Bias Tap on PT?

[Clear Guy]

I am rebuilding a Princeton Reverb reissue using the Hoffman AA1164 turret board (latest version). My question is if I have a 50V bias tap (Red/Green stripe) on my power transformer do I use that instead of the red wire coming off pin 6 from rectifier to the bias tap point on the board layout. Or just tie off the red/grn wire from the PT to nothing and not use it al t all. Is the Hoffman layout already designed to not use the PT bias tap??? Thx in advance

[plexi50]

If you have a 50 Volt bias tap then yes that's what you should use for the bias feed.

[mresistor]

If you have a 50 Volt bias tap then yes that's what you should use for the bias feed.

I second that motion - connect your PT bias tap to the terminal that the purple line called "bias tap" goes to on the visio layout.

[sluckey]

I would not use the bias tap. The Hoffman board is designed to expect the AC supply to come from one of the red PT leads just like the original. You will need to modify the circuit to use the bias tap. It's a simple mod, but why? There is no advantage to using the bias tap. It's simpler to just tape it off.

[Clear Guy]

"I would not use the bias tap. The Hoffman board is designed to expect the AC supply to come from one of the red PT leads just like the original. You will need to modify the circuit to use the bias tap. It's a simple mod, but why? There is no advantage to using the bias tap. It's simpler to just tape it off."

I kind of had a feeling this was the case. I am going to leave it out, tied up with heat shrink and and nicely buried in the chassis. Thx for the the help, guys>   

[plexi50]

Now this is an authentic second motion!!!
Just kidding guys. I find many amps that use the diode/resistor combo for the bias tap.

 

[Paul1453]

It is nice not to need a special PT with a bias tap.

Sluckey has lots of experience, and most likely has given you the best option to take.

I might look at which way is easier and requires the least amount of new components and space.

If you already have the HT tap bias circuit built on your board,
then you don't need any new parts or space for them,
and just taping up the bias tap and connecting a wire to the HT tap for bias,
would seem to be the simplest and best, to me.   

[sluckey]

I wasn't trying to make a point about one way is better than the other. Either way works just fine. My point was for a Princeton Reverb built on a Hoffman turret board ........ well, read my first reply. If you ain't familiar with a Princeton Reverb or Hoffman's PR board, then maybe take a look at the schematic and then look at Hoffman's documents.

[plexi50]

I am not familiar with the Hoffman PR board.  I will look at it. The Princeton Reverb is a great amp. That lead me to show you one in a new post that was way past a basket case. My pic's are not the greatest but you will see the general insanity of the mods etc.

[sluckey]

Yes, I'm eagerly waiting to see that. Quit teasing us! 

[plexi50]

 

[tubeswell]

I'm going to endorse using the bias tap.


If you have one, use it for sure.


If you have a separate bias winding, just as good if not better.

[plexi50]

I'm going to endorse using the bias tap.


If you have one, use it for sure.


If you have a separate bias winding, just as good if not better.

I third that motion!  I ask for a recess while i confer with counsel!

[sluckey]

Gonna have to start calling you flipflop.

[plexi50]

Gonna have to start calling you flipflop.

Three is the charm. 

[kagliostro]

I think that you can observe a difference wiht a bias tap separated circuit only if the bias voltage isn't "tapped" and there is a

dedicated/separated winding for the bias voltage and you apply a bridge rectifier instead of a single diode

this way you'll have both half-waves, the attached image shows an example

Franco

[trobbins]

Are the two options both half wave rectifier, but from different voltage taps, and hence different R in the RC filter?

[kagliostro]

Are the two options both half wave rectifier, but from different voltage taps, and hence different R in the RC filter?

This is an interesting question and I suppose I know where you want to arrive

Franco

[sluckey]

Are the two options both half wave rectifier, but from different voltage taps, and hence different R in the RC filter?

Yes. The PR gets the AC from the PT HT winding and uses a 100K to drop the voltage to a usable bias voltage level. The 50VAC bias tap uses a much smaller resistor, typically 470Ω for a Fender amp.

[mresistor]

Sorry guys - I actually couldn't remember if I used the bias tap or not, and I did not, in my HPR I tied off the bias tap just as Sluckey has stated. I had to pull the chassis and take a look this morning. You can see the blue wire coming from the rectifier and the green/red bias wire tied up next to the PT end bell. It's an Allen TP-25.

[drgonzonm]

I have to agree with Steve L, on this, do not use the bias tap.  The turret board you are using is already designed for a PT with out a bias tap. Build the amp to the original schematic, get it operating.  IMO, you will have enough fun, starting the amp, without having to worry about pre startup modifications to the amp.