Help me troubleshoot my in progress Princeton build.

[morcey2]

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?

The problem with that is that the bias voltage needs to be negative.

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.

If you follow the link that martin manning posted, there's a schematic there in a post by talbany that shows exactly how to do it.

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.

Since a 6L6G is a 23W tube w/ a max plate voltage of 360, voltage-wise, you should be ok w/ this tranny. When you say that each of the secondary measured at 375VAC, is that in reference to each other, ground, or some other winding?

What strikes me as odd is that if this has a 5V rectifier heater winding and supplied power to a set of 6L6G's, the odds are that there is a tap that you're missing somewhere. The only situation that I can think of where you would have a non-CT winding on a PT from that era is if it used a half-wave rectifier instead of a full wave bridge since a full wave bridge w/ tube rectifiers, while possible and do-able, are complicated and unwieldy.

Do you know what the rectifier originally used was? Any way you can post some pictures of the tranny?

Matt

[Structo]

The 5U4 rectifier is a directly heated cathode, in other words the heater is the cathode.

Depending on the current rating of that PT, that voltage might dip quite a bit after a tube load is put on it.

Early Fender power transformers didn't have a separate bias winding so they had to take the high voltage from the HT and rectify it negatively for the bias supply.
The amps with vibrato or more accurately tremolo, use a varying bias voltage to change the amplitude of the signal to the power tubes for the tremolo effect.

Later Fender used PT's with a separate bias tap at a lower voltage for the bias circuit.

[tribi9]

Sorry I posted the reply before reading Martin manning's link. I will go ahead and read it.

The 375 VAC is in reference to ground. The rectifier the tranny powered was a 5Z4.

Structo thanks that's what Im saying so you have the 5V tap running from the PT but if you measure the wire running out of the cathode according to the schematic it should measure about 420V.

I added and extension to one side of the primary and that's what that strand is in on ine side of the primary wire.

Pictures

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... C02594.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... C02596.jpg[/img]

[Structo]

The two yellow wires should be the 5v tap.

The striped wires are usually the center taps.

Do you have two red wires?

If it has two black wires that is the primary side.

Is that number 22892?

That doesn't ring a bell for a Fender transformer.

Can't recall, what is the PT out of?

[tribi9]

The PT is from that bell howell amp I had posted before. On one side of the primary There's a red wire and a black wire. On the other side there are 2 red, 2 green plus 1 yellow/green. Also there are 2 yellow plus a 3rd tan colored wire I guess that would be the 6.3 CT

[tubeswell]

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?

The problem with that is that the bias voltage needs to be negative.

What morcey2 said. The grid needs to be at a negative voltage with respect to the cathode in order for the tube not to end up merely functioning as a 1/2 wave rectifier diode. As this is a fixed-bias amp, the cathode is sitting at 0V (ground) potential, and therefore the grid has to be made lower than 0V - i.e.; it has to be supplied with a negative voltage in order for the grid to be negative-with-respect-to the cathode.

The negative grid bias voltage functions to allow the tube to amplify the full swing of the signal on the grid. Without being at a decent negative voltage, the swing at the grid will only alter tube current at the plate during the negative part of the grid signal swing. If the grid voltage were at 0V, then when the grid voltage swings positive, there cannot be any further change to tube current other than what is already otherwise flowing between the plate and the cathode because the cathode is also at 0V, therefore the positive part of the grid signal swing will just result in a cut-off at the plate.

What's more, if the bias isn't cool enough, then tube current has nothing limiting its rush between the plate and cathode. Electrons will stream from the cathode to the plate (and the grid) (and the screen) without mitigation, causing the plate (and the grid) (and the screen) to glow red hot, then white hot, before they cause the whole envelope to melt down.

[tribi9]

Thank you all makes perfect sense now.

[morcey2]

A 5Z4 is effectively the same as a 5Y3, so I don't think the tranny will support a 5U4 because of the excess current draw.

Which lead are you using for the ground when you're measuring the 375V? That should be the one that's the center tap for the main winding. If you're measuring against anything that's not part of that same winding, it won't give a meaningful measurement. Is there continuity between the red leads and any of the others?

Matt

[tribi9]

I figured I wouldn't be able to use a the 5U4. I looked it up and saw that it draws 3 amps.

I measured the red leads after I cut all the leads while removing I was removing the pt from the chassis. I had another chassis on the side that was plugged in so I just clipped the black probe there.

[tribi9]

I measured with the meter and..........

There's continuity from the HT wires to that tanned wire I was talking about so I guess that is the CT. I guess the 5.3 V winding has a CT tap but the 6.3V doesn't have one. So i guess I can just create an artificial tap.

Thx for all the help.

[Phil_S]

In a directly heated rectifier tube, the filament is also the cathode. So, in this case, pins 2 and 8 are not just the filament. You wire them for the 5V filament supply, but you also tap just one of them for the rectified high voltage to supply the amplifier circuit. It seems like it might not work, but, somehow it does.

I always feel much better using an indirectly heated tube like a 6CA4 or GZ34. Having a separate filament and cathode is somehow a comforting thought. In practice, though, either type works equally well.

[Structo]

With the indirectly heated rectifier such as the GZ34, it warms up slower, right?
So you get a slower ramping up to the full B+.

[tribi9]

Aaaah..... Thats how it happens then Phil S. Thx all!

[tribi9]

I just tried measuring a transformer and as soon as I wired the live and neutral to the primary and plugged in it started humming/vibrating softly. Never seen that before....

[tubeswell]

I just tried measuring a transformer and as soon as I wired the live and neutral to the primary and plugged in it started humming/vibrating softly. Never seen that before....

Some PTs hum more than others.
Are you using a light bulb dimmer?
If you connect a 1k5 10W resistor from the PT case to the mains ground wire, is there any voltage across it? (looking for shorts to the case/ground. There should be no voltage across the resistor if the PT is safe.)
What do the secondary VACs measure?