haven't dabbled with the principles of nfb with tubes, only a little bit with transistors and stuff.

Could someone show how to do the nfb network for the first picture I posted?

Also, with the first picture, this is of the amp alone, does it need a preamp ( the 14gt8 section ) if I am going to use a cd player with it or can I leave that part out?

Tubelab_com 17th November 2005 03:24 AM

Whatever amp you decide to build, make sure that there is a transformer between your circuitry and the wall outlet. It was common practice in the old days to connect the power line DIRECTLY to the chassis. I have seen guitar amps where this is done. I have an Electrolab amp from the 50's (using a 50C5) that has shocked the S*** out of me. After investigating there was an old paper capacitor connecting from the power cord to the guitar jack. This cap was shorted puting 120 volts DIRECTLY on my guitar. The jolt could have been lethal.

Since that incident I add an isolation transformer and a 3 wire cord to any amp like this that I see. I would think that an isolation transformer is mandantory for any circuit that uses series wired filaments. You can use any tubes that the voltages all add up to 120 volts. They must all draw the same current.

anatech 17th November 2005 12:35 PM

Hi ThSpeakerDude88,
I pulled the 5W figure out of the air, that's the plate dissipation, not the output power.

Your heaters are 0.15A. The 14GT8 needs 14 V on the heater. The 50C5 needs 50 V on the heater. Now you know why they are in series. From the book, the max plate dissipation for the 50C5 is 7W, not a bad guess depending on how you run it.

With these tubes you are further ahead running them in series with a dropping resistor. Now, this circuit is screaming to be made push pull by adding another 50C5 to make your series heater string 114V and a dual triode to provide gain and a phase splitter. This will give you more power with lower distortion still.

I think everyone is determined to tell you to run an isolation transformer with this. We'd like to keep you around a while.;)


ThSpeakerDude88 17th November 2005 04:01 PM

LOL ok I have been told about 40 times already in several other areas I need an isolation transformer... so let me get this straight.

I have mentioned SEVERAL times that I am going to use one, I am not stupid as many of you may think I am. I know what to play with and what not to. I've been shocked by it before so I know. Just because I posted a circuit without an iso tranny does not mean I am going to use it that way! So if anyone would like to give me other information not pretaining to the 20 facts about me getting the daylights shocked outa me if I don't use an iso tranny it would be greatly appreciated. *phew* :)

ok anyways...

Here was the premisis for the project, I only need some simple info. I just wanted to take my 50c5 and 14gt8 laying around and throw it together on the breadboard for fun. I also have a similar 35c5 and 12at7, but I don't like the 35c5's sound as much. I may order another 50c5 later, but right now I just want to see what stinky low power amp I can come up with using my 50c5 and 14gt8.

anatech: I see how having two 50c5's and a twin triode would get my voltage right. However, I would like to keep the plate current as low as possable to be safe. For right now I will be running the 50c5 and 14gt8 off of seperate power supplies. :)
I'm glad the heater only needs 150ma of current, that should keep the power supply size down when I do build one.

Ok I have a question : Do I need the 14gt8 behind the 50c5 to drive it or can I run a line out directly into the input of the 50c5 (capacitor coupled) ?

Does the first picture I posted include all the parts I need JUST to make the amplifier section, or do I need to follow the other guys circuit?

I hooked up the first circuit last night and took some measurments. After using an 18vac 300ma transformer ran into a 12v primary, I measured about 117 volts on the secondary. I accidentally touched it and it gave me a little zap but not anywhere as bad as if it were wall current :hot:

*slef note, don't work on projects at 2 in the morning -_- )

I tried using a 2.7kohm resistor to drop the voltage but came up with 117v again. I also could not measure any negative bias ( -7 volts)

anatech 17th November 2005 04:23 PM

Hi ThSpeakerDude88,
Just the 50C5 might have enough gain from line out. Just couple it with a capacitor to the grid. You will want a volume control. So the first diagram. You may need to fiddle with this resistor value to get the proper operating point. Your final decision will be made with new tubes.

Your 7 V bias is not negative. Measure the cathode of the 50C5, it should be positive wrt ground. The grid is therefore negative this reading wrt the cathode. Cathode bias.

Yup, working alone and tired at night is not the smartest thing I've done either. Someone should be available to get help if you need it.


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