50C5 Triode strapped?

[ThSpeakerDude88]

I want to run the 50c5 in my guitar amp in triode connection, for about 1/8 watt of power ( great for low volume overdrive!.) I'll have a switch so I can switch between pentode for ~2 watts or triode.

I did this in my 6bq5 SE amp, but the 50c5 datasheet shows no specs for using it as a triode...


Do I just run a 100 ohm resistor from the grid to the plate like one would normally do?

[JohnAtwood]

Yes, this should work just fine. On these kinds of amps, the screen voltage is normally about the same as the plate voltage, so the bias should be close to normal. As a double-check, measure the plate current and the plate-to-cathode voltage and multiply these to get the plate+screen dissipation. On a 50C5 this should be 5.5 Watts or less. The 100 ohm resistor is to damp out any possible RF oscillations.

[ThSpeakerDude88]

The screen on this one is run after a 2.2k resistor, and so is the preamp. I don't have it built yet but I assume the voltage is around 128v or so. Plate voltage is 140-160v. This is a harmony h303A clone. Do I just measure my screen current and make sure its not drawing more than the alotted current in the datasheet?

BTW, my bias resistor is 180 ohms. Schematic says 1 watt... I'm a bit skeptical about that but I'll try it.

[JohnAtwood]

The screen current is a little hard to predict. What is most important is that the sum of the screen current and plate current not exceed about 40mA (which roughly gives 5.5W at 150V). In the triode-connected mode, the screen current is always going to be way less than the plate current. Never run the tube with voltage on the screen grid and no voltage on the plate! All the current will then go to the screen, and you will get to see how hot metal gets before it melts!

Measure the 180 ohm resistor (they often drift if they are carbon composition types). If it is reasonably close to 180 ohms, then the voltage drop across it should be around 7 volts, which corresponds to a bit less than 40 mA. This gives a power dissipation of about 0.3W in the resistor, so a 1W resistor is good.

If the current is too high, you can increase the cathode resistor. You will, of course, have to re-calculate the appropriate voltage across it, using Ohm's law. I hope this helps.

[ThSpeakerDude88]

Thanks! I just recieved my parts today, so I will do a follow up once its up and running. I'm using a hammond 120vct transformer for the b+, and the ct on it to run the 12au6 and 50c5 heaters in series. I ordered a NOS sprague Blue Jacket, as it was the only thing I could find on the sites I ordered from that was 180 ohms and above 1/2 watt. I could have ordered from another place, but I wanted to combine shipping.