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Old 17th May 2011, 10:48 AM   #1
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Default GZ33 in place of 5U4 can it be done?

I blew the 5u4 in my Fender. I have a lot of Mullard GZ33's and looking at the data sheets it looks like it would be ok as a substitute. What do you guys think?
Thanx.
Mario.
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Old 17th May 2011, 11:10 AM   #2
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One thing to watch is whether the GZ33 is a 53KU like a Cossor, which takes a smaller input capacitor like 16uf. The GZ33 is considered as a CV378 type but so is the 53KU, and I've never quite understood the differences here. I've swapped GZ33 and 5U4 in my amp, but that's an audio amp, and my input cap is 10uF. I don't know the cap in your circuit.

Andy
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Old 18th May 2011, 12:16 AM   #3
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The GZ33 is not = CV378. The GZ37 is = CV378. While it looks like a GZ37, the GZ33's electrical behavior lies between that of the GZ34/5AR4 and the GZ37. The GZ33 draws 3 A. of heater current, which is what a 5U4 draws. I don't know if a GZ33 can stand a 60 μF. cap. I/P filter, like the GZ34 can, but I doubt the 40 μF. 5U4 limit is a problem.

FWIW, I think the biggest issue in substituting a GZ33 for a 5U4 is reduced forward drop. In a guitar amp, a somewhat higher B+ rail might be a good thing, provided the filter cap. WVDC rating is not exceeded.
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Old 18th May 2011, 01:08 PM   #4
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The GZ33 Mullard is actually a short bottle on this datasheet - not the tall bottle GZ37 type. http://frank.yueksel.org/sheets/129/g/GZ33.pdf

On the other hand, the GZ33 here is a tall bottle -
GZ33 @ The National Valve Museum

There it says the GZ33 is "sensibly equivalent" to 53KU - CV378 - CV5745 - U54

The Cossor 53KU is called "sensibly equivalent" to:- GZ37 - CV378 - GZ33 - U54

I agree that the GZ37 and GZ33 are different, but there's a lot of confusion in the way these are referred to!

Andy
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