Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: catnine on July 04, 2011, 07:09:06 pm

Title: This really baffles me , I wanted to measure the resistence on my champs 2.7k
Post by: catnine on July 04, 2011, 07:09:06 pm

 NFB resister . I don't get a reading across it and I have a dpdt slide switch so i can add a 100k pot is seriers with the NBF or switch to back to the stock value. There is a schem attached . Now if I switch it to the pot position then I read 2.7K ohm .

Title: Re: This really baffles me , I wanted to measure the resistence on my champs 2.7k
Post by: PRR on July 04, 2011, 07:34:00 pm

> I don't get a reading across it

Yes you do. It may not be what you expect, but you MUST respect reality. What does the meter say?

What is on one end of the 2K7? A 47-ohm resistor to ground.

What is on the othe end of the 2K7? The OT secondary, under 1 ohm DC.

So the 2K7 is in parallel with 47+1 or 48 ohms. The reading is something under 48 ohms, correct?

Title: Re: This really baffles me , I wanted to measure the resistence on my champs 2.7k
Post by: catnine on July 04, 2011, 07:42:04 pm

Quote from: PRR on July 04, 2011, 07:34:00 pm

> I don't get a reading across it

Yes you do. It may not be what you expect, but you MUST respect reality. What does the meter say?

What is on one end of the 2K7? A 47-ohm resistor to ground.

What is on the othe end of the 2K7? The OT secondary, under 1 ohm DC.

So the 2K7 is in parallel with 47+1 or 48 ohms. The reading is something under 48 ohms, correct?

yes correct. It's 46.2 ohm . I see now that it is in parallel not in series . The only way i would see the 2K7 value is if I lifted one/ either end out of the circuit .

Title: Re: This really baffles me , I wanted to measure the resistence on my champs 2.7k
Post by: catnine on July 05, 2011, 07:07:37 pm

I understand the parallel resister reading since both ends of the 2k7 ohm NFB reister go to ground one end through  the 47 Ohm to ground and the other end through the OT coil to ground @ 1 ohm . This is what makes it parallel.

 Ok I add 1k ohm in series to the 2k7 NFB resister on the OT 1 ohm side , then I read 2k7 across the resister.

 From what I understand adding more resistence to the NFB allows less voltage back to the second gain stage so it is closer to not having NFB  a more open sound.

 Being in parallel the actual NFB value is 48 ohm and not 2k7 ohm . So and this is a question. I hear a great difference when I dial the 1k pot in series yet I am lost as to what value the NFB resistence is at that point.

 Is it 1k ohm + 1 ohm in parallel with 2k7 ohm + 47 ohm ? is it 733 ohm?