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

SilkWilk

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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!
 

Chris-in-LA

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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.
 

william vogel

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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!
The impedance selector on 16 ohms and running into a 16 ohm cabinet is what would be called matching impedance. Changing the NFB by moving the wire from the 8 ohm tap to the 4 ohm tap simply reduces the NFB voltage and reduces the amount of NFB. A multi tap output transformer will output separate and different voltage/current depending upon which tap is chosen and loaded with an impedance (speaker). All of the taps will output voltage even without the load being attached to them. Meaning if your speaker is on the 16 ohm tap, the 4 and 8 still output voltage but less per tap. The 8 ohm outputs .707 volts of the 16 and the 4 ohm outputs 1/2 the 16 ohm voltage. By moving the NFB from the 8 ohm tap to the 4 ohm tap, you reduced the NFB voltage by 29.3%.
 

thetragichero

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yup, whichever secondary tap you take your nfb from is simply a design choice so if sounds like you want it it's the right choice
 


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