Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: worth on September 17, 2012, 10:45:45 am

Title: Princeton Reverb question
Post by: worth on September 17, 2012, 10:45:45 am

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 ?

Title: Re: Princeton Reverb question
Post by: phsyconoodler on September 17, 2012, 11:10:47 am

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.

Title: Re: Princeton Reverb question
Post by: sluckey on September 17, 2012, 11:30:38 am

Quote

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

NFB

Quote

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.

Title: Re: Princeton Reverb question
Post by: worth on September 17, 2012, 01:12:43 pm

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 ?

Title: Re: Princeton Reverb question
Post by: jojokeo on September 17, 2012, 01:29:19 pm

Quote from: worth on September 17, 2012, 01:12:43 pm

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.

Title: Re: Princeton Reverb question
Post by: worth on September 17, 2012, 01:46:01 pm

I meant to say , "4 ohm secondary".

Title: Re: Princeton Reverb question
Post by: sluckey on September 17, 2012, 01:47:46 pm

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.

Title: Re: Princeton Reverb question
Post by: worth on September 17, 2012, 02:05:21 pm

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

Title: Re: Princeton Reverb question
Post by: sluckey on September 17, 2012, 02:16:54 pm

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

Title: Re: Princeton Reverb question
Post by: worth on September 17, 2012, 02:46:36 pm

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

Title: Re: Princeton Reverb question
Post by: Willabe on September 17, 2012, 06:12:41 pm

Quote from: worth on September 17, 2012, 02:46:36 pm

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       :l3:

Title: Re: Princeton Reverb question
Post by: PRR on September 17, 2012, 08:00:20 pm

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.