5C3 style amp with 275-0-275 PT?

[armstrom]

I know the stock power transformers for the deluxe amps are 325-0-325... I have access to a 275-0-275 PT that I would like to use. I understand the amp would be down on power compared to a stock build, but would I run into any other major "gotchas"? Any idea what (if any) other components would need to change? I'm not 100% confident in my understanding of how the power tubes are biased in that amp so I'm not sure if I would need to change the cathode resistor or any other parts... Should I just bite the bullet and buy the correct PT?
-Matt

[Bruce / Mission Amps]

I know the stock power transformers for the deluxe amps are 325-0-325... I have access to a 275-0-275 PT that I would like to use. I understand the amp would be down on power compared to a stock build, but would I run into any other major "gotchas"? Any idea what (if any) other components would need to change? I'm not 100% confident in my understanding of how the power tubes are biased in that amp so I'm not sure if I would need to change the cathode resistor or any other parts... Should I just bite the bullet and buy the correct PT?
-Matt

I'd just use it anyhow but do it with a solid state full wave rectifier circuit into a 47uF@450v first filter cap and a small 5 to 10 watt, 68 to 100 ohm sag resistor in the center tap leg to ground.
I'd try a standard 250 ohm 5 watt biasing resistor first. You might find that 220 ohm is better later, but due to the fact that the Deluxe runs the power tubes hot anyhow, I'd still use the 250 ohm resistor.
With respect to the rest of the B+ rail, I'd use a pair of 22uF@450 filter caps (the last one can be 10uF to 16uF to be honest) and consider using slightly lower values for the two dropping resistors, maybe a 3900 and 18K or 20K.
I bet it will not be down on volume as much as you'd think.

[armstrom]

Hmm.. I was considering retaining the tube rectifier in order to still get the voltage "sag" when pushed hard... Do you think the lack of a tube rectifier will be "missed' In the tone of the amp at all? Or does most of the deluxe's unique sound come from the grid leak preamp biasing and the paraphase PI?

Edit: Oops, somehow I missed the part about the sag resistor on the CT.. How closely does that mimic "real" tube sag?

[armstrom]

Would something like this match what you describe? The fender schematic lists 10K for the series resistors in the power supply so I left them alone rather than make them any smaller.
Edit: Opps.. I just realized I had the first leg labeled as going to the "PT" center tap and it should be OT Center Tap... I'm sure you got my meaning.

[Bruce / Mission Amps]

Would something like this match what you describe? The fender schematic lists 10K for the series resistors in the power supply so I left them alone rather than make them any smaller.
Edit: Opps.. I just realized I had the first leg labeled as going to the "PT" center tap and it should be OT Center Tap... I'm sure you got my meaning.

Yes that's it.
I forgot the 5C3 used way larger plate to screen node resistor... I'd use nothing larger the 5K anyhow since your overall voltages are going to be a little lower with a FW SS rectifier and that PT. The other 10K resistor is probably OK.
No resistor will mimic a tube rectifier sag 100% but it does a decent job of it.
The power tubes are idling in very high current in a Deluxe so the majority of the "sag" through the rectifier has already occurred... unlike a lower idle current, fixed bias amp where the power supply is lightly loaded by comparison until you really rock out.
The resistor will use up more B+ voltage as the current increases, similar to the soft 5Y3GT rectifier.