NFB taps, impedance mismatch - am I doing it wrong?


Oct 1, 2019
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I stumbled on an old post regarding power tubes getting hot recently and it got me wondering.

(backstory) I was having a no sound issue with my amp (1969 Rockitt Retro super lead clone) a few months ago and figured out it was a bad ground connection on the impedence selector. At the same time I replaced the tube socket on v6 due to previously shorted power tube that caused arcing. I replaced the socket, the screed grid resistor and the 1 ohm to ground on pins 1 and 8.

The amp is working pretty good again with no weird noises or cutting out but I have noticed that my V6 and V7 power tubes seem to be running quite hot (temp wise), no red plating even at high volumes but I can physically feel the difference in heat coming off of these tubes vs V4 and V5.

I'm waiting on a new bias probe as I suspected the bias might need checking, perhaps its to hot or I have a mismatched tube. The tubes are JJ EL34's and were purchased as a matched quad about 1 1/2 years ago after the tube failure in V6.

Once I get the bias probe I'll be checking my readings with plate voltage, bias and all that to look for any obvious issues. I have moved tubes around and still only notice the heat issue from V6 and V7.

The post I read was talking about impedance mismatching. A couple years ago I did the one wire mod (which I've since gone back to stock preamp) and at the same time I changed from a variable NFB and wired the NFB directly to the 4 ohm tap. I currently have a 47k NFB resistor.

My cab is a 1960a 4x12 that I connect to via 16 ohms and I run my head on the selector at 16 ohms. Am I creating a mismatch? should I be running my head and cab at 4 ohms to match the NFB circuit I wired?

thanks guys!


Well-Known Member
Jul 19, 2018
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Los Angeles
You don’t need a bias probe if you have 1ohm resistors from pins 1 and 8 to ground. Everything can be checked using only a multimeter.

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