Princeton Reverb Transformer Windings Question

[Magnatron]

I’m building a Princeton Reverb clone using a Hoffman layout in a Weber chassis. I originally ordered the 125P1B type transformer from Weber known as the W022772. That transformer did not match the mounting holes in the Weber chassis, so I was recommended to buy the 125P23B Weber model which would fit. The question I have is what windings to use. The Hoffman layout calls for the red wires that deliver 660 volts from the 125P1B. But the Weber layout on their PR clone with the 125P23B calls out for the red/white wires that deliver 540 volts. The red wires on that transformer deliver 680 volts.

For the Hoffman build using the 125P23B transformer, which wires and voltage should I use?

[sluckey]

For the Hoffman build using the 125P23B transformer, which wires and voltage should I use?

I would use the red/white wires that deliver 540 volts.

[Magnatron]

I would use the red/white wires that deliver 540 volts.

What would be the implications of that considering that the voltage difference is 120 volts lower from one to the other?

Thanks for the help. Just trying to understand for future reference.

[dwinstonwood]

Assuming my math is right:

680V/2 = 340V x 1.2 (5U4G tube rectifier) = ~408V B+

540V/2 = 270 x 1.2 (5U4G tube rectifier) = ~324V B+

But, if you ever decide to change to a SS diode rectifier:

680V/2 = 340V x 1.414 (SS rectifier) = ~481V B+

540V/2 = 270 x 1.414 (SS rectifier) = ~382V B+

481V is maybe too high for 6V6's?

Also, if you ever change to a GZ34 tube rectifier:

680V probably ends up around ~442V B+

540V probably ends up around ~351V B+

[sluckey]

The 125P23B transformer is for the Deluxe Reverb. The 125P1B transformer is for the Princeton Reverb. I suggest using the 125P1B and drilling new screw holes.

[dwinstonwood]

I hope I'm not just muddying the water by posting these numbers. 

"660V from the 125P1B" = 330 x 1.2 = ~396V B+

With today's higher wall voltages that looks right. Around 400V give or take.

I tend to follow sluckey's suggestion on my builds.

[mresistor]

I think a better alternative is a Hammond 290AX.

[Magnatron]

The 125P23B transformer is for the Deluxe Reverb. The 125P1B transformer is for the Princeton Reverb. I suggest using the 125P1B and drilling new screw holes.

Thanks Sluckey. So let me be an idiot for a moment — something I’m good at.

I took advice from Weber and ordered the 125P23B prior to this thread, and sent back the 125P1B. They may have assumed I was using their build and not the Hoffman. If I’m stuck with this transformer, and connect the 540 volt winding to the rectifier, would that be a problem? What about the 680 volt winding? Would that be a problem as well? If they are problematic, why would that be? What would I expect?

This is a bit new to me, so I apologize for my newbie-ness.

[dwinstonwood]

Hi Magnatron, in my two posts I was trying to give a ballpark idea of what B+ voltages you could expect with those transformers.

The original Fender PR schematic notes 410V on the plates. So, it looks like the 125P1B would put you very close to that, even if your wall voltage was a bit high (123VAC, etc.)

There are other important factors with power transformers, too, like the current it can safely supply. But, in my experience, an amp using two 6V6's and three or four preamp tubes like 12AX7's is fine with a power transformer rated at 100mA.

[mresistor]

Hi dr    the original Princeton Reverb OT I believe was only rated at 80-85ma and consequently many of them failed in the long term under heavy usage.   290AX is rated at 100ma.  Many people have used them in PR builds including myself. 
 
Notice as well there are choices of 120v and 125v inputs which thrown into the mix creats other possible voltages on the output..

I looked at the weber 6A14 kit and it looks like they are using a higher rated power transformer in it. It probably has the cutout dims to accomodate the Hammond 290AX.  It's rated at 150ma.  Might be a better transformer to use although it's probably made in an asian country.

[sluckey]

Thanks Sluckey.

See reply #1.

[tubeswell]

Just to muddy the waters, I built one of those 6A14 kits about 10 years ago and used the higher voltage secondary with a 5U4 and JJ 6V6s.  The thing still works fine.

[Magnatron]

Thanks Sluckey.

See reply #1.

I'll go with that, but for my education, what would be the functional difference in the amp between using 540 VAC and 680 VAC?

Thanks much so far!

[ac427v]

I used the WO25130 in my Blackface Princeton (no reverb) and cathode biased 6V6GT output.
Red taps and 5U4GB gave me 410volt B+  I liked the punchy sound and kept this as the final version

Red/White taps and 5AR4 gave me 333 volts B+  I liked the browner sound but felt the bass was too mushy at anything over bedroom levels
I have a vague memory that Weber used this 150ma power transformer to allow optional 6L6 power tubes???

[Magnatron]

I used the WO25130 in my Blackface Princeton (no reverb) and cathode biased 6V6GT output.
Red taps and 5U4GB gave me 410volt B+  I liked the punchy sound and kept this as the final version

Red/White taps and 5AR4 gave me 333 volts B+  I liked the browner sound but felt the bass was too mushy at anything over bedroom levels
I have a vague memory that Weber used this 150ma power transformer to allow optional 6L6 power tubes???

ac427v, that is a perfect description and just what I was looking for! Thank you! You mention cathode bias, would adjustable grid bias have any impact on this voltage selection? Sorry if that sounds newbie -- cause I'm newbie.

[ac427v]

I don't know. I haven't kept a record of trying that. and I do not have systematic training of electronic theory to predict whether the bias system would change B+. I know that B+ changes with the bias voltage so I'm thinking it will be complicated to calculate a precise prediction.
Hopefully others here can answer that question.

[jordan86]

Thanks Sluckey.

See reply #1.

I'll go with that, but for my education, what would be the functional difference in the amp between using 540 VAC and 680 VAC?

I’m a fairly new learner myself but built a Princeton and walked your path. So let me give it to you in fellow layman terms. You only use one of those windings. Not both. Whichever you don’t use, tape off and tuck away. Don’t let them touch each other, or anything else. Side note: you can experiment using one at a time if you like. See which you prefer. Again, just tape off the one not in use.

Now as for choosing which one, the functional difference is higher B+ Voltages on the power tubes (and other tubes down the line) with that 680v winding or lower voltages with the 540v winding. That’s all. A higher voltage amp will feel a bit quicker in response and crispier on the high end. A lower voltage amp will sag a bit and have a somewhat smoother/warmer feel. Higher voltages will also produce more wattage, all other things being the same.

If you’re putting an adjustable bias pot in your Princeton you can compensate the power tube bias to offset the wattage factor. If you’re not doing that, I’d probably play it conservative and do the 540v per Sluckey and use a 5AR4 rectifier. Ie get the voltage as close to the schematic as possible.

But with different tube rectifier types (or a SS plugin rectifier) you can sort of offset (“adjust”) the way the winding voltage influences the amp. Like use the high 680v winding with a low voltage 5y3 rectifier. That’s what dwinstonwood was trying to explain to you. He was kind enough to even do the math. Those options really make it a total personal preference issue and one you can “dial in” to your liking with the different rectifiers. I assume you know that different rectifier types will raise or lower the voltage. If not, look that up. One disclaimer, You probably don’t want 450v+ on the 6V6’s though. It won’t sound like a Princeton.

There’s no right or wrong here but you’ll get the best results I think with somewhere between 380-415v on the 6V6 plates. You just need find the voltage you like. If you have a 5U4 or 5Y3 rectifier, I’d probably personally do the 680v winding. If you have a 5AR4 I’d do the 540v winding. I’ve tried both windings on my Hammond 290ax (550v/650v) and I prefer the higher one, and then knocking the voltage down with a tube rectifier. I think I’m floating somewhere around 410v on my 6V6 plates. I do have an adjustable bias pot though fwiw.