With our Push Pull output transformers we try to hive as much detail as possible on the actual transformer to assist the user in wiring it into their circuit.
However it can seem daunting when looking at the number of wires. How do I wire up this thing?
Below is a circuit representation of a typical PP output transformer. The wire colours may be different from yours, but the results will be the same if you go by the label on your transformer.
Your amplifier will have 2 output valves or tubes. You should wire the A wires to an anode connection on each output valve. The same goes for the G wires. These get wired to the same grid/screen terminal on the corresponding valve.
The HT will be wired to the HT voltage.
The outputs will go to either a speaker connection, or be safley tied away. I like to use a choc block that I screw the unused wires into to keep them safe.
Now here is a problem! How do we know which valve is A1/G1 and which is A2/G2. If you wire them to the incorrect valve the amplifier will oscillate if you have global feedback.
In the circuit above we have added the global negative feedback connection, this is the thing that can cause the amplifier to oscillate if the valves are wired up incorrectly. This is down to the phasing of the transformer primary and secondaries. One way around will be correct and give negative feedback, the other will produce positive feedback and so oscillate. You could find out the phasing of the transformer yourself, and work out the various phase inversions in your circuit, and then you should be able to work out how to connect the transformer without oscillation.
But the most of us have more important thigs to do, so a tray and see approach is what most people revert to.
The try and see approach.
With the amp switched off for more than 5 minutes to allow the HT to die away, connect up the primaries to your best guess output valves. Now connect a dummy load across the speaker output. This should be sufficient for the power output of the amplifier.
Never turn on the amplifier with a speaker attached for the first power up. The amplitude of the oscillation can quite easily ruin your prized speaker.
Short the input to the amplifier to earth.
What we are going to do is turn on the amplifier and wait as it warms up, listening to it and the output transformer. As it warms up, if the phase is incorrect you will be able to hear the oscillations in the output transformer. The amplitude will increase rapidly as the amplifier gets to normal temperature. As soon as you hear the oscillation switch off the power. Do this quickly as soon as you can detect a problem.
If there is no phase problem, then you will not hear anything even when the amp is hot. You are good to go.
Fixing the problem
OK, you heard the oscillation, and you turned your amplifier off. Wait for a good 5 minutes to allow the HT to die away, and disconnect the mains.
You have 2 options now. You can swap the connections to the output valves. So what you chose as V1 will now be connected to A2/G2 and V2 will now be connected to A1/G2.
If you don’t want to do that you can swap the output connections. However this is very difficult with a tapped secondary as the circuit above.
Normally I would suggest swapping the primaries.
Once you have made the changes you can do the above test again. All being well you should not hear any oscillation.
If you still hear something then there is probably something incorrect in your circuit. You should check all your connections and wiring.