Rectifier tube substitution

[Randy]

Can I safely substitute the GZ34 rectifier in my JTM-45 for a 5U4GB or similar? Will I need to re-bias if I do? I'm trying to achieve earlier breakup, more sag and lower volume.

 

[Blue Strat]

You can sub a GZ34 (isn't that the stock tube? It certainly was in the original JTM-45) but you MUST rebias. Making this swap will do exactly the opposite of what you're trying to do. The amp will be cleaner with less sag and more headroom.

What you want to use is a 5R4GB rectifier (and rebias for maximum effect). The great thing about these is that they're cheap (about 1/8th the price of a NOS GZ34).

 

[johngalt]

Mike. One of us has misread his post. I think he wants to substitute a 5U4GB for the stock GZ34. That should increase sag a bit.

 

[Blue Strat]

Ah, you're correct! My response is the same though...the 5R4 will give more sag and NOT increase heater current demands on the power transformer (as the 5U4 will).

 

[Randy]

Originally posted by Blue Strat
Ah, you're correct! My response is the same though...the 5R4 will give more sag and NOT increase heater current demands on the power transformer (as the 5U4 will).

Thanks for the info! So I do need to re-bias if I change it at all. Which type is more desireable for this - a 5R4GB or a 5U4GB?

 

[Blue Strat]

A good generic answer is, YES, you should rebias whenever changing rectifier types. The 5R4GB will sag more and is safer (in your amp) than a 5U4.