I just recently tried to use LTspice to model the KT88 push pull amp and have a few questions on the output wave form in general. My amp will be based on 3 stages. Input stage (6SL7) will direct couple to driver stage (6SN7) then capacitor couple to KT88 to drive my 5K output transformer. So far the simulation indicate that output waveform taken from output tranny 8 ohm load showed a very clean sine wave with distortion less than 1% from 20 to 20 Khz with max Power output at about 50W RMS with global feedback from 8/16 ohm to input stage. I also checked both output of the input and driver stages wave form. They both showed very clean sine wave output if output power is less than 35W. However, both input and driver output stages sine wave start to distort if power output is driven above 35W and get worst with increasing in power output. However, as mentioned earlier my output sine wave from output transformer still showed a clean output sine wave and total distortion less than 1%.
Is this normal what you would see or expected from input and driver stages when using Feedback? Or I need to fix the input and driver stages? Thanks in advance for the input.
Is this normal what you would see or expected from input and driver stages when using Feedback? Or I need to fix the input and driver stages? Thanks in advance for the input.
Sounds normal; the feedback loop distorts the signal going to the output stage such that the signal coming out of the output stage is clean. That's the way it largely regulates away the output stage's distortion.
For example, if the output stage's input-output relation were a small part of a quadratic curve and the feedback loop had a huge loop gain, the feedback loop would control the input signal of the output stage to whatever is needed to get a clean output. The input signal of the output stage would therefore be distorted in a square-root-like fashion.
For example, if the output stage's input-output relation were a small part of a quadratic curve and the feedback loop had a huge loop gain, the feedback loop would control the input signal of the output stage to whatever is needed to get a clean output. The input signal of the output stage would therefore be distorted in a square-root-like fashion.
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MarcelvdG,
Thank you very much for a quick response. It made me feel better. Now I need to find out why I have some type of modulation riding on some portion of the output waveform at low frequency of 10 Hz.
Thank you very much for a quick response. It made me feel better. Now I need to find out why I have some type of modulation riding on some portion of the output waveform at low frequency of 10 Hz.
This is because of Class AB1 operation. Each tube will conduct in a half period (with some overlap). So you will see a half sine on each anode. In your case Class A mode of operation (both tubes conducting) is about up to 35W, above this level only one tube is conducting. The output transformer combines the currents of both tubes.