tokl wrote:
First thing there are cracking noise in the speakers when the music is playing especially around the bass. Speakers are quiet without music. The louder the music the louder are the cracking noises.
Are the cracking noises in one or both channels?
If there are crackling noises in an amp, the first thing I look for is cold solder joints. So inspect all the joints to make sure they all look ok.
After that, in this design, look at the heater lift for the 5687. It should be around 85 volts according to the schematic, However, according to the Tung-Sol and Raytheon data sheets, the maximum heater to cathode differential is +/- 100v. With the upper cathode at ≈208v this puts the differential at 208-85=123v. The cracking you're hearing could be heater-to-cathode breakdown in the SRPP.
With the voltages in this circuit, you want that heater lift to be about 108v. This is right at the limit for both the upper and lower triodes. To get this lift, based on schematic values, R2 needs to be about 109kΩ, not 87kΩ. The closest 1% value is 110kΩ. This may or may not be your problem, but it is an error on the schematic.
The other thing to look for is overdrive. The 6SN7 stage has a gain of about 14.5. So depending on your input voltage and your volume setting, you could be overdriving the SRPP. You might try a lower input signal. Since the SRPP is biased to about 8.5v, I would suggest an input signal of not more than 8.5/14.5 ≈ 0.586 Vpeak or ≈0.415 Vrms.
I hope this helps.
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