Class A Tube BJT Hybrid

[jerluwoo]

Just thought I'd throw this little circuit over here for the tube guys and gals to have a look at. It is just a work in progress and none of the values are optimum but it does work as I am listening to it now. I will later build a better unit with ccs's and higher voltage for more power but as this circuit is it puts out a very nice sounding 5 watts.

[Geek]

A FIWA

Bipolar's are finicky little guys, but don't to too bad once you get them going right. Should be a good project!

[tschrama]

sorry, but you cann't just post projects like this....

reality warning

[smoking-amp]

12AU7 at 15 Volts B+ !!
What kind of distortion levels is this producing?

[Wavebourn]

Originally posted by smoking-amp
12AU7 at 15 Volts B+ !!
What kind of distortion levels is this producing?

By the way. I don't mean this kid's project. He will defenitely learn a lot, at least he don't hesitate to think and create. Now he needs to learn how to evaluate.
I what to tell you that 12AX7 works really well at 15 Volts B+. A first, I used it in microphone amplifiers. And second, I use them in my receiver, it is miliraty beast Collins R-392 with 28V B+.

[smoking-amp]

"to tell you that 12AX7 works really well at 15 Volts B+. "

Umm..., well, for microphone signal levels this could be OK since the signal voltage level is so tiny it would not see much curvature in the tube response.
The RF stage in the Collins would put 2nd harmonic and above outside the IF bandpass, so is safe.
But for an audio power amplifier, one will hear large signal tube curvature distortion up to many harmonics. Maybe could get away with this low B+ by using a more suitable low voltage tube like 6GM8 or some of the car radio tubes.

Don

[Eli Duttman]

I agree that the 6GM8 is a MUCH better choice for a LOW voltage situation.

Using the "sand" only for current amplification is THE proper choice. Paired with EFFICIENT speakers to avoid clipping, a refined version of the design will sound DECENT.

[jerluwoo]

Always have to bust my chops eh tschrama lol. The circuit works just fine, as I said the first schematic was just a test build to check some theory. Since it worked out I went ahead and built a little better version. As for using the 12au7, I had one laying around collecting dust as I usually only work with bjts, a different may work considerably better. So here is the new build.

[smoking-amp]

Nice touch with the current source loads. But it appears that the 12AU7 still only has 40 milliVolts of bias headroom between grid and cathode. Will be entering A2 grid current region with 1V input signal, causing significant distortion. Needs more plate current and higher grid bias. You'll need more B+ on a 12AU7 to get the plate current. Try a 6GM8 tube or maybe a high gm tube like 6DJ8.

Don

[tschrama]

Allways happy to do you a favor ...