Help me troubleshoot my in progress Princeton build.

[tribi9]

I got a power transformer that measures about 375 V on each of the secondary wires. There is no center tap tho'

Do I need a center tap if I want to go with a tube rectifier or is diode rectification my only option?

[morcey2]

http://www.freewebs.com/valvewizard/bridge.html

You can do a hybrid bridge w/ 2 silicon diodes and a tube rectifier, but you can get the same effect w/ 4 SS diodes and a sag resistor.

Matt.

[tribi9]

Thank you for the link Matt.

What voltage difference do you get from AC in to DC out? Is it volt by volt the conversion? What VDC should I be expecting with a 375 VAC on the secondary?

[morcey2]

A bridge rectifier w/ a cap input load will give you a DC of approximately 1.4 * VAC. In this case, that's 375 * 1.4 or 525VDC. Once you have a load on it, it'll drop some. In any case, that's a pretty high B+. (a couple of EL34's )

If you wanted to switch to a choke input load, it'd drop to about 330VDC with lots more current available, but that would require a pretty hefty choke.

http://www.hammondmfg.com/pdf/5c007.pdf

Matt

[tribi9]

So what size choke would I need to get the DC at 420V?

Could I not use a resistor before or after the rectifier to bring the B+ down?

[morcey2]

First, if the 375 is measured w/ the tranny totally unloaded, it'll drop significantly when there's any load on it, probably closer to 325. (that's just a guess; others here will have a more accurate guesstimate) . That'll give you about 450VDC rectified (cap input load), which is pretty close to what you want.

Again, that's just a guess, but should be pretty close. If you use a hybrid SS/tube bridge w/ a GZ34, it'll give you about 420VDC, depending on load. A 100-ohm sag resistor would give about the same voltage.

What type of amp are you going to use this for?

Matt.

[tribi9]

I have a bunch of spare parts, enough to build an AA1164 so I'm hoping to use this. Yhank you Matt.

[tribi9]

In the example below...

See the secondary wire that connects to pin 4 on the 5U4GB and then goes to the 100K resistor and then a diode. Is the diode there acting as a rectifier?

What is the need for that couldn't that just run off the other rectifier filament (Pin 2) eliminating the need for the diode there?


http://www.el34world.com/charts/Schemat ... AA1164.pdf

[tubeswell]

You do need a 5V winding for a tube rectifier tho'. Something your PT may not have if the High Tension winding doesn't have a CT. So you might need a 2nd (5V) PT

[tribi9]

My PT does have a 5 V winding tubeswell.
What about the question on the schematic. Can anyone help me with that.

[morcey2]

That 100k + diode + other-stuff is the bias supply. The diode is backwards to create the necessarily negative bias voltage. With a bridge rectifier, the bias supply can't be connected that way, but must run through a cap. Some of the later marshalls have the same bias-supply setup. If you can't find one, I'll get one for you later today when I get back.

Do you have the full specs on the tranny, or what it was powering in a previous life? Power tube type & number, etc? (or a manufacturer/model #)

Matt

[dynaman]

Marshall JCM900 and others use a similar PT. I recently used one and had no problems with their bias circuit.

[martin manning]

Recent relevant discussion with several circuit examples here.
https://tubeamparchive.com/viewtopic.php?t=11613

[tubeswell]

Yes the 'backwards' single diode is a 1/2-wave negative rectifier. You will note that the electrolytic filter cap for the bias is also installed +ve end pointing to ground. This smooths the 1/2-wave -ve rectified AC voltage to a sufficiently stable -ve DC voltage. The -ve bias voltage is supplied to the 6V6 grids via the 2 x 220k grid load resistors, via the trem intensity pot.

The voltage divider in the stock BFPR is comprised of the 100k and the 22k. In other amps with a secondary 50VAC bias tap, the voltage divider is nowhere near as low, but in the BFPR the bias supply is being tapped off on of the (340VAC) HT winding ends so it needs to be low.

You can't run the bias supply off one of the 5VAC rectifier windings, because you need about -34VDC for the bias supply and in practical terms you'll wont get that with a 5VAC source

[tribi9]

Thank you tubeswell. See what confuses me a bit is that Pin 2 and 8 are both filaments on a 5U4GB. Pin 8+ supplies 420VDC to the OT. So wouldn't Pin 2 which is tied to Pin 8 be able to supply 420V minus whatever resistor+cap combo needed to get the appropiate rectified voltage needed to the grid eliminating the need for that diode?

How could I wire the bias cap if I decide to build this amp using diode rectification and this PT with no center tap. The only way I know how to do it is with a CT on the PT.


Morcey the PT used to power 2 6L6G's. It was from a movie projector amp.
It has 5 and a 6.3 V taps each of the secondary measured 375V. That's all I know.