6L6 Circuit

[JJH0906]

Anyone care to chime in about this circuit? This is from a popular amplifier electronics book and I'm trying to make a little sense out of it.

I guess the two questions that I have is this some sort of multi-stage circuit which is creating distortion? Also, not really sure what's going on with the output tubes, I don't see any phase inverter circuit there and not sure how the two 6L6 tubes are driving the output transformer.

I'm sure a number of you have had the same experience I have had with these books, that the author world go off on a tangent talking about one kind of configuration and then show something completely different with no explanation as to why it was done that way!

[Stevem]

This is not a schematic for a class AB amp that would need a phase splitter / gain stage to drive each tube, this is a parallel signal ended output stage so that one section of the 12AX7 before the output tubes is just there to drive them.
With this 6L6GC type parallel output stage the most RMS wattage your going to have is about 22.

[Stevem]

Also note that if your going to build this that the 82 ohm output tube cathode resistor is being run at 4.76 watts out of the 5 watts that it is rated at!

The rule of Thumb that I like to go by in terms of power resistors is to over rated them by 100%!

If you wanted to save 10 cents you could build with a 7 watt resistor.

[Stevem]

Sorry, my mistake!
That resistor is only running at 3.54 watts, so a 7 watt resistor is what I would fly with.

Also note that a output transformer for a build such as this new will run you well over 175 bucks.

[JJH0906]

Thanks for the great insight! No, I'm not planning on building this particular circuit, but in studying the authors theories on various tube circuits, he presented this one that you see. He probably talked about what you mentioned but I was on a different path and either glossed over it or just missed it all together.

[JJH0906]

Attached is a schematic from another popular amp building book for a Push-Pull Class AB output stage circuit. Only some specific values for components were given. I'd appreciate any input that members might have regarding this circuit. Once again, the author of the book ran through a few basic concepts, tossed the schematic out there and didn't delve into many details about the "how-to's". Thanks!

[sluckey]

That phase inverter will not work as drawn. The grid cannot operate at zero volts. Either needs a resistor connected between grid and B+ to form a fixed bias voltage divider or needs to be bootstrapped to the cathode by adding a 47K resistor between the junction of R2 and R3 and ground. And change R3 to approx 1K.

[martin manning]

Attached is a schematic from another popular amp building book...

Which book? Why redraw it? If it's a popular book it might be easier for people to just check that reference.

[JJH0906]

The circuit that I was referring to is from a book by EJ Jurich. Attached is the photo of the page of that circuit that I had attempted to redraw.

The reason for redrawing the circuits is that my intent is to use various circuits that I come across that I want to use in my build, so I transfer the drawings from other sources into a CAD program in order to make a complete schematic set for myself down the road. I don't do my due diligence sometimes to make sure that the information from the reference circuit is getting transferred accurately into the CAD software, something I need to be attentive to with future drawings.

[sluckey]

The circuit that I was referring to is from a book by EJ Jurich.

Never heard of him. What's the name of the book?

The circuit in the photo is wrong for the same reason I posted earlier.

[wpaulvogel]

This is not a schematic for a class AB amp that would need a phase splitter / gain stage to drive each tube, this is a parallel signal ended output stage so that one section of the 12AX7 before the output tubes is just there to drive them.
With this 6L6GC type parallel output stage the most RMS wattage your going to have is about 22.

Also if you look at the output voltage noted on the schematic, it’s 7.9 volts A/C across a 8 ohm load. That’s 7.8 watts RMS. Not very efficient.

[JJH0906]

The name of the book is "Vacuum Tube Amplifier Basics", publish date of 2014.

Thanks for the observations, probably steered me away from a disaster, ha ha!

Not sure what to say, he's got this drawing in his book but as I mentioned earlier, very little explanation of the circuit prior to revealing the schematic. He's got another one right after that that uses a center-tapped transformer to invert the signal, which looks interesting but again, offers little details as to the how's and why's.

[JJH0906]

That phase inverter will not work as drawn. The grid cannot operate at zero volts. Either needs a resistor connected between grid and B+ to form a fixed bias voltage divider or needs to be bootstrapped to the cathode by adding a 47K resistor between the junction of R2 and R3 and ground. And change R3 to approx 1K.

If these modifications were made, is there a consensus that this circuit would work, or are there better methods?

Also note I'm looking to get somewhere between 30 to 50 watts output. 30 would really be fine, just trying to create enough headroom for a clean sound for the amp.

[sluckey]

If these modifications were made, is there a consensus that this circuit would work, or are there better methods?

Talking about the changes I said need to be done to make EJs circuit work?

[JJH0906]

Yes, you pointed out if you things that you believe were incorrect. You made it sound if those were edited correctly, possibly the circuit would be viable?

If there's a better solution, I'm completely open to learning about it. This was just another reference source I was trying to use, which seems to steer me in the wrong direction each time I do that!