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Continuing issues with a Knight Basic 60

rudedogg

rudedogg

AK Member
Subscriber
So i posted awhile back about a hacked up Basic 60 amp. Mainly the power supply had been badly modded to lower B+. So that has been fixed, and the supply and the power section are stable and seemingly working great. However i have an anomaly on one driver board. The amp will play fine awhile, then suddenly one channel will distort and go lower in volume a few seconds. This corresponds with a jump in voltage of 100+ on the 12ax7 pin shown in the photos. The voltage will drop back to normal and all is well again all within say 15-20 seconds. Both boards have been recapped prior to my possession. I replaced an open resistor in the unaffected driver board. What could cause these quick spikes in DC? All voltages elsewhere “seem” stable and fine. Tubes are also good. And i have tried others there.
 

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So pin 6 jumps up by 100V? Either the cathode resistor (r33/r10) the tube socket, or the 12Ax7 are the issue. Since the amp is DC coupled, it could also be an issue with the driver but you did mention other voltages are stable
 
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If the voltage jump isn't also happening on pin 1, seems like probably not a cathode resistor issue since it's shared with the other half of the triode. Does the DC voltage at pin 1 stay steady when these transients occur? If that voltage were to drop significantly, that half would be biased super cold, causing the signal to cut out and plate voltage to increase. Also agree that tube socket is a possible culprit
 
maxhifi said:
So pin 6 jumps up by 100V? Either the cathode resistor (r33/r10) the tube socket, or the 12Ax7 are the issue. Since the amp is DC coupled, it could also be an issue with the driver but you did mention other voltages are stable
Click to expand...
Ok, sooo after i replaced the 1meg resistor earlier, and put the tubes back in i had the issue sporadically. So after reading your response i simply swapped the two 12ax7 between channels. Now the amp has been running for 5.5 songs and zero issues! Maybe it was a contact issue with the pin? Fingers crossed! Also the voltage at pin 6 on the offending tube did constantly swing 10vdc prior. Now it is very stable..
 
rudedogg said:
Ok, sooo after i replaced the 1meg resistor earlier, and put the tubes back in i had the issue sporadically. So after reading your response i simply swapped the two 12ax7 between channels. Now the amp has been running for 5.5 songs and zero issues! Maybe it was a contact issue with the pin? Fingers crossed! Also the voltage at pin 6 on the offending tube did constantly swing 10vdc prior. Now it is very stable..
Click to expand...

Order new sockets and swap them out when you have time. Life is too short to fiddle with intermittents. I suggest Belton, they're the best new ones.
 
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