With the upper cathode at ≈208v this puts the differential at 208-85=123v. 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. So inspect all the joints to make sure they all look ok.Īfter that, in this design, look at the heater lift for the 5687. If there are crackling noises in an amp, the first thing I look for is cold solder joints. ![]() The louder the music the louder are the cracking noises.Are the cracking noises in one or both channels? Otherwise only one of the two triodes will be heating up.Ī picture of the circuit boards, both top and bottom, would be helpful as well.įirst thing there are cracking noise in the speakers when the music is playing especially around the bass. For this to work properly pins 4 and 5 should be shorted together and the 6.3v winding connected to pins 4/5 and pin 8. The heater is marked as pin 4 and pin 8 on the tube, but the note on the transformer winding say 4 and 5 on the 6.3v winding. There is also something on the schematic that is unclear. But a common problem with some circuit boards is that the tube socket gets soldered to the wrong side of the board. Now I'm going to suggest you check something and I don't want you to take offense. If it's happening on both channels there is a more systemic problem. You were not clear as to whether this was happening in one or both channels. ![]() The second clue is that your grid voltage on the upper triode (pin 2) is essentially the same as the plate (pin 1) and I assume the cathode voltage (pin 3) as well. The only way this can be is if there is no current through R17. What this means is that there is no current flowing in the SRPP stage.
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