Hammond transformer primary question

[Diverted]

Hi all,

I just mostly finished up the build of a new Princeton reverb. I’m using a Hammond power transformer that is rated at 300-0-300@173 ma. I’m planning to use it with a pair of old but good 5881s for about 23-25w.
The primary winding has two taps, one for 115VAC, the other for 125VAC. I initially wired it with the 125V tap, but that is giving me a B+ of around 395 or so. So I swapped the primary leads and now have it hooked up with the 115V lead. This is giving me about 415 to 420. Right where I wanted it.

My question is how detrimental will this be to the transformer’s current capability, since it will be running at probably 120 or 121VAC. I would imagine the overall current rating is going to go down a little bit. How much wiggle room will I have? Am I playing with fire switching to that 115V tap?

Thank you.

[mresistor]

I don't see any problem with using it on a modern 120VAC line input. Personally I'd prefer the 125v tap using a GZ34 rectifier. If the PT has those inputs then it was intended to be used on a 120VAC line and it presents no problems.


Were your readings unloaded? If so then the 115 tap might be better.

[sluckey]

What is your measured filament voltage? 6V6 pin 2 to pin 7? I'd let that be the determining factor for which primary tap to use. Ain't much difference between 395 and 415 for B+.

[Diverted]

Good call Sluckey. For some reason this completely slipped my mind. With the 115V tap it's up close to 7v (6.99). So that seems a bit high for my liking, I'm going to bring it back to the 125 tap. Thanks.

[HotBluePlates]

... build of a new Princeton reverb. I’m using a Hammond power transformer that is rated at 300-0-300@173 ma. I’m planning to use it with a pair of old but good 5881s for about 23-25w. ...

You will probably need a 6.6kΩ or 4kΩ OT to allow the output tubes to draw the extra current, and develop the higher output power.

The Princeton Reverb's stock 8kΩ OT will prevent the output tubes from drawing substantially more current than what 6V6s do stock in a Princeton Reverb.