5AR4 vs 5U4

[rfgordon]

The guy whose 66 Pro Reverb I just fixed wants to know if he can safely run a 5U4 instead of 5AR4. Now I know that a 5AR4 draws 2 A, and the 5U4 draws 3 A of 5 V filament current. So, the question is, can the PT in this old Fender take it?

I measured the voltage with the 5AR4 as 5.12 VAC, with the amp full on and warmed up. The 5U4 measured 5.17 VAC. That's a fairly small drop in voltage, given a 1 A change in load, so is it safe? Seeing as how there aren't any specs on these old trannies, I figured I'd seek the wisdom of the collective.

Thanks.

[jelle]

Hi,

I never cheched he voltages but I gigged a lot with a 5U4 in my superreverb. Worked for me! If the stage was to ig I'd put the GZ34 back in. I liked the option of increasing headroom. It makes a big difference. Be carefull with bias....

Have you solved the bass note problem? What was it?

Jelle

[rfgordon]

Jelle,
Yeah, the problem was a parasitic oscillation. I moved the tone caps to the pots, added a bunch of shielded cable and redressed the reverb control wires.

[jelle]

Glad you've found it!

The 5U4 can be cool in these amps. I'd use the straight glass one. They sound best to me.

Jelle

[mhuss]

I measured the voltage with the 5AR4 as 5.12 VAC, with the amp full on and warmed up. The 5U4 measured 5.17 VAC.

So, since the voltage is (slightly) higher with the 5U4, that would imply *less* current being drawn, which makes no sense (unless the B+ is changing a lot, decreasing the overall watts going through the PT).

The safest way would be to measure the PT temp after idling for an hour with each, if there is little or no difference, it should be OK.

--mark

[rfgordon]

I think I got my numbers backwards! Oops. The voltage dropped 0.05 VAC with the 5U4 in there, which seems to be fine, according to my favorite Blackface guru.