Need help on first tube amp rebuild -trans problems

[DJNUBZ]

I had a knight amp that blew and I decided to use the parts to make a new amp. This is my first amp and I am planing on fallowing the the simple el84 amp on diyparadise.com. My first problem is that it seems like my transformer doesn't have a center tap to connect to the ground. How do I use this trans? The knight amp used a 6ca4 and I am planing on using SS rectification like the original plan.

Here are the colors on the trans former.
side A
red/black + black
yellow/red + yellow/red

Side B
green + green
yellow + yellow/green

[sklimek]

This may help, I found it on a search awhile back. It would be good though if someone else could confirm it.

Stan

[DJNUBZ]

Looks like that works for modern transformers but not for ones from 1965.

The black + black/red were connected to the mains.

[porkchop61]

I have the same information as sklimek posted in my RCA Radiotron Designers Handbook. It even say's it's from a May 1935 R.M.A. standard, so it's not a modern transformer issue.

Must be a special color code used for their transformers?

[DJNUBZ]

Could someone please tell me how to build the PS using this trans? I should also add that I will not be using a choke in the PS. I will be using a resistor instead.

[dhaen]

You can determine what the connections are if you do the following tests.

Primary:
If you are sure that the black - red/black were connected to the mains then connect them and carefully check the voltages on the other windings. Make sure there is a fuse in circuit. Anything between 400mA to 5A should protect against damage.
Secondary:
Across the heater pair there will be about 7V AC.
Across the high voltage pair there will be 300V AC - ish.

The third pair may in fact be a second primary winding. Check the voltage; If it's the same as your mains, then that's likely. It will need to be connected in parallel with the other primary winding, but watch out, the phase (or polarity) matters. You'll blow the transformer up if you get it wrong.
How to check? Connect just one of the pair, and then measure the AV voltage between the remaining wire and the point where you intend to connect. If the voltage is low (10V or less) you've got it right. If the voltage is twice your mains voltage then it's the wrong way.

Once you have worked out the connections, post the value of the high voltage, and I'm sure someone will guide you through a PSU design.

[alexistheo]

it seems strange to me that a tube-rectified power supply wouldn't have a center-tapped transformer. are you sure there's no CT? if you really don't have one, you can just use a full-wave bridge (four diodes).

as for the amp, i've not built it but i've heard that the current sink under the output tubes is nearly mandatory for decent performance.

[DJNUBZ]

ok with red and black connected to 120V here is what I am getting

yellow/red + yellow/red - 600VAC

green + green - 7 VAC

yellow + yellow green - 0 VAC

On a hunch I measured

yellow/red + yellow and got 300V (tried with both and got the same)

green + yellow/green and got about 3.5 VAC

I think I have found my center taps, Am I right?

If so I guess my next question is how do I build the PS w/o the choke?

[Sherman]

Originally posted by DJNUBZ
ok with red and black connected to 120V here is what I am getting

yellow/red + yellow/red - 600VAC

green + green - 7 VAC

yellow + yellow green - 0 VAC

On a hunch I measured

yellow/red + yellow and got 300V (tried with both and got the same)

green + yellow/green and got about 3.5 VAC

I think I have found my center taps, Am I right?

If so I guess my next question is how do I build the PS w/o the choke?

You've got it. "Standard" practice has green as the 6.3V heater secondaries and green with yellow stripe is the 6.3V CT (if present).

Also the HV tap is most often red and the HV CT is red with yellow stripe. In your case it sounds like yellow/red is the HV secondary and yellow is the HV CT.

[dhaen]

Yes, the 600V is 300+300V, but check the centre tap.

What tubes are you intending to use? We need to know the voltage and current requirements. Do you have a schematic of the original and of the new build?