Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jrusto on March 17, 2024, 01:40:14 pm

Title: Hoffman Princeton build in a PRRI Chassis
Post by: Jrusto on March 17, 2024, 01:40:14 pm

This is a rather crazy problem for a Princeton Reverb as far as I know.  It is too clean.  Breakup doesnt happen till I get to 6.6 and sounds great at 8 but obviouly too loud for home use. 

Its a Hoffman build using the PRRI power transformer, reverb transformer, and an Allen TO20 Output transformer. All other parts came from the Hoffman kit.

The voltages seem a little high. The B+ is at 475V if I remember right.  I have it biased at 23mA with matched JJ Tubes.

The amp really sounds great and the cleans are very warm and nice.  Highs are pretty pronounced but I attribute a lot of that to my Tele.  Seems like break up used to begin around 4.5 on the volume when it was a PRRI.  I have a Weber 10f150T that has been in the amp for 12 or so years. 

Anyone got any ideas.  I could put the old PRRI output transformer back in but when it was in there the amp was pretty fartty sounding.

Thanks
Rusty   

Title: Re: Hoffman Princeton build in a PRRI Chassis
Post by: jim on March 17, 2024, 02:40:41 pm

Be loud and proud.  You built a great sounding Princeton exactly the way it wants to sound.  Push it with a pedal.  Are you using a 5U4 rectifier?--if not give it a try.  Jim

Title: Re: Hoffman Princeton build in a PRRI Chassis
Post by: sluckey on March 17, 2024, 03:15:38 pm

Quote from: Jrusto on March 17, 2024, 01:40:14 pm

Breakup doesn't happen till I get to 6.6 and sounds great at 8 but obviously too loud for home use... Seems like break up used to begin around 4.5 on the volume...

If you changed the pots that would explain the breakup difference.

Quote

The voltages seem a little high. The B+ is at 475V if I remember right.

Did you check voltages ***BEFORE*** the conversion?

Title: Re: Hoffman Princeton build in a PRRI Chassis
Post by: Jrusto on March 17, 2024, 04:38:46 pm

I am using a 5AR4 rectifier in it now.   I just got a 5U4GG in but haven’t tried it yet. 

I did change the pots and the new one doesn’t let sound pass until it reaches 2 so that could be some of it for sure. 

I wish I had taken the voltages prior to  disassembly but I did not.  The PRRI schematic call for 445V on B+.  I can’t remember if the 475V I measured on B+ was before or after populating the tube sockets.  I’ll have to disassemble it and measure again. Be back to you tonight.

It does sound glorious though.

Rusty

Title: Re: Hoffman Princeton build in a PRRI Chassis
Post by: Jrusto on March 18, 2024, 11:52:11 am

Looks like everything is fine.  I have attached the layout with some voltages on it and I am closer to the PRRI voltages rather than the AA164.  435 on the B+.  Guess I have to dig out my boost pedal.

Thanks for all your help.

Rusty

Title: Re: Hoffman Princeton build in a PRRI Chassis
Post by: sluckey on March 18, 2024, 11:59:17 am

I'm happy with those voltages.