Princeton Questions

rdchapman · June 6th, 2006, 02:38 PM

I took the plunge and bought a 67 BFPR. so, I get the amp and in the process of looking it over and replacing the power cord, I note the amp has a 5u4 rectifier tube and the chart in the amp calls for a 5ar4/gz34 tube. So, I get a gz34 and put it in as I have read it is easier on the PT and soft starts the tubes. I put it in and the amp seems warmer with a little more headroom. More like the others I have played as it was brighter/harsher with the 5u4 or so it seems.

So, anyway, I also have a SF Princeton, that by the transformer dates, seems to be a 69. It does not have a frame around the grill cloth. The tube chart in the amp calls for a 5u4 but there is no circuit numbere listed on it. Can I put a gz34 in this also and get the same results? Would I hurt the amp? I understand Fender changed tubes to keep costs in line as the gz34 was a more expensive tube. any insight would be greatly appreciated
Richard

PhatTele · June 6th, 2006, 05:50 PM

The voltages in the silverface Princeton/Princeton Reverbs run a little hotter than the blackface amps because they use a different power transformer. Because of that, Fender started using the 5U4 rectifier to drop the plate voltages on the 6V6s (it's a fixed biased amp). So, the 5U4 is correct for that particular amp.

If you want to use a 5AR4/GZ34 rectifier and keep that power transformer, you need to change the bias range resister to help drop the plate voltages down. I actually add a small bias trim pot as described by Doug Hoffman on his site:

http://www.hoffmanamps.com/charts/Biascircuits.htm

Otherwise, you can replace the power transformer to something that is designed to run with a 5AR4/GZ34. I've done both. I actually like the sound of the stock 5U4 setup (though I usually put in the bias mod anyway).

Tremo · June 6th, 2006, 06:17 PM

No problem running a 5AR4 if you have JJ 6V6s installed. Rebias will be mandatory.

rdchapman · June 7th, 2006, 08:37 AM

Thanks Guys!
PhatTele, I too like the sound of the SF Princeton amp, but i was interested in easing the strain on the transformer if that was the case. That amp is nothing like the blackface with the 5u4 in it. It has the most beautiful clean I think I have ever heard. Tremo, the JJ thing is something that might be fun to try. Thanks. I love this place!
Richard

June 6th, 2006, 02:38 PM	#1 (permalink)
rdchapman Tele-Meister Join Date: Apr 2004 Location: Saratoga, NY Age: 55 Posts: 418	Princeton Questions I took the plunge and bought a 67 BFPR. so, I get the amp and in the process of looking it over and replacing the power cord, I note the amp has a 5u4 rectifier tube and the chart in the amp calls for a 5ar4/gz34 tube. So, I get a gz34 and put it in as I have read it is easier on the PT and soft starts the tubes. I put it in and the amp seems warmer with a little more headroom. More like the others I have played as it was brighter/harsher with the 5u4 or so it seems. So, anyway, I also have a SF Princeton, that by the transformer dates, seems to be a 69. It does not have a frame around the grill cloth. The tube chart in the amp calls for a 5u4 but there is no circuit numbere listed on it. Can I put a gz34 in this also and get the same results? Would I hurt the amp? I understand Fender changed tubes to keep costs in line as the gz34 was a more expensive tube. any insight would be greatly appreciated Richard

June 6th, 2006, 05:50 PM	#2 (permalink)
PhatTele Tele-Holic Join Date: Jun 2003 Location: Richmond, VA Posts: 756	The voltages in the silverface Princeton/Princeton Reverbs run a little hotter than the blackface amps because they use a different power transformer. Because of that, Fender started using the 5U4 rectifier to drop the plate voltages on the 6V6s (it's a fixed biased amp). So, the 5U4 is correct for that particular amp. If you want to use a 5AR4/GZ34 rectifier and keep that power transformer, you need to change the bias range resister to help drop the plate voltages down. I actually add a small bias trim pot as described by Doug Hoffman on his site: http://www.hoffmanamps.com/charts/Biascircuits.htm Otherwise, you can replace the power transformer to something that is designed to run with a 5AR4/GZ34. I've done both. I actually like the sound of the stock 5U4 setup (though I usually put in the bias mod anyway).

June 6th, 2006, 06:17 PM	#3 (permalink)
Tremo Banned Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,803	No problem running a 5AR4 if you have JJ 6V6s installed. Rebias will be mandatory.

June 7th, 2006, 08:37 AM	#4 (permalink)
rdchapman Tele-Meister Join Date: Apr 2004 Location: Saratoga, NY Age: 55 Posts: 418	Thanks Guys! PhatTele, I too like the sound of the SF Princeton amp, but i was interested in easing the strain on the transformer if that was the case. That amp is nothing like the blackface with the 5u4 in it. It has the most beautiful clean I think I have ever heard. Tremo, the JJ thing is something that might be fun to try. Thanks. I love this place! Richard