negative feedback question

[paul525715]

I've just acquired a set of transformers for a hi gain amp I'm building. Can anyone tell me if there is a way to determine which of the OT primary leads goes to which of the output tube plates so that the neg feedback loop is the right way round. Both of the primary leads that go to the plates are the same colour so I can't tell from the colour. Is it just a matter of trial and error (i.e. if it squeals, swap them)?

Thanks,

Paul

[loudthud]

First of all look at the secondary. If there are any impedance taps, there has to be one side that is intended to be grounded. If there are no impedance taps, choose one wire an call it ground. Now you have to determine which plate wire is in phase with the output. The easiest thing is to feed a 1KHz sine wave into the speaker wire and look at the primary with a scope (temporarily ground the B+ wire). On most schematics where there is a long tail phase inverter with the signal going into the top half of the dual triode and the feedback going into the bottom half, the in-phase primary wire goes to the top power tube.

Don't have a scope and signal generator? Try this. Feed a low voltage AC from a wall-wart type power supply into the speaker wire. Be careful, there could be 100V or so on the primary wires. Ground the B+ wire and verify that the AC voltage on each plate wire is the same. Now carefully measure the AC volts from each plate wire to the speaker wire. One will measure slightly higher and one will measure slightly lower. The lower one is the in-phase wire.

[paul525715]

Thanks for your swift and informative reply.

I can get my hands on a signal generator and two input oscilloscope easily enough.

Can you just confirm for me...

1) Find ground and 16 ohm taps on secondary side (I can do that bit)
2) Connect sig gen to these points, and also connect up to oscilloscope input 1 to show original signal).
3) Connect OT Primary B+ connection to oscilloscope gnd.
4) Connect one of the OT primary anode wires to the oscilloscope input 2 and see if the waves are in phase (constructive) or out of phase (destructive).
5) Double check with the other primary anode wire.
6) Constructive = top tube. Destructive = bottom tube.

Thanks,

Paul

[loudthud]

1) Find ground and 16 ohm taps on secondary side (I can do that bit)

Any tap will do.

2) Connect sig gen to these points, and also connect up to oscilloscope input 1 to show original signal).

Set trigger select to input 1.

6) Constructive = top tube. Destructive = bottom tube.

Generally true but it depends on the phase inverter.

Hint: If your amp needs to have the phase reversed because of oscillation, you can reverse the connections at the power tube inputs or at the phase inverter outputs if it's too much trouble to reverse the output transformer leads.

[paul525715]

Thanks for all your help

[Jazz P Bass]

Negative Feedback Check

A down & dirty check of your output transformer polarity can be accomplished by disconnecting & reconnecting the feedback signal while observing the output signal on a scope.
If the signal increases when the negative feedback is reattached, the output transformer wires need to be reversed. (positive feedback)