Multitape OT

[bluescaster]

Hi,

I bought a Weber Multitap OT for my handmade AB763 amp. I load it with a 8 ohms cab (2x10" Jensen C10Q 16 ohms in //). Super Reverb schematic use 820/100 NFB resistors with a 2 ohms OT. Can i use 8 ohms tap for speakers and 2 ohms tap for NFB please ?

Thank you.

[xtian]

Bonjour! In English, "tape" is an adhesive. "Tap" is the word you're looking for.

You can use any tap on the OT secondary to get NFB into the circuit, and you can experiment with which one sounds best.

What's the theory? I don't know! Maybe someone will explain.

[bluescaster]

Oops, Sorry for my bad english. I changed it. Thank you.

[David Root]

The voltage off each OT secondary tap varies by the square root of 2, ~1.414.
So if the 2 ohm tap gives x volts, the 4 ohm tap gives 1.4x and the 8 ohm tap gives 2x. The higher the voltage, the greater the NFB voltage. So using the 2 ohm tap gives the lowest NFB, and the correct AB763 Super Reverb NFB too.

Are you sure two C10Q speakers can handle the 50W from a Super Reverb? There was a reason Fender used four of them.

[bluescaster]

So, i use 820 and the 2 ohms tap for the NFB and the 8 ohms tap for the speakers ?

If I use 1k5 (2*820) and use the 8 ohms tape for NFB and speaker, it will be the same, no ? Deluxe reverb use 820/47 with 8 ohms tap. Same ratio !

The 2 speakers see half of the power each and are 35 W speakers.

[bluescaster]

I found this on http://www.tedweber.com/:

[img2313]http://www.tedweber.com/media/kits/6a40_schem.jpg[/img]

2700 for 8 ohms tap ! I don't understand.

[pdf64]

I don't understand the Weber rationale either.
As the amp / feedback is configured as a voltage amplifier, it makes more sense to me to adjust the feedback ratio to return the same voltage to the 'other' input.
For the same power level, as the impedance doubles, the voltage increases by 1.414.
So if 820/100 at 2ohms, then 1k2/100 at 4 and 1k6/100 at 8 etc.

[robrob]

[EDIT: I changed my error of 2.14 to 1.41]

Since we are on the topic of equivalent feedback, my understanding is when moving the feedback tap from the 2 ohm tap to the 4 ohm tap to keep the feedback the same you would increase the feedback resistor value by 1.41 (sqrt of 2), and do it again from 4 to 8 ohms and again to move from 8 to 16 ohms.

But I've seen from multiple sources that the JTM45 has 2.8 times more feedback using the 16 ohm tap versus the 5F6A Bassman using a 2 ohm tap (2 x 1.4). Shouldn't it be 3 x 1.4 = 4.2 times more feedback?

[bluescaster]

Sqrt(2)=1.41 not 2.14

[robrob]

Math in public

I meant to write 1.41 x 2 = 2.8, shouldn't it be 3 x 1.41 = 4.2?

[martin manning]

SQRT(16/2)=2.83, or SQRT(2)^3 if you prefer.

[gui_tarzan]

no math! no math! it makes my head hurt! argh!!!

[robrob]

Thanks Martin, now it all makes sense.

So to make sure I got it right, when moving from 2 to 4 ohms the factor is 1.41, 2 to 8 ohms the factor is 2, and 2 to 16 ohms 2.83.

[martin manning]

Correct!