Difference between GZ34 and 5U4GB

[Alan0354]

The older Fenders used GZ34 and the newer SF used 5U4GB. What is the difference between the two. Are they interchangeable?

[Gingertube]

Not really interchangable.

The GZ34 is equivalent to a 5AR4, specs are 2 Amp @5V heater, 60uF max 1st Cap, good for about 160mA DC with an internal resistance of about 175 Ohms causing about a 28V drop across the tube with that 160mA current.

The 5U4 is closer to a 5AS4, specs are 3 Amp @ 5V heater, 40uF max 1st cap, good for 225 mA with an internal resistance of about 195 Ohms causing a 44V drop across the tube at that 225 mA current.

Putting a 5U4 into a GZ34 socket is not recommended because of the higher heater current requirement

Putting a GZ34 into a 5U4 socket MAY be OK but you need to check if the lower current rating (160mA instead of 225mA) will be OK. You will also get a few more volts on the B+ due to lower internal resistance. If you don't know the current draw then don't do it.

The 5AS4 is a "forgotten" tube and can often be found dirt cheap. It is actually slightly higher rated than the 5U4. I put one into a Fender Bandmaster in lieu of the 5U4 and it worked beautifully.

Cheers,
Ian