NFB and switchable impedance speaker switches

[norburybrook]

While I wait for my pots to arrive so I can update to the skyliner EQ I was having a think about the NFB on Dumble amps.

now, as HAD used fender Twin OT there was only a 4ohm tap which of course the NFB went to.

How does this now work with having a switchable impedance selector?

If I use say an 8 or 16 rather than a 4 ohm most of the time do things need to be adjusted to get the same effect?



Marcus

[martin manning]

Each step in secondary impedance brings a 1.4 factor in voltage. To return the same FB voltage to the PI using say the 8 ohm tap instead of the 4 ohm, you would for increase the value of the FB resistor 1.4x. Of course you want to connect the NFB directly to the secondary tap of choice and not to the speaker jack.

[aflynt]

I believe that if you have a 4 ohm tap you can just connect the NFB wire to where that tap meets on the impedance selector and it will behave as intended. If not I think you'd need to do some calculations to adjust the nfb/presence circuit.

-Aaron

[norburybrook]

Martin, are you saying that if I use the 8 on Tap mostly then I should adjust the FFB value and connect it to the 8 on connection on the switch?


M

[martin manning]

Sorry, I didn't make myself very clear. If you connect the FB directly to the 4 ohm tap (solder it to the impedance switch lug where the 4 ohm tap is soldered) the FB voltage will remain the same regardless of the tap in use, and you can use the FB resistor value shown in the layout.

[norburybrook]

yes, that makes sense. It was late and I wasn't thinking about it properly.

As Aaron said , as long as there is a 4 ohm tap then the circuit is working as expected, you'd only need to change things if there were only an 8 or 16 ohm tap.


Thanks


M

[metalguy]

I think there's another situation when you build a 50Watt version of a 100W amp and have a multitap OT.
If in the original 100 Watt version the FB resistor was tied to the 4 Ohm tap then in a 50 Watt version then the same value resistor should go to the 8 Ohm tap, right?
I've seen some TwoRock amps having a 3 way toggle switch at the back so you can choose which tap to connect the FB resistor to or it has another purpose?

[martin manning]

I think there's another situation when you build a 50Watt version of a 100W amp and have a multitap OT.
If in the original 100 Watt version the FB resistor was tied to the 4 Ohm tap then in a 50 Watt version then the same value resistor should go to the 8 Ohm tap, right?

Correct, or the same tap could be used if the value of the feedback resistor is reduced.

I've seen some TwoRock amps having a 3 way toggle switch at the back so you can choose which tap to connect the FB resistor to or it has another purpose?

I think that would be for letting the player choose the level of feedback, although it would also be a way to compensate for a half-power situation.

[norburybrook]

Interesting so my late night, after a couple of glasses of wine, hunch was indeed worth following thanks Metal guy

I'm going to wire mine to my 8 ohm tap right away.

I should do it to my #102 as well.

M

[metalguy]

I have couple of ON-ON-ON toggle switches so I may add one for more options.

[Synchu]

If in the original 100 Watt version the FB resistor was tied to the 4 Ohm tap then in a 50 Watt version then the same value resistor should go to the 8 Ohm tap, right?

I think this is a bit more complicated:
http://www.aikenamps.com/index.php/desi ... e-feedback

It may well turn up that the dependency is linear, but it may not as well.

What I am doing though is that I am playing with the global NFB, until I like it and the scope says there is no weird oscillation going on:)
Niki

[martin manning]

With the caveat that the power amp voltages are the same and all you are doing is compensating for the use of a different secondary tap or half or twice as many output tubes, is is that simple. Nothing wrong with adjusting the feedback to your liking though.

[metalguy]

Theory of course is much more complicated but for our purposes I don't feel like diving so deep and would prefer to make it simple.
It sounds like you already went this way - play with it until you like it and in the mean time make sure weird things don't happen.