Output transformer & Rectifier Tube Question

[joe1968]

I built a Deluxe Reverb Clone using a Mojo761Ex power transformer. The plate voltage has been a bit high (up to 40 VDC over specs) when I hooked up to the 230 VAC tab (wall voltage is 230 VAC exactly). I changed to the 240 VAC Tab to bring it down a little. I am in the specs now, but the heater voltages also dropped to 4.75 VAC and 5.95 VAC. I was told that this would be still in the ballpark.

Does anyone has an opinion on that one?

Also I would like to try to change from a GZ34 to a 5U4 rectifier tube which has a lower voltage output so I can use the 230 VAC tab again. I heard that the current draw would be much higher then. I cannot find any specs of the Mojo PT I am using. It's getting quite got already. Should I be worried to fry it when I put in the 5U4?

Would appreciate any comments.

Thanks.

Joe

[RedMan]

The 5U4 would put less of strain on the power transformer if anything. If that doesn't get your plate voltage low enough try a 5Y3.

[hogy]

The 5U4 would put less of strain on the power transformer if anything.

No, that's wrong. A GZ34 draws 1.8A of heater current, a 5U4 draws 3A. That could very easily burn up you power transformer's heater section if it isn't designed for it.

[RedMan]

No, that's wrong. A GZ34 draws 1.8A of heater current, a 5U4 draws 3A. That could very easily burn up you power transformer's heater section if it isn't designed for it.

The heater current isn't an issue. Many DR's came with 5U4's. The 5V tap on that PT can handle it fine. The current limiting the 5U4 does on the plate voltage will make the PT have an easier life overall.

[hogy]

The heater current isn't an issue. Many DR's came with 5U4's. The 5V tap on that PT can handle it fine. The current limiting the 5U4 does on the plate voltage will make the PT have an easier life overall.

It is indeed an issue. "Many" DRs didn't come with 5U4s, only AB868 and later, and those had power transformers designed for that. I have exchanged plenty AB763 PTs that had been run with 5U4s and failed.

[brad347]

The heater current isn't an issue. Many DR's came with 5U4's. The 5V tap on that PT can handle it fine. The current limiting the 5U4 does on the plate voltage will make the PT have an easier life overall.

But he is using an aftermarket PT with an unknown safety margin on the heater winding.

I'd say pop the 5U4 in and measure the voltage 5V winding where it connects to the tube. If it is steady at 5V or more, you are probably good. If it looks low or something looks funny, you're probably putting strain on it.

[joe1968]

Thanks for the comments. Sorry to say so, but I am even more confused now. Does anyone used the MOJO PT or can provide a data sheet?

Does anyone has an opinion on the lower heater votages? Will it hurt the tubes? I guess there is a certain range that would be acceptable.

Thanks.

Joe

[Blue Strat]

If heater current is a possible issue, use a 5R4GB. It has nearly the same performance as a 5u4 but with 2 amp heater current requirement.