'65 Fender Princeton Reverb Reissue (PRRI) Mod questions

[Groberts]

I've spent a bunch of time hand selecting some great NOS tubes for my Fender Princeton Reverb Reissue (PRRI). I also installed a 12" speaker on a new speaker board. Sounds killer.

• I would like to modify the Tremolo so it can be set slower. What needs to happen to do this? Is it a capacitor change?
• I am considering an Output Transformer swap, but only if it enhances the tonal response. What effect will an OT upgrade have? Which OT should I chose? I looked at Mercury Magnetics website and was overwhelmed with the choices. Is that the brand to go with?
• Is the PRRI really a Fixed Bias amp? If so, how can I bias for different tubes? I've changed Rectifier tube and Power Tubes. I installed NOS Tung Sol 6V6GT's and a Mullard 5AR4 Rectifier is going in tomorrow.
• Can the PRRI be modded with a Bias pot? Where is the best place to locate the BIAS pot? What is needed?

Thanks! Gary

[Dawg76]

Output Transformer: David Allen-designed Heyboer OT; model TO22 might be an option: http://www.allenamps.com/parts.php#transformers
I had one of these put in a Blues Junior, but their also noted to be great for a 2-6V6 amp.

[Groberts]

Originally Posted by Dawg76

Output Transformer: David Allen-designed Heyboer OT; model TO22 might be an option: http://www.allenamps.com/parts.php#transformers
I had one of these put in a Blues Junior, but their also noted to be great for a 2-6V6 amp.

Re-bias: When you change output tubes in this amp, you should have the amp re-biased. This can be done in a couple of ways: Have an adjustable bias pot installed; or have a resistor installed to change the value (this is what my amp tech did for my silverface princeton reverb).

Thanks Dawg. yeah I know I need to have the amp re-biased for the power tubes I have in place. I would rather have a Bias Pot so if I decide to try different tubes, I can break out my volt meter and set it up myself.

I read good things about those heyboer transformers in another thread. I am pretty handy with a soldering iron having been an electronic tech in my early career. So I might give that a try myself.

Thanks! Any other feedback is most welcome!

[TweeDLX]

Originally Posted by Dawg76

Output Transformer: David Allen-designed Heyboer OT; model TO22 might be an option: http://www.allenamps.com/parts.php#transformers
I had one of these put in a Blues Junior, but their also noted to be great for a 2-6V6 amp.

Re-bias: When you change output tubes in this amp, you should have the amp re-biased. This can be done in a couple of ways: Have an adjustable bias pot installed; or have a resistor installed to change the value (this is what my amp tech did for my silverface princeton reverb).

David has some great trannies at good prices. Depending on the sound you want from this amp, consider going to a BIGGER OT. This will give you extra clean headroom, better bass response. I used a bigger PT & OT (Hammond 1645) in my SF Princeton and love the sound. Allen's TP25 is a nice upgrade for a PT, allows you to go with 6L6's if you wish.
Go here for how to install a bias pot in your Princeton: http://www.el34world.com/charts/Biascircuits.htm

Mike

[Groberts]

Originally Posted by TweeDLX

David has some great trannies at good prices. Depending on the sound you want from this amp, consider going to a BIGGER OT. This will give you extra clean headroom, better bass response. I used a bigger PT & OT (Hammond 1645) in my SF Princeton and love the sound. Allen's TP25 is a nice upgrade for a PT, allows you to go with 6L6's if you wish.
Go here for how to install a bias pot in your Princeton: http://www.el34world.com/charts/Biascircuits.htm

Mike

If I go with the TP25, can I still use my 6V6GT Tubes? Or is that a 6L6 Transformer only. The website doesn't say anything about gV6's with that transformer.

[TweeDLX]

Originally Posted by Groberts

If I go with the TP25, can I still use my 6V6GT Tubes? Or is that a 6L6 Transformer only. The website doesn't say anything about gV6's with that transformer.

You're still good with the 6V6 tubes, just get them biased correctly. My Princeton & my Accomplice (both using that PT) ran around 416VDC on the plates, which is OK for 6V6 tubes. If you also go with David's TO30, you'll get the headroom and bass response I mentioned previously. If you want typical Princeton breakup, go with one of the smaller output trannies. Both of these work with 6V6's. For myself, I like the idea of a small amp that can keep up with the bigger guys...I use a Barber Small Fry when I want OD.
http://www.allenamps.com/parts.php#transformers

Mike

[Groberts]

I am now thinking if I change the OT, I may as well replace the PT as well.

Are the Transformers from Allen on par or better than some of the others? What about Mercury Magnetics? Where do they fit in the scheme?

[mark norwine]

Question 1: trem speed.

There are 3 capacitors which form the RC time constants which yield 360 degree (positive) feedback. They are .02uF.....01uF......01uF.

Change the middle .01uF caps to a .02uF That will slow things down considerably. Go higher & you run the risk of stalling the oscillator.

[TweeDLX]

Originally Posted by Groberts

I am now thinking if I change the OT, I may as well replace the PT as well.

Are the Transformers from Allen on par or better than some of the others? What about Mercury Magnetics? Where do they fit in the scheme?

MM's are "supposed" to be the best, but I have no experience with them to back up that claim. Allen's are made by Heyboer to his spec. They sound good to my ear. There are people here who will argue one way or another about the quality of any certain part, but in the end, if it sounds good to you, that's all that matters.

Mike

[notpetrucci]

The PRRI has a bias pot.

Look at the back of the amp. It's in the back left corner behind the PT.

As far as mods go, go for RCA preamp tubes. You'll shit a brick.

Disclaimer: I love me some Shiner Bock

-Jon

[Groberts]

Originally Posted by notpetrucci

The PRRI has a bias pot.

Look at the back of the amp. It's in the back left corner behind the PT.

As far as mods go, go for RCA preamp tubes. You'll shit a brick.

Disclaimer: I love me some Shiner Bock

-Jon

Already drinking the NOS RCA Preamp tube Kool aid! You're RIGHT! I shit a brick! STunning. Just STUNNING. Also tried a pair of RCA Black Plate Grey glass 6V6GT's ...but need to get them biased. Without biasing, doing quick listens, the NOS Tung Sol 6V6GT's were just a bit more stunning than the RCA's. But I think its not fair to make that call without biasing the tubes properly.

Did a gig last night outside and felt the bass and low end fullness was a bit light. Granted, it was outside. But I think I want to go for the larger Output Transformer that will give a little more headroom and bigger bass. already have a 12" Mojo tone speaker in place with great NOS tubes. Mostly done!

Didn't know the PRRI had a Bias pot! Damn ...I never opened it. Just read that Princeton reverbs were fixed bias. Maybe that was only true for BF and SF Princetons, but not the reissue. Cool if the pot is there! Thanks!

[mark norwine]

Originally Posted by Groberts

Didn't know the PRRI had a Bias pot! Damn ...I never opened it. Just read that Princeton reverbs were fixed bias. Maybe that was only true for BF and SF Princetons, but not the reissue. Cool if the pot is there! Thanks!

Cool that the pot is there....true.

But the presence or absence of a pot is not what defines "fixed bias". "Fixed" bias does not mean.....nor has it ever meant...."lacking a pot".

Fixed bias means that the bias of the tube is controlled by a permanent negative voltage applied to the control grid. Whether or not that permanent negative voltage has an adjustment control [pot] in the circuit is of no bearing.

[Groberts]

Originally Posted by mark norwine

Cool that the pot is there....true.

But the presence or absence of a pot is not what defines "fixed bias". "Fixed" bias does not mean.....nor has it ever meant...."lacking a pot".

Fixed bias means that the bias of the tube is controlled by a permanent negative voltage applied to the control grid. Whether or not that permanent negative voltage has an adjustment control [pot] in the circuit is of no bearing.

Thanks for the info. I honestly had no idea what Fixed Bias meant. I assumed it meant there was no bias pot. My bad. In any event, I am delighted to learn I can get my amp biased without having to change resistors. It would be so much easier just using a Voltmeter and getting the voltage and current set correctly. (So I think) Laughs.