princeton PT with bias wire

[ryanburton]

i accidentally bought a mercury PT with a bias wire for my blackface princeton build.
FBFPP-M Blackface -- 50V bias tap -- lower B+ -- 325-0-325

i'm adding a bias adjust pot, but don't know how to integrate the bias wire from the PT into the build. what does that diode do on the bias board?

[Structo]

That diode and 25uf cap provide the negative bias voltage for the power tubes.

That is why the diode and cap are connected in reverse polarity.

I don't know that much about the trem circuits but I believe that 27K resistor there is the bias resistor.

On Fender trem amps, the tremolo is achieved by undulating the bias voltage to the power tubes.

So I believe you would use a 25k pot and a 10k resistor in series with the diode to ground.

But don't hold me to that and wait for somebody else's opinion.

[Wayne]

The bias wire would be hooked to the 100k, then the 100k and the 27k values juggled to give the proper voltage (100k would wind up much smaller).

If that gives you a headache, you could always ignore the bias tap and hook up your build exactly like the diagram above. The bias tap wire, when not connected to anything and properly insulated, won't hurt a thing.

W

[ryanburton]

The bias wire would be hooked to the 100k, then the 100k and the 27k values juggled to give the proper voltage (100k would wind up much smaller).

Is there a formula to use to calculate?
Original voltage is 340 bias tap is 50

[jjman]

Looks like the Fenders that use a bias tapped tranny use a 470 ohm where the 100k is on your schematic. Your schematic does not have adjustable bias. You can "fix" that by using a pot and a resistor in place of the 27k resistor. The BFDR uses a 10k resistor + a 10k pot.

Presumably the "50v bias tap" provides a max strength of 50 VDC after the diode/cap. I would use the bias tap if it was my build. It just seems "safer" to me to keep the high voltage away from the bias circuitry if easily available. And make it adjustable.

This would be the Fender bias-vary trem, Fender bias tap component values, and Fender adjustability approaches combined. You may need to play with the 470 and/or the 10k resistors but my guess is these values would work well. I'm guessing a range of 22 to 45 negative volts would results here.

Use good pots. You bias is depending upon them.

[drew]

http://www.el34world.com/charts/Biascircuits.htm

[ryanburton]

thanks drew

[Bob-I]

Funny, I just finished up the same amp.

I used a 10k > diode > 50uF/100V to ground > 50kL pot > 22K ? ground. Take the bias from the + side of the diode.

Worked great and the amp sounds great as well. Princeton is a sweet sounding amp.

[ryanburton]

is the 50k pot sensitive/precise enough?

[Bob-I]

is the 50k pot sensitive/precise enough?

Yes

[ryanburton]

got any pics bob?

[Bob-I]

got any pics bob?

Not that'll help you, but maybe this drawing will.

Oops, I screwed up the resistor values, the 4.7K should be 10K, the 15K should be 22K

[tubeswell]

what does that diode do on the bias board?

The diode rectifies the VAC coming off the bias winding to a half-wave negative voltage, which then gets smoothed by the RC filter to a negative DC voltage to supply your output grid load resistors with a negative voltage.

FWIW if you bought the PT for PR build, then you have a wee bit less HT voltage than the original. But if the PT's HT winding is rated at 200mA, you might be able to use it for a 5F6A build (assuming the heater winding is about 4A)

[ryanburton]

what would the lower HT do? i am under the impression that a lower voltage will make the amp sound better

[Bob-I]

what would the lower HT do? i am under the impression that a lower voltage will make the amp sound better

It depends on what your ear hears as "better". For an amp like a Princeton where usually the goal is fairly clean, just a little grit and fairly low volumes lower B+ voltages work well. Higher voltages will tighten up the output stages and add volume. With the lower voltage and cathodyne PI it'll put out about 12-15 watts. Increase the voltage to a more "normal" 420-450 and use a Long Tail Pair PI and you're up to Deluxe Reverb power at about 22-25watts.

All of that said, my Princeton Reverb build is damn loud and gutsy. I used an Allenamps.com Princeton xformer and I'm getting about 390V on the plates of the 6V6's. It's a damn gutsy little amp, great for small bars. I crank it to about 5-6 and use a Zen drive as my only pedal. Nice and clean with the guitar down below about 6-7 on the volume, gritty with the guitar on 10, screaming with the Zen.

Good luck with the build.