Uncommon Power Output Tubes

Make your amp stand out

Using a strange or uncommon power tube is a way to make your amp stand out. I am going to take you thru a process to use those uncommon tubes, and today I am going to use a 6146B/8298A. We need to determine if this tube is going to be practical.

Is it practical?

My first stop is https://frank.pocnet.net/sheets/049/6/6146B.pdf. I am looking to see what this tube is used for. I need to know what type of tube this is, plate dissipation in watts, and does it have curve data. Plate dissipation is 27 Watts! It’s a beam power pentode, and curve data is provided.

Decisions

You could play with the parameters to design an awesome power output stage with this calculator https://www.vtadiy.com/loadline-calculators/loadline-calculator/ under 8298A. Use the links at the bottom of this article to design a preamp and match the output transformer.

For those who want to learn how to use plate characteristics (curve data), I will go thru the steps.

Charting the data

The red line above is our do not pass line (plate dissipation). I like to operate at 85% of the 27watt plate dissipation or about 23 watts which could give us a 7-10 watt Single Ended (SE) amp. The red line is 23 watts divided by plate volts to give plate milliampere. Now we just need a load line to find operating voltages, impedance for output transformer, and the value of our bias resistor.

Operating point

The operating point is a red dot in the middle of our purple load line. The operating point is approximately on the -20v grid line, 80 plate milliamperes, and 180 plate volts (just below the curve of the plate dissipation line). to make the purple load line, I simply start at twice the plate milliamperes 160mA. My load line starts at 160mA, crosses thru the red operating point, and ends at 370 plate volts at 0mA. My load line crosses the black cut off line at approximately 155plate milliamperes and 20 plate volts. This means the amp will operate between 0-155mA and 20-370v (350v swing). our impedance is R=V/I (350v/0.155A=2258 Ohms impedance or 2200z). 2200z is close enough and will get us north of 7 watts!

Biasing

We need to know 3 things to bias a power pentode. Grid volts, plate current in amps, and grid #2 current in amps. We know from our operating point that we have -20 grid volts and .08A on the plate. To find the missing grid current we can use the chart above. Line up 180 plate volts from our operating point on the -20 grid volt line and we find 4mA (.004A) on the grid. to find our bias resistor value we just ignore to negative on the 20 grid volts and add plate and grid amps together. Solve V/I=R 20v/(.08A+.004A)=238ohms. The common 220 ohms will be close enough. 220ohm x .084A=18.48v, and now we need to how many watts for our resistor V x I=W (18.48v x .084A=1.6 watts). Use 3-5 watts so the 220 ohm resistor doesn’t overheat.

Let’s put it all together

We need 200vdc to plate for 180v operating point to be correct after we bias the cathode at -20ish volts. We can achieve these voltages using a 6CA4 rectifier tube and this power transformer https://crunchytubeamps.com/product/40w-r-core-audio-transformer-40va-180v-0-180v-0-6-3v2/

Now you can use this same technique to find and employ just about any strange tube you come across, and make your amp stand out.

More on matching transformers here: https://crunchytubeamps.com/matching-your-output-transformer/

Look here if you’re interested in designing a preamp section: https://crunchytubeamps.com/use-the-preamp-tube-of-your-choice/

Here is completed draft schematic for those interested in building this amp.

40W R Core Audio Transformer 40VA 180V-0-180V 0-6.3V*2 – Crunchy Tube Amps

Multi Tap Universal SE Audio Output Transformer – 15W Chassis Mount – Crunchy Tube Amps

(Use 5 watt tap for on the transformer above)

10PCS Ceramic 8.8mm Vacuum Tube Anode Cap Grip Plate Cap MCZ for 807 FU7 FU25 6146B 310A 24A Vintage amplifier DIY – Crunchy Tube Amps

8 pin tube socket (crunchytubeamps.com)

5pcs 9PIN Tube Socket 24BB – Crunchy Tube Amps

6N2 Valve Vacuum Tube Replace 6H2N 6AX7 6AV7 ECC41 – Crunchy Tube AmpsVacuum Tube, 6CA4 / EZ81, JJ Electronics, Rectifier – Crunchy Tube Amps