Rectifier Swap in new '68 Custom Princeton Reverb
Hey Skip--I just bought a '68 Custom Princeton Reverb, one of Fender's newest entries of Princetons after listening to David Barrett's smal amp reviews. Marvelous amp, by the way. David recommends swapping the GZ34 (or 5ar4) rectifier for a 5u4GB for what I assume is the additional sag the tube creates. The only issue I see is that the 5u4GB has a filament draw of 3amps vs 2amps from the GZ34. I know that Princetons at one point in time ran the 5u4GB, but I also know that the new Fender has a custom transformer. Think it can handle it?
Hey Skip--Thanks for the reply. The filament current is really my main concern. My thought is that Fender put a transformer in that can handle the 5u4. The Princeton's bounced back and forth between the two rectifiers over their history and I'm sure Dave's original '68 ran with those. I'll try it out. I do have a 5R4 on hand. It's the old potato masher variety, so I don't I'll have to see if it will fit to try it out. I'm actually liking the stock GZ34.
I'm pretty sure that the Groove Tubes in the amp are all JJs, which mean the 6v6 act more like a 6l6 and can be run pretty darned hot, which is probably where they are biased with the GZ34. Will swapping the rectifier require a re-bias on those?
Hello Bushdog- I don't agree that JJ 6V6s "act more like a 6L6". Swapping rectifiers can affect the bias, but in your case I would say that you are safe. 25 to 30 milliamps would be a good range for the 6V6s.
Thanks for your response Skip. I've tried out the 5u4 but think that the GZ34 gives the amp more balls.
Hello Bushdog- Another good question! You are pretty sharp to notice that the 5U4 draws more filament current than the GZ-34! In vintage Fender amps it is no problem. People have used 5U4s in amps that call for GZ-34s for many years. In fact, I have a Fender tech sheet from 1967 saying that all amps using GZ-34s should use the 5U4 for replacement purposes. It is obvious that a vintage Fender transformer can supply the three amperes of filament current needed for the 5U4 without any problems.
I imagine that your re-issue amp can handle the 5U4, but I really can't say for sure. Two other rectifiers you might consider are the 5R4 and the 5V4. Both have two ampere filaments, but will run at a reduced "B+" voltage. This reduced voltage is another reason (along with sag) why the can sound warmer and more full with a different rectifier.