Oscillation in clone Ampeg B15N

[dbchamblee]

Just finished a CtG FCBN 51B clone of an Ampeg B15N, schematic attached. I have 2.5 kHz oscillation when I power up. I substituted a Hammond 1750Q output transformer because the Weber WOT45HHR-UL they called for was out of stock. Also added a speaker selector switch since the Hammond has 4, 8 and 16 ohm outputs. I plugged in a guitar and I get signal through the amp but its unusable because of the oscillation. I tried substituting an 8 ohm resistor for the speaker so I wouldn't have to listen the noise while troubleshooting and the oscillation went away. If I pull all the preamp and phase inverter tubes I get no oscillation. As soon as I plug in the phase inverter I get it again. Any ideas?

[M Fowler]

NFB so reverse the OT primary wires feeding the 6L6 power tubes.

[Stevem]

If the primary and or secondary OT wire swap does not kill the issue than you need to pin down what stage its coming from!
In general it takes the signal passing thru at least 2 coupling caps, or 3 stages of gain ( a cathode follower does not count!) to get positive reinforced oscillation.
One of the most common issue with home builds is to have the drive wires off of the PI too long, on some layouts anything longer than 3 1/2" will make for a oscillation issue.

[Structo]

NFB so reverse the OT primary wires feeding the 6L6 power tubes.

Or if those wires are cut too short you can reverse the wires on the PI coupling caps.

[tele_player]

I'd first disconnect the NFB to see if it's the cause. That's easier than swapping the wires, which might not even be necessary.

Robert

[dbchamblee]

Thanks for all the responses. It was the transformer primary windings. I swapped them and everything's cool. What causes that?

[Reeltarded]

Negative feedback becomes positive feedback. Eeeeeeeeeekk

[Stevem]

If you ever see that issue again with another amp you can swap wires on the primary or secondary side and get the cure to take place.

[tubeswell]

Thanks for all the responses. It was the transformer primary windings. I swapped them and everything's cool. What causes that?

Primary and secondary transformer windings have a phasing in relation to each other, whereby the phase of the VAC at each end of the secondary can either be in phase or in opposing phase to the VAC phase that is supplied to the primary, depending on which end of the winding you are looking at. Global NFB is taken from one end of the secondary and supplied to a point in the pre-amp which is in opposing phase, in order to reduce distortion in the amp. If the side of the secondary winding that the VAC is taken from happens to be in the same phase as the NFB insertion point in the pre-amp, you will get positive feedback (which results in self-oscillation). The leads in factory made OTs are normally colour-coded to indicate the PR:Sec phasing, but sometimes mistakes are made at the factory, so you really don't know what you get until you hook it up and see.