Princeton Reverb question

[worth]

Question 1 ; Why is the 25uf/1.5K grounded through a 47 ohm resistor instead of going straight to ground ?  Question 2 ; If I'm using an OT with a 4K primary ( using 6L6s' ) should this 47 ohm resistor be changed to 100 ohms ?

[phsyconoodler]

It forms a voltage divider for the negative feedback.You can change it to 100 ohm and see what it sounds like.It will work just fine as is with 6L6's.It's all about the negative feedback the amp was designed with.
  The AB763 amps have a different phase inverter so leave it as is and see what it sounds like.I've built lots with 6L6's and they sound great as is.

[sluckey]

Question 1 ; Why is the 25uf/1.5K grounded through a 47 ohm resistor instead of going straight to ground ?

NFB

Question 2 ; If I'm using an OT with a 4K primary ( using 6L6s' ) should this 47 ohm resistor be changed to 100 ohms ?

No. The OT secondary tap determines the value of that resistor, not the primary impedance or type tube.

[worth]

My OT has a 4k primary and a 4K secondary... so, should I use a 100 ohm instead of the 47 ohm  in the Princeton Reverb ?

[jojokeo]

My OT has a 4k primary and a 4K secondary...

That must be incorrect? Don't you mean 4 ohm secondary? Just use a test lead and try a few different values to hear what the different amount of feedback amounts sound like and what you prefer? No set rule there.

[worth]

I meant to say , "4 ohm secondary".

[sluckey]

I should have said that the speaker impedance determines the value of that resistor. But as long as you're deviating from the PR circuit you may use whichever value you want. Just don't go so big that you start messing with the bias of that tube. I would just use the 47 ohm. Larger values will give more NFB.

[worth]

I replaced the 47 ohm with a 100 ohm.... not something that I would recommend.. sounds like a blanket over the amp.

[sluckey]

Would that be a light weight synthetic blanket or a heavy woolen blanket? 

[worth]

A heavy Civil War era woolen blanket soaked from a driving rain.

[Willabe]

A heavy Civil War era woolen blanket soaked from a driving rain.

Well alright then Worth, now we know what direction NOT to go with the FB R on a PR!


             Brad      

[PRR]

Leave the 47r alone. Change the 2.7K to 1K (too mellow), 5K (a bit brasher), 10K (nearly naked tube sound).

On-off-on switch with 2.7K, 7.5K and disconnect (center) may be useful.