Metropoulos Forum

Hey Folks,
So I have noticed that on certain layouts the NFB resistor is wired to a different impedence. Such as, on a JTM 45, it is wired to 16ohm and a JTM 50 is wired to the 8ohm lug. What is the determining factor for which one it is wired to?

Thanks

Personal taste. Depending on where you put it, the amount of NFB varies. Least NFB would be 4ohm tap, which makes for a more aggressive, slightly gainier (at volume) amp.

100k@4ohms=200k@16ohms=141k@8ohms

demonufo wrote:Personal taste. Depending on where you put it, the amount of NFB varies. Least NFB would be 4ohm tap, which makes for a more aggressive, slightly gainier (at volume) amp.

You need to move it around, as I moved mine around on my JCM800 combo, (4010) and it made a huge difference in gain/feel. I kept messing around with other areas of the circuit, but once I moved the NFB, it was perfect. I forget which lug I moved it to, but it pays to try the various lugs.

Mike

use a pot in series with a 27k resistor

What value pot you think Roe?

Big Mike wrote:What value pot you think Roe?

usually 100k or 250k

250k log in series with 27k from 8ohm tap is a good starting point, as you'll have 50k at noon, pretty common, and then you can go up to 280k. Rivera used something similar (called focus), and Mesa too. I like that control more than depth control. Just because works better where the guitar needs more nuances.

I'm going to try installing a 250K pot in series with the 27K resistor on the 16 ohm tap. As Roe points out this allows me to keep the vintage NFB value plus full flexibilty for other values. Where would be the best place for installing the pot?? I was going to install it on the rear faceplate next to the impedance switch. Any suggestions here would be appreciated. Thanks!

keep in mind that some of the old presence pot measure 6k and 7k. and that the 100w amps have 1.4 more nfb than the 50w

Roe, I'm not entirely sure what you're telling me (I'm just a newbie). Just for clarification I'm building a JTM45 following the Metroamp instructions and thus the presence pot is 5K. From other posts on the forum guys seem to like NFB resistor values from 48K to 100K on the 8 ohm tap. So in my case, with the 250K pot in series with the 27K NFB resistor (on the 16 ohm tap) I'm able to maintain the vintage original NFB value plus the flexibility to achieve the other values that members seem to prefer ... and more. If I'm missing something here please feel free to enlighten me. If you have any comments type real slow so I can understand.

my 2 cents - build the amp according to the stock spec values (27k -16ohm or 8ohm NFB) then play the amp for couple of weeks or months thus letting everything burn in nicely and your ears get used to the sound etc... then start tweaking slowly, step by step (lower NFB, try some NOS mustard caps and iskra or piher resistors for example, tubes etc..)

mutts wrote:Roe, I'm not entirely sure what you're telling me (I'm just a newbie). Just for clarification I'm building a JTM45 following the Metroamp instructions and thus the presence pot is 5K. From other posts on the forum guys seem to like NFB resistor values from 48K to 100K on the 8 ohm tap. So in my case, with the 250K pot in series with the 27K NFB resistor (on the 16 ohm tap) I'm able to maintain the vintage original NFB value plus the flexibility to achieve the other values that members seem to prefer ... and more. If I'm missing something here please feel free to enlighten me. If you have any comments type real slow so I can understand.

I'd suggest 22k and a pot with the 5k pot. also a .1 cap over a 5k pot gives a different frequency than a .1 over 7k

Roe, I like your suggestion of lowering the NFB resistor from 27K to 22K in case I want to tighten the amp up a little more. I am considering mounting the pot between the impedance switch and AC input on the rear faceplate. I'm not sure if this is the best mounting location in terms of noise reduction. Is this where you would put it or do you have a better suggestion??

Just as close to where it is going as possible. It is prone to pickup shitloads of noise all around that area so chopstick the wire around for least noise after it is in. If your power supply is tied up nicely it should be fine. Sloppy dress will be punished sometimes.