Do I have classic motorboating??

[casey73]

I have a Princeton Reverb SF that I just put back to original specs. I had done the longtail PI conversion a long time ago and didn't like it. So the chassis sat on a shelf for the last 9 years or so. I finished it today and fired it up. I brought the volume up slowly and everything works, tone stack, reverb, vibrato. As soon as I hit a cord, it went into a low frequency oscillation, and nothing I did stopped it until I turned it off. I'd heard of "motorboating" but never actually experienced it. The filter cap is my main suspect, as everything I've read say motorboating is usually caused by the PS filters, and hitting the bass strings definately set it off. And, after I thought about it a minute, it does sound kinda like a motor boat. I double checked all my wiring and it is on the money. All tubes were tested in another working amp.

I replaced the original filter cap with a Mallory replacement (made in Mexico) about 10 years ago, but as I said the chassis has sat unused in a hot garage for 9 years. I've heard those Mexican made Mallory multi section caps were notoriously unreliable. Before I buy a replacement from AES, is there anything else that might cause this?

[Amp Kat]

Any electrolytic cap could cause the problem so changing them all would be doing yourself a big favor even if they aren't all bad. A cold solder joint on the ground side of a cap could cause it also. Take your meter and measure the gound on the E-caps for DC voltage. It should be zero and if there is voltage on a cap it may be the likely candidate but any bad ground connection in the filter supply will do it also.

[stokes]

Any electrolyics that sit for 10 years without being charged up at all,I would consider suspect,like Amp Kat says,change them all.

[casey73]

Yeah, I should have known better, and maybe brought the power up slowly, I do have a variac. But I got impatient. May or may not have made a difference. I bought home all new caps today. Spragues atoms for the cathode bypass and a CE FP drop in replacement for the old Mallory. Thanks for the feedback.

[Enzo]

Before you do that, try reversing the plate leads on the output tubes.

[Mr. Droopy-Drawers]

If it is motorboating, the coupling capacitor to the phase inverter can be made smaller, and this usually clears that problem up. Ditto for the capacitors from the phase inverter to the output tubes.

[casey73]

Thanks to all who posted on this one. Enzo had the answer. I reversed the plate leads and it works fine! I'd thought of that earlier, but figured since I hadn't messed with them, I shouldn't have to reverse them. But, I did change the PI configuration, which obviously changed the signal phase to the output tubes, right? I'm going to change the caps out anyway because I have them and because they are old, a preventative maintenance thing. Thanks Enzo.