Different resistor in a Deluxe Reverb?

[Figaro]

A Deluxe Reverb has a 47 ohm resistor connected to the 820 ohm feedback resistor and other Fenders have a 100 ohm instead of the 47 ohm. Why, and what difference does this make?

[Stevem]

To provide more feedback voltage into the preamp section.
my guess would be that this was done by Fender due to the increase amount of feedback voltage from the less powerful output stage of that amp in regards to keeping all of there preamp sections across there whole amp line reacting the same to being played thru.
The whole blackface line of amps that started off in 1964 was to be very much the same by design with the main differnces being in output power and speakers!

[martin manning]

To provide more feedback voltage into the preamp section.
my guess would be that this was done by Fender due to the increase amount of feedback voltage from the less powerful output stage of that amp in regards to keeping all of there preamp sections across there whole amp line reacting the same to being played thru.
The whole blackface line of amps that started off in 1964 was to be very much the same by design with the main differnces being in output power and speakers!

Decreasing the PI tail resistor (100R to 47R) while keeping the FB resistor (820R) constant will reduce the feedback voltage by half. The combination of output power and speaker load impedance determines the source voltage, and if you look at the BF DR, SR, and TR, you will see that the FB signal in the DR and SR are similar (DR is a bit lower), but the TR has more than twice as much FB voltage going to the PI.

[sluckey]

A Deluxe Reverb has a 47 ohm resistor connected to the 820 ohm feedback resistor and other Fenders have a 100 ohm instead of the 47 ohm. Why, and what difference does this make?

All the AB763 amps that use an 8Ω speaker load use 47Ω for NFB. All the AB763 amps that use 4Ω or 2Ω speaker load use 100Ω for NFB. An 8Ω OT secondary will produce a larger voltage than a 4Ω OT secondary. Fender used the different value resistors to provide approx. the same amount of NFB for each configuration.

Check out the note on this Showman schematic. This amp was available with single or dual speaker cabs.

[martin manning]

Interesting. But note my example above- the TR has twice the FB voltage that the SR has.

[sluckey]

I'm aware of that. Double power into double impedance gives double voltage across that impedance. I'm not disagreeing with you. I think we are mostly saying the same thing.

I have wondered why the SR did not use a 200Ω resistor, but I suppose then the SR would not have it's unique sound and feel.

Oh, the Vibroverb was the other amp I was trying to remember that used the 47Ω resistor.

[martin manning]

The Twin is the odd one at 2x all the others. The Vibroverb and Super Reverb are almost identical.

[sluckey]

The Vibroverb and Super Reverb are almost identical.

Except for the OT and speaker loads. And that 47Ω v. 100Ω feedback resistor.

The Dual Showman uses the same PT and OT as the TR. Both have a 4Ω speaker load and both use the 100Ω resistor. But the Single Showman uses a different OT with an 8Ω secondary and it has the 47Ω resistor.

[martin manning]

The Vibroverb and Super Reverb are almost identical.

Except for the OT and speaker loads. And that 47Ω v. 100Ω feedback resistor.

...and those differences make the feedback signal almost exactly the same (~1V RMS) at rated power.

The Dual Showman uses the same PT and OT as the TR. Both have a 4Ω speaker load and both use the 100Ω resistor. But the Single Showman uses a different OT with an 8Ω secondary and it has the 47Ω resistor.

So then the dual Showman is like the Twin, with the high FB voltage (~2V RMS), and the single Showman is in-between (~1.4V RMS).

With the DR being the lowest at about 0.7V RMS, that makes four different FB levels in the BF family.