Can I use a GZ-34 in place of a 5U4GB? (super reverb AB568)

[Tonegeek]

I know it will change the feel of the amp, but is it safe and will I have to change the bias resistors? This is a circuit that has mixed cathode bias and fixed bias with the "balance" control. The original 5U4 is working but after 39 years ...
thanks

[Firestorm]

The power transformer will certainly support the GZ34, which draws less heater current than the 5U4, but the GZ34 will raise the voltages in the B+ supply. The PT in that amp was cranked up a little when Fender went to the 5U4, so you'll likely see something around 485VDC on the plates rather than the 460VDC on the schematic when you go to the GZ34. Make sure your filter caps are up to it. Good tubes will handle it fine. You will probably have to tweak the bias supply for the higher voltage. In that circuit, the 15K load resistor could be made slightly larger, but you'll likely need a non-standard value.

Actually, that whole circuit has so many things wrong with it (that was the circuit that totally destroyed Fender's reputation), you should consider blackfacing the thing. At the very least, I'd get rid of the cathode resistors on the 6L6s and make a truly adjustable bias supply. Just my opinion.

[Tonegeek]

The power transformer will certainly support the GZ34, which draws less heater current than the 5U4, but the GZ34 will raise the voltages in the B+ supply. The PT in that amp was cranked up a little when Fender went to the 5U4, so you'll likely see something around 485VDC on the plates rather than the 460VDC on the schematic when you go to the GZ34. Make sure your filter caps are up to it. Good tubes will handle it fine. You will probably have to tweak the bias supply for the higher voltage. In that circuit, the 15K load resistor could be made slightly larger, but you'll likely need a non-standard value.

Actually, that whole circuit has so many things wrong with it (that was the circuit that totally destroyed Fender's reputation), you should consider blackfacing the thing. At the very least, I'd get rid of the cathode resistors on the 6L6s and make a truly adjustable bias supply. Just my opinion.

Yep I had recommended the blackface mod to the customer but he is holding off since this amp was DOA when he got it and it is costing a bit to get it going. I did the caps so they can take the GZ-34 but I don't see any point in putting back the 5U4. I would love to rip all that tone robbing junk off the outputs too. Actually the amp does not sound too bad as is - probably the fact that it has what appears to be the original RCA tubes and getting lots of sag from the wear. Hopefully the guy will go for the BF mod. I at least convince him to get new outputs and recto.
Thanks

[jelle]

If you plunk in that GZ34 you'll have to lower the screen voltage for those 6L6GC's as well. The max spec for those powertubes is 450V on the screens. I did a similar amp recently (which was used in a stadium concert right after the mod ) and I put in a new 5U4, redid the bias supply to BF specs, got rid of the high end snubbers on the powertubes, replaced the brown turd signal caps with 6PS, and cleaned up leaddress. Owesome amp after the mods.

Have fun,

jelle

[Firestorm]

Some 6L6GCs are rated for 500V on G2. SED/Winged C's from St. Petersburg in particular. There is also a growing consensus that RCA 7027As were really nothing more than rebased 6L6GCs and those were rated 600V plate 500V screen.