help with the Impedance selector?

[Peeps]

I built my 2204 with the help from Neil and SDM's layout, thank to both of these two for their time.

I'm just have a question about the impedance selector.
I wired my nfb purple wire to the output tip like the 50 watt kit instructions showed. so why do I see post of people putting the purple wire to a specific tap on the on the selector. Is that just if you don't want to use a Impedance selector?
Isn't putting the selector on each 4, 8 and 16 ohm doing the same thing?

[neikeel]

Putting your resistor wire to the selector will vary the NFB according to your speaker cab, so an 8 ohm cab gives x k ohms into 8 ohms or 16 ohm cab more NFB with same resistor.

Putting it to specific tap will give a fixed NFB regardless of cab.

[Peeps]

Putting your resistor wire to the selector will vary the NFB according to your speaker cab, so an 8 ohm cab gives x k ohms into 8 ohms or 16 ohm cab more NFB with same resistor.

Putting it to specific tap will give a fixed NFB regardless of cab.

So with my cab being a 1960a stereo, it has 4 and 16 ohm selection imputs, would I be better off just using the Impedance selector to go from 4,8 and 16 then set the cab to match the output?

[toner]

The NFB wiring has no effect on how you should set the amp impedance. Always match the amp impedance to the speaker cab. (16 ohms on amp into 16 ohm cab) or (8 into 8 ) or (4 into 4)

When the NFB is wired to a specific output transformer tap, like 16 ohms for example, you will always have the same amount of negative feedback, no matter which impedance you set the amp on. The amp will sound more consistent with different speaker loads.

When the NFB is wired to the impedance switch output (or the speaker jacks that connect to it), the amount of NFB will change when you change the impedance switch. 16 ohms = most NFB, 8 = medium NFB, 4 = least NFB. Given the same NFB resistor, the 16 ohm tap will have twice as much NFB as the 4 ohm tap.

More NFB = less PI and power tube gain (cleaner), somewhat tighter bass, flatter frequency response and a more "controlled" feel

Less NFB = more PI and power tube gain (more overdrive), usually more mids and a "looser" feel

The difference in sound and feel is more noticeable at higher volumes.

[neikeel]

If you use the wire to the speaker jack and use your 1960A cab you can use 4ohm (4 parallel speakers) 8ohm (two speakers in series) or 16ohm (4 speakers series parallel) negative feedback but you will of course need to match the selector setting to the cab.

If you hook up the wire to the 8ohm tap you will only ever have that resistor (47k ??) into 8ohms which is my personal favourite for a split cathode plexi. Same if you go to the 4 or 16 ohms tap.

Hooking up to the selector is exactly the same as the speaker tap assuming you will always be matching the selector impedence to the cab.

Sure you can select the 16ohm tap for 47k/16ohms but then you will be using the 16ohm cab, unless you go for an impedence mis match with an 8ohm cab, then you have 47k/16ohms but an 8ohm cab. This has a significant effect to my ears, at least as mutch as different NFB.

Doh, Toner beat me too it - too long composing

[Peeps]

Thanks fellas, i think between both of your post...I have a much better Idea now. I did use the 47k resistor on the NFB.....the amp sounds really good with no noise at all. It just gets a little too harsh when I drive the preamp with the master volume turned up. I also installed the lar/mar, but even with that...it's not pleasing to my ear. The clean sound is perfect, but I guess I need to try a few different taps and tubes. I had 3 mullards laying around and used them for the preamp tubes. I'm starting to think in need to try some different tube too.

[toner]

...It just gets a little too harsh when I drive the preamp with the master volume turned up...

I'm not sure exactly what you're hearing but you may want to try adding a 100k resistor on the pre-PI master volume wiper (middle lug) inline with with existing wire. It helps keep the frequency response more consistent at different settings (helps avoid a reduction in bass and low-mids when the MV is set around 5).

Also, adding a 220k resistor on pin 2 of the PI socket (V3) can reduce harsh clipping in the PI when running the pre-PI master volume at higher settings.

More info about both here:
http://forum.metroamp.com/viewtopic.php ... 17#p234417" onclick="window.open(this.href);return false;

[Peeps]

...It just gets a little too harsh when I drive the preamp with the master volume turned up...

I'm not sure exactly what you're hearing but you may want to try adding a 100k resistor on the pre-PI master volume wiper (middle lug) inline with with existing wire. It helps keep the frequency response more consistent at different settings (helps avoid a reduction in bass and low-mids when the MV is set around 5).

Also, adding a 220k resistor on pin 2 of the PI socket (V3) can reduce harsh clipping in the PI when running the pre-PI master volume at higher settings.

More info about both here:
http://forum.metroamp.com/viewtopic.php ... 17#p234417" onclick="window.open(this.href);return false;

Yeah Toner, guess what I'm trying to say is I can't get it to break up early enough to get some good crunch. I'm going to keep tweaking in the morning while the wifes at work. I'll look at the thread you posted too, thanks.