300B Cathode Feedback + Cathode Bias

[korneluk]

Folks,

I ran across this schematic online and was wondering if there are any drawbacks to using both cathode bias and cathode feedback at the same time.



The schematic can be found here:

http://www.tubebuilder.com/schematic2.html

TIA,

-- josé k.

[SY]

4% THD at 3W? Now that's some fine design.

[Andrewbee]

4% THD at 3W? Now that's some fine design

.....but its all second harmonic so its sooooo pleasing to the ear, and, because its so pleasing you don't even think about its in-efficiency

[korneluk]

Originally posted by SY
4% THD at 3W? Now that's some fine design.

Those figures caught my eye as well and made me wonder about the cathode feedback implementation.

This technique was used in push-pull amplifiers to reduce the plate resistance and the distortion level, so I am trying to figure out what went wrong with this design.

-- josé k.

[jlsem]

The output tube is only being run a 15.7W plate dissipation so the 3W 4% distortion would be pretty normal for a single ended amp of this size. I don't see any cathode feedback in this circuit. I do see an RF choke in the cathode circuit in lieu of a grid stopper to prevent parasitic oscillation, though.

John

[rtsang]

The 300B is running in the specs of a 2A3

Plate dissipation is only around 15.5W
So even without cathode feedback, the amp could only make around 4W, assuming 25% efficiency.

Is it a good way to run an output tube in such a low plate dissipation (<60%).
I used to have a Sun Audio 300B amp that runs a 300B like that, never sounded good to me. But the driver stage of that amp also have problems, so I am not sure if the driver or the output stage was the culprit.

Raymond

[korneluk]

Originally posted by jlsem
...I don't see any cathode feedback in this circuit. I do see an RF choke in the cathode circuit in lieu of a grid stopper to prevent parasitic oscillation, though.

John

It is not an RF choke but rather a separate winding off the output transformer being used for feedback.

-- josé k.

[jlsem]

I stand corrected although the schematic is a bit ambiguous.

John

[Michael Koster]

I don't think the distortion is being generated in the output stage...

Michael

[45]

Originally posted by korneluk


Those figures caught my eye as well and made me wonder about the cathode feedback implementation.

This technique was used in push-pull amplifiers to reduce the plate resistance and the distortion level, so I am trying to figure out what went wrong with this design.

-- josé k.

I will never understand why people have to make things complicated more than necessary....
I guess that a 300B (run like that) in a Loftin-White circuit performs better!

IMO, the first wrong thing in that schematic is the input tube. The 12AT7 in SE mode (both RC and transformer coupled) is good only if you don't need too much swing. Let's say, it is good for driving EL84's.
In this case the swing is low as well but the static load is 10K only and the step-up transformer will have a lot of stray capacitances....with a 40K secondary, I think the 12AT7 will see some nF's!
Then have a look at the characteristic curves of the 12AT7 and it will be clear it is working in a quite non linear mode...
I think much of the distortion comes from it and at the output you will see a lot of 3rd harmonic because of some harmonic cancellation ( basically 2nd harmonic cancellation).
Harmonic cancellation can help, however it works (sounds good) only if it is a small effect, in my experience.

Cheers,
45