rectifier tube sub? GZ34-5U4GB

[homerayvaughan]

In a Blackface, AB763 Fender amp that takes a GZ34 rectifier, could you sub a 5U4GB?
In a silverface Fender with a 5U4GB could you sub with a GZ34?

[glahnb]

I don't think you can, doesn't the voltage change or something? I'm sure a tech or someone in the know will answer better.

[Old Tele man]

two (2) major differences:

GZ34 "drops" about 17-20 Vdc while 5U4GB "drops" about 40-44 Vdc. <--so, idle bias adjust is mandatory!

GZ34 filament current is 1.9 Amps while 5U4GB filament current is 3.0 Amps. <--about 58% more current!

So, the question is: can the PT handle the extra filament current?

...most of the "later" silverface models were equipped with 5U4GB so PT was certainly capable of the current, but the "early" blackface amps were equipped with 5AR4/GZ34 and might NOT be able to supply the filament current without heating up (and possibly dying).

[trdlasvegas]

I can verify what the Tele Man just said as I've been measuring these two rectifiers in a customer's old Fender Super Reverb, and obtained those exact results.

The bottom line is I just ordered a new power transformer from Mercury Magnetics for the customer. Their transformer will handle the 5U4G with room to spare.

-Tony

[homerayvaughan]

two (2) major differences:

GZ34 "drops" about 17-20 Vdc while 5U4GB "drops" about 40-44 Vdc. <--so, idle bias adjust is mandatory!

GZ34 filament current is 1.9 Amps while 5U4GB filament current is 3.0 Amps. <--about 58% more current!

So, the question is: can the PT handle the extra filament current?

...most of the "later" silverface models were equipped with 5U4GB so PT was certainly capable of the current, but the "early" blackface amps were equipped with 5AR4/GZ34 and might NOT be able to supply the filament current without heating up (and possibly dying).
So then you cannot sub one for the other, either way...right?:confused: Sorry, but I'm not up on a lot of technical stuff.

[Old Tele man]

..no, it all depends upon the ACTUAL power transformer installed in your amp (you don't say what year):

1) is it a true blackface AB763 PT which originally used 5AR4/GZ34, in which case it MAY NOT be able to "safely" use a 5U4GB, or...

2) is it a later blackface AB763 PT which originally used 5U4GB, in which case it WILL safely use a 5U4GB, since that's what it was designed for.

...how can you tell? Have your tech measure the filament voltage with each rectifier installed, it should be 5.0 VAC ±0.5VAC or better (remember: too high voltage is just as bad as too low voltage)!

...if the measured filament voltage "sags" to less than 4.5-4.6VAC with the 5U4GB installed, you're probably gonna have PT "heating" problems immediately and possible PT failure in the future...meaning your PT was designed to provide 1.9 Amps current for 5AR4/GZ34, not 3.0 Amps for 5U4GB.