Using a CT PT in place of a non-CT PT

[beasleybodyshop]

Had a center tapped power transformer wound for an amp that it's previous PT didnt have a CT. It keeps shorting out whenever i connect it to the bridge rectifier.

What reason would you not have a center tap for a power transformer? And is it ok to lift the CT and connect the outer windings ?

[cbass]

Don't use ct with bridge recto. If the amp was designed for a fwb recto then it doesn't need a ct.just hook up the repayment pt to a regular full wave recto and use the ct

[cbass]

And is it ok to lift the CT and connect the outer windings ?

Yes but that fwb will be about double the voltage

[beasleybodyshop]

And is it ok to lift the CT and connect the outer windings ?

Yes but that fwb will be about double the voltage

you alright Cbass I don't care what they say about you

[eniam rognab]

center tapped transformers with a bridge rectifier generate a negative potential on the negative side. damn right you were shorting out!! made the same mistake myself, luckily on a light bulb.

lesson learned for me, something that sticks with you kinda knowledge

[beasleybodyshop]

So the diode setup for say, a blackface fender (three in a row on each leg) is a different type of diode rectification than a bridge setup?

So that's why a blackface twin uses a center tap? Because it's not a full wave bridge?

[eniam rognab]

yeah, bridge uses four diodes in a diamond pattern to either give a ground reference for a non center tapped winding, OR generate a negative power rail on a center tapped winding

the diode string is to increase the peak inverse voltage rating

[cbass]

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

[beasleybodyshop]

Ok. Well damn.

My transformer is a 250-0-50-250 primary center tapped. MY bias winding is tapped from the HT. So now when i measure the voltage going into the bias circuit im getting 280vDC

Would a 5 watt resistor be big enough to dissipate the heat to drop it down to 50-60 volts?

[rp]

http://el34world.com/charts/CommonHookups.htm -> scroll down for more info

[rp]

BTW while here, and a bit related, if you ever get a PT with a 5V CT eg Hammonds, don't hook that up!

[sluckey]

BTW while here, and a bit related, if you ever get a PT with a 5V CT eg Hammonds, don't hook that up!

You would not want to connect it to ground. But you can certainly connect it to the B+ rail. Rather than take the B+ from pin 8, you would take the B+ from the 5V CT. Lot's of 50/60/ year old equipment used a 5V CT like this.

[martin manning]

Using the CT would be fine for directly heated cathode (i.e. 5U4 or 5Y3), but for indirectly heated cathode pin 8 would be better.

[beasleybodyshop]

When i measure the bias winding disconnected from the circuit to ground, it's giving me 70vAC. The only place the secondary HT touches ground is through the " - " end of the bridge rectifier to ground. Would this mean that my ground is not referenced and the 70v is a bogus reading? whenever i attach it to the circuit i read almost 280vAC

[potatofarmer]

You need to use the full-wave center tap rectifier (like a blackface Fender like you mentioned) in order to use that bias tap.

Trace both current paths through the bridge to see why. On one cycle it works fine, but on the other...

250-0-250 through a bridge will give you, what, 700V unloaded? If you want to use a bridge rectifier, you'd want a 250-0, or a 125-0-125.

With the FWCT rectifier, the center tap is needed to complete the circuit. With a bridge rectifier, it's not needed; the circuit is completed by the other half of the bridge. On the later Twin Reverbs (135W and I think through the Evil Twin and Red Knob) they used a bridge rectifier into stacked filter caps, then connected the center tap to the junction of the two filter caps for a handy "half voltage" node for the "low power" option.

But since you have a bias tap, just use the FWCT and abandon the bridge.